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Aspects of the feeding ecology of the garden cross-spider, Araneus diadematus Clerck Smith, Risa Barbara 1984

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ASPECTS OF THE FEEDING ECOLOGY OF THE GARDEN CROSS-SPIDER, ARAMEUS DIADEMATUS CLERCK by RI.SA BARBARA SMITH B. Sc., UNIVERSITY OF TORONTO, 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (DEPARTMENT OF PLANT SCIENCE) and (INSTITUTE OF ANIMAL RESOURCE ECOLOGY) WE ACCEPT THE THESIS AS CONFORMING TO THE REQUIRED STANDARD THE UNIVERSITY OF BRITISH COLUMBIA May 1984 © Risa B. Smith, 1984 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements fo r an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission f o r extensive copying of t h i s t h e s i s for s c h o l a r l y purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or publication of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) i i ABSTRACT A f i e l d study on the feeding ecology of juvenile cross-spiders confirmed the work of other researchers that "boom and bust" best des-cribes the a v a i l a b i l i t y of prey for these s p i d e r l i n g s ; most of the A r a -neus diadematus Clerck webs observed captured no prey but occasionally prey catches were very high. During the "bust" phases of prey a v a i l a b i l i t y cross-spiderlings were adept at switching to alternate food sources l i k e the micro-organisms that are f i l t e r e d out of the a i r by t h e i r sticky orb-webs. This study established, for the f i r s t time, that spiderlings are able to gain s u b s t a n t i a l n u t r i t i o n from this a e r i a l plankton by eating i t when they consume the i r old webs. Birch pollen, Betula papyrifera, lengthened the l i f e s p a n of both second and t h i r d i n s t a r s p i d e r l i n g s . Feeding on b i r c h pollen also resulted i n an increased frequency of webspinning for second ins t a r s and a decline i n the number of 12-hour i n t e r v a l s required to spin a new web. Fungous spores, Cladosporium  herbarum, however, provided no n u t r i t i o n a l value and may have been delet e r i o u s . Third i n s t a r spiderlings feeding on fungus spun less frequently than both the pollen-fed and starving groups. During the "boom" phases of prey a v a i l a b i l i t y a functional response of cross-spiderlings was their a b i l i t y to respond to increased prey density, as determined i n the laboratory experiments. When an hymenopterous p a r a s i t o i d Aphidius n i g r i p e s , was the prey species i i i hungry s p i d e r l i n g s with no predatory experience exhibited an exponential f u n c t i o n a l response curve. However, once they had a day of capturing prey they responded with a Type 2 response. When an aphid, Macrosiphum euphorbiae, was offered as the prey species predatory experience did not have an e f f e c t and the spiderlings exhibited a Type 2 response. i v TABLE OF CONTENTS page ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS ''Ix CHAPTER 1. INTRODUCTION 1 The Spider 3 CHAPTER 2. FIELD STUDY Introduction 6 The F i e l d Site 6 Methods 1 Results 9 Discussion 13 CHAPTER 3. FUNGUS AND POLLEN CONSUMPTION IN A.DIADEMATUS SPIDERLINGS Introduction 16 The Fungus 18 The Pollen 19 The Spiders 19 Cages and Laboratory Setting 20 Experimental Design 20 Results 22 Discussion 34 Web-Spinning 36 Molting 37 A b i l i t y to Survive Starvation 38 Summary 38 CHAPTER 4. FUNCTIONAL RESPONSE TO TWO PREY TYPES Introduction 39 4A. FUNCTIONAL RESPONSE WITH A BENEFICIAL INSECT AS PREY Materials and Methods 45 The Prey 45 The Spiders 46 Materials 46 Experimental Design 47 Results 50 A n a l y t i c a l Methods 50 Number of Prey Captured 51 Frequency of Web-Spinning 71 Prey Captured Per Web 95 Discusssion 104 Inexperienced Spiderlings 104 V Experienced Spiderlings 110 E f f e c t s Not Explained by Predatory Experience .. I l l 4B. FUNCTIONAL RESPONSE WITH A PEST INSECT AS PREY Materials and methods 113 The Prey 113 The Spiders 113 Materials 113 Experimental Design 114 Results 115 Number of Prey Captured 115 Frequency of Web-Spinnning 137 Prey Per Web 137 Discussion 147 EPILOGUE 149 REFERENCES CITED 152 v i LIST OF TABLES Table No. page 2.1 Summary of prey types captured i n webs 11 2.2 Summary of prey types captured on s t i c k y traps 12 3.1 ANOVA Summary Table - l i f e s p a n by treatment and time i n the communal web 23 3.2 ANOVA Summary Table - frequency of web spinning by treatments and time i n the communal web 25 3.3 ANOVA Summary Table - l i f e s p a n by treatments and molting group 26 3.4 ANOVA Summary Table - frequency of web spinning by treatments and molting group 27 3.5 E f f e c t s of pollen and fungus feeding on web spinning and l i f e s p a n 28 3.6 E f f e c t s of pollen and fungus feeding on t o t a l web exposure 29 3.7 E f f e c t s of pollen and fungus feeding on web spinning behaviour 31 3.8 P r o b a b i l i t y of surviving under d i f f e r e n t treatments 32 4A.01 ANOVA Summary Table - number of parasitoids captured by density and day 53 4A.02 SNK Test for density e f f e c t s on parasitoids captured 54 4A.03 SNK Test for day e f f e c t s on parasitoids captured 55 4A.04 SNK Test for parasitoids captured on each day, segregated by density 58 4A.05 SNK Test for parasitoids captured at each density, segregated by day 60 4A.06 ANOVA Summary Table - paras i t o i d s captured by density and day - for 1st day of prey captured 62 4A.07 SNK Test for parasitoids captured at each density -1st day of prey captured 63 4A.08 ANOVA Summary Table - paras i t o i d s captured by density - experienced s p i d e r l i n g s 67 4A.09 ANOVA Summary Table - paras i t o i d s captured by density - experienced s p i d e r l i n g s - without density=l 68 4A.10 ANOVA Summary Table - parasitoids captured by day 69 4A.11 ANOVA Summary Table - new webs by density and day 74 4A.12 SNK Test for new webs by density 75 4A.13 ANOVA Summary Table - t o t a l web exposure by density and day 77 4A.14 SNK Test for t o t a l web exposure by density 78 4A.15 SNK Test for for new webs by day 79 4A.16 SNK Test for new webs by day - density = 60 81 4A.17 SNK Test for t o t a l web exposure on Days 4 and 5, as affected by density 85 4A.18 ANOVA Summary Table - new webs by density and day -experienced s p i d e r l i n g s 87 4A.19 SNK Test for new webs by density - experienced s p i d e r l i n g s 88 v i i 4A.20 ANOVA Summary Table - new webs by prey captured on the previous day and experimental day 90 4A.21 ANOVA Summary Table - new webs by density and day - before any prey captured 92 4A.22 SNK Test f o r new webs by density - before prey captured 93 4A.23 ANOVA Summary Table - web spinning by day, density=0 96 4A.24 ANOVA Summary Table - prey per web by density and day 99 4A.25 SNK Test f o r parasitoids captured per web by density 100 4B.01 ANOVA Summary Table - aphids captured by density and day 117 4B.02 SNK Test for aphids captured by density 119 4B.03 ANOVA Summary Table - % predation by density 121 4B.04 «SNK Test for aphids captured by day 123 4B.05 ANOVA Summary Table - aphids captured by day 126 4B.06 ANOVA Summary Table - % predation by day 129 4B.07 ANOVA Summary Table - aphids captured by density and day - 1st day of prey capture 130 4B.08 ANOVA Summary Table - % predation by density and day - experienced s p i d e r l i n g s 132 4B.09 SNK Test for % predation by density and day - 1st day of prey capture 133 4B.10 ANOVA Summary Table - aphids captured by density and day - experienced s p i d e r l i n g s 134 4B.11 Percent predation by day - experienced s p i d e r l i n g s 13 4B.12 ANOVA Summary Table - web spinning by density and day 138 4B.13 ANOVA Summary Table - t o t a l web exposure by density and day 140 4B.14 ANOVA Summary Table - new webs by density and day - before aphids captured 141 4B.15 SNK Test for new webs by day 142 4B.16 ANOVA Summary Table - prey per web by density and day 143 4B.17 SNK Test for prey per web by density 145 v i i i LIST OF FIGURES Figure No. page 2.1 Number of prey captured per web 10 2.2 Number of prey captured per web as a function of the period of exposure 14 3.1 Survival rates for each treatment 33 4.1 Summary of four types of functional responses 41 4A.01 Number of prey eaten p r i o r to a molt 49 4A.02 Functional response of cross-spiderlngs with A. nigripes as the prey 52 4A.03 No. of parasitoids captured each day, segregated by density 56 4A.04 No. of parasitoids captured at each density, segregated by day 59 4A.05 Days on which the 1st pa r a s i t o i d was captured 61 4A.06 Functional response on the f i r s t day of prey capture 65 4A.07 Functional response - experienced sp i d e r l i n g s 66 4A.08 Comparison of number of parasitoids captured on 1st day of prey capture and a f t e r 1st day of prey capture 70 4A.09 Percent predation 72 4A.10 Web spinning as a function of pa r a s i t o i d density 73 4A.11 New webs on each day, segregated by density 80 4A.12 T o t a l web exposure per day, segregated by density 82 4A.13 New webs at each density, segregated by day 83 4A.14 To t a l web exposure at each density, segregated by day 84 4A.15 New webs as a function of pa r a s i t o i d density - experienced s p i d e r l i n g s 89 4A.16 P r o b a b i l i t y of spinning a new web as a function of number of parasitoids captured on the previous day 91 4A.17 New webs as a function of pa r a s i t o i d density - before any prey captured 94 4A.18 New webs per day - density = 0 97 4A.19 Comparison of new webs spun 98 4A.20 Parasitoids captured per web 101 4A.21 Comparison of parasitoids captured per web 103 4B.01 Comparison of no. of aphids captured and no. of parasi t o i d s captured 116 4B.02 Functional response of cro s s - s p i d e r l i n g s with aphids as prey 118 4B.03 Percent predation 120 4B.04 No. of aphids captured as a function of day 122 4B.05 No. of aphids captured per day, segregated by density 125 4B.06 Day the f i r s t aphid was captured 127 4B.07 Percent predation as a function of day 128 4B.08 Comparison of percent predation • 131 4B.09 No. of aphids captured as a function of day 135 4B.10 Web spinning as a functon of aphid density 139 4B.11 No. of prey per web 146 4x ACKNOWLEDGEMENTS I would l i k e to thank the people who provided support throughout t h i s study. Dr. William Wellington, my research advisor, provided me with the philosophical perspective necessary to keep me on track and the enthusiasm for outrageous ideas which forced me to be daring. He also generously provided f i n a n c i a l support which allowed me to concen-tr a t e f u l l y on the t h e s i s . Towards the beginning of this study Dr. Bryan Frazer provided invaluable moral support as I struggled with the p o l i t i c s of a g r i c u l t u r e ; towards the end of the study he p a t i e n t l y provided s t a t i s t i c a l advice as I struggled with t e c h n i c a l i t i e s . He also read the f i r s t draft and provided the thoughtful suggestions which helped me improve the t h e s i s . Drs. V i c t o r Runeckles and Robert Copeman generously allowed me to use their equipment and were often w i l l i n g to engage i n discussions concerned with my research. Dr. Tom Mommsen taught me to open my eyes and love science. His c u r i o s i t y about the natural world was in f e c t i o u s and I thank him for the many discussions about natural h i s t o r y , the p o l i t i c s and philosophy of science and p a r t i c u l a r l y the moral r e s p o n s i b i l i t y of s c i e n t i s t s . Special thanks to Dr. Deborah (Hood) Henderson, Ms. Maryanne Kingma and Ms. Barbara Henderson for a l l the help when the "crunch" came. I would also l i k e to thank the "Friday Lunch-Group" for their h e l p f u l suggestions and excitement about their own work. The Department of Plant Science provided the greenhouse space without which the work could not have been conducted and Agriculture Canada provided f a c i l i t i e s to do some of the rearing. 1 CHAPTER 1 INTRODUCTION There i s a growing awareness among arachnologists that spiders represent an underexploited source of potential predators for b i o l o g i c a l control programs (Sasaba et a l . 1973, Post and Riechert 1977, Riechert and Lockley 1984). Considering numbers of individuals, spiders are the dominant predator group i n several a g r i c u l t u r a l ecosystems. Despite our growing knowledge about thei r ecology, r e l a t i v e l y l i t t l e applied interest i n them has been generated. In the late 1940's and 1950's, the most renowned arachnologists agreed that spiders offer no potential for the control of pest species (Gertsch 1949, Bristowe 1958). This idea was based mainly on i n t u i t i o n as few experiments were done to ve r i f y the hypothesis. The b e l i e f was, however, closely linked to a bias for using monophagous parasitoids and predators i n b i o l o g i c a l control programs. This bias p e r s i s t s . Most of the simulation models of predator-prey interactions which are used to gauge the effectiveness of pa r t i c u l a r parasitoids and predators are based on two-species interactions ( i . e . one predator or parasitoid and one prey species) (Riechert and Lockley 1984). Models involving multi-species relationships are s t i l l i n thei r infancy (Hassell 1978, 1979, 1981, Luff 1983, Mace 1983). Several a g r i c u l t u r a l researchers agree that the contributions of general predators (such as spiders) to the natural 2 control of insect pests i s probably underestimated (Dondale et a l . 1979, Knipling 1979, Newsome et a l . 1980, Luff 1983). For example, b i o l o g i c a l control i s usually r e s t r i c t e d to perennial crops i n which monophagous predators or parasitoids can establish themselves over time and s t a b i l i z e the pest population at low densities. Polyphagous predators may, however, be effec t i v e as control agents i n ephemeral crops. In these situations rapid colonization and a rapid rate of increase are more important than s p e c i f i c i t y (Hassell 1981). As w e l l , mechanisms such as density-dependent switching (Murdoch 1969, Murdoch and Oaten 1975), and an a b i l i t y to adapt to the hazards of annual disruptions so common i n agriculture, may further enhance the usefulness of some generalists as b i o l o g i c a l control agents. Many aspects of the predatory response are important i n understanding how a pa r t i c u l a r predator may contribute to the control of a pest species. This thesis examines two aspects of the predatory response of a polyphagous predator, the orb-weaving spider, Araneus diadematus Clerck: the a b i l i t y of the spider to survive long periods of very low prey a v a i l a b i l t y by switching to alternative n u t r i t i o n a l sources (Chapter 3) and the a b i l i t y of the spider to adjust i t s consumption rate i n response to changing prey densities ( i . e . i t s functional response, Chapter 4). A f i e l d study establishes the natural rate of predation to be expected (Chapter 2). The remainder of this chapter w i l l introduce the spider. 3 THE SPIDER A. dladematus , the garden cross-spider, i s native to Europe and was probably f i r s t introduced into the northeastern United States 100 years ago. In Eastern North America i t occurs i n a narrow belt from Newfoundland south to Rhode Island (Levi 1970). I t was more recently established on the West Coast of North America from Oregon to B r i t i s h Columbia (Gertsch 1978). This species i s abundant i n suburban, r u r a l and forested areas i n both Europe (Pfletschinger 1979) and North America. A. diadematus spins a v i s c i d , v e r t i c a l , orb web t y p i c a l of the genus Araneus . Although juveniles are abundant from late A p r i l in the Vancouver area, they usually go unnoticed because of thei r small size ( f i r s t webs of second instars range i n diameter from 2.5 to 4 cm). The adults, however, are very conspicuous i n the f a l l and easily i d e n t i f i e d by the white cross-like mark on the dorsal side of the opisthosoma. The early juveniles do not resemble the adults, but are also easily i d e n t i f i e d . The second to the fourth instars are small yellow spiders, weighing 0.75 + 0.02 ( + S.E.) mg as second instars and possessing a d i s t i n c t i v e black mark at the posterior end of the opisthosoma. By the f i f t h instar they have the same markings as the adult. A. diadematus undergoes 7 to 8 molts, i n both sexes, before reaching maturity (Gertsch 1978). Females put down si l k e n cocoons containing 200-700 eggs two to s i x weeks after copulation. The females soon die and developing spiderlings overwinter i n the cocoon. When development i s complete, the spider cuts i t s way out of the embryo with "egg teeth" and molts 4 simultaneously (Pfletschinger 1979). These f i r s t instar spiderlings remain i n the cocoon, feeding on yolk reserves u n t i l another molt i s achieved. Second instars leave the cocoon i n the spring and spin a communal web. They spend a few days to a few weeks i n a tight cluster before dispersing and building individual webs (Pfletschinger 1979). The juveniles emerging i n the spring develop throughout the summer, overwinter as f i f t h or si x t h i n s t a r s , and mature the following season. There i s some evidence, from my own laboratory experiments, that the males take one season to develop and the females take two seasons. Second to fourth instars may be seen as late as August, perhaps the result of late emerging cocoons. A. diadematus i s as well known to researchers as i t i s to backyard gardeners. In the 1960's Dr. Peter Witt studied the changes i n web building behavi our of the cross—spider i n response to the administration of a variety of drugs (Witt et a l . 1968). In the course of these experiments Witt developed the techniques necessary to rear them i n the laboratory (Witt 1971). The species also received some notoriety for being one of the f i r s t invertebrates on a space shuttle (Witt et a l . 1977). Although the adults eat a variety of insect prey, including wasps, syrphid f l i e s and honeybees, the diet of the early instars i s quite d i f f e r e n t . This i s t y p i c a l of many predators that consume different types and amounts of prey at different stages i n their l i f e cycle (Luff 1983). The juveniles, present when pest species begin to build up, are most l i k e l y to have an impact on pest species. This study was, therefore, r e s t r i c t e d to'the second, t h i r d and fourth i n s t a r s . 6 CHAPTER 2 FIELD STUDY INTRODUCTION Fi e l d studies on the quantity and species spectrum of prey captured by orb-weaving spiders are abundant (e.g. Kajak 1965, Nyffeler and Benz 1982, Janetos 1982, Rypstra 1982). Generally, orb-weaving guilds p a r t i t i o n resources through variations i n web mesh size (Uetz et a l . 1978), chelicera size (Olive 1980), leg length and with v e r t i c a l s t r a t i f i c a t i o n (Enders 1973, Lubin 1978, Uetz et a l . 1978, Olive 1980). However, most f i e l d studies concerned with the prey types eaten by sp e c i f i c spiders focused on adults and sub-adults or the age of the spiders was not mentioned. The f i e l d study described i n this chapter was conducted to establish baseline data for the prey types and quantity most commonly captured by juvenile cross-spiders. The laboratory studies that followed, and which constitute the bulk of this thesis, explored i n d e t a i l some of the results found i n the f i e l d study. THE FIELD SITE The f i e l d s i t e was a suburban backyard garden about 5 km from the University of B r i t i s h Columbia. It had not been sprayed with pesticides for at least 10 years and so contained a diverse arthropod fauna, including several species of orb-weaving spiders. 7 The backyard was divided into 4 d i s t i n c t sections to make census taking eas i e r . They were: a vegetable garden (6.4 m X 6.0 m), four f r u i t trees, a potato patch (4.0 m X 3.0 m) and a fence (22.6 m long X 1.2 m high). The t o t a l volume available to spiders for making webs changed through the summer, as the plants grew and as some crops in the vegetable garden were harvested and replaced with others. METHODS A census of second to fourth inst a r cross-spiderlings was taken three to f i v e times a week, for 11 weeks, i n the summer of 1982. The census time was alternated between dawn (4 observations), noon (6 observations), dusk (14 observations) and midnight (23 observations) because preliminary observations determined that juvenile cross-spiders were feeding at a l l times of the day. A water sprayer was used to make webs more v i s i b l e (Tolbert 1977), and a f l a s h l i g h t was also used at night. The number of prey items In a web and the taxonomic order of prey was determined with the aid of a 10X hand lens. The Homopterans were further separated, into two groups: "aphids" (Family: Aphididae) and "other Homopterans". The Hymenopterans were separated into two groups based on whether or not they were p a r a s i t o i d s . On the rare occasions when i t was impossible to determine the taxonomic order of a prey, the prey was c a r e f u l l y removed from the web, stored in a v i a l of 75% e t h y l alcohol, and i d e n t i f i e d l a t e r i n the laboratory. When the prey were u n i d e n t i f i a b l e , because they had been macerated by the 8 sp i d e r l i n g s , they were recorded as such. A l l prey items that were retained by a web for more than 2 minutes were counted as captured, whether or not the spiderlings wrapped them i n s i l k . This assumption was made by other authors studying orb-weavers (notably Nentwig 1982). However, laboratory experiments bore out that occasionally aphids were able to escape a f t e r a 45 minute delay. Therefore, the f i e l d r e s u l t s probably overestimated the number of aphids a c t u a l l y eaten. With most predator-prey studies the types of prey consumed are measured by an analysis of stomach contents. This i s not possible with spiders because they regurgitate digestive f l u i d into t h e i r prey and then ingest a pre-digested l i q u i d mixture. The species composition of the diet can be determined by s e r o l o g i c a l techniques (Loughton et a l . 1963, Greenstone 1979). However, these were not adopted because i t was simpler, and as accurate, to record the prey found i n webs. In order to gauge the a v a i l a b i l i t y of prey groups, eleven glass pl a t e s , 15 cm square, were coated with "stick-em", a commercial preparation often used i n sampling small insects, and placed i n the same general areas where juvenile spider webs were found. Each trap was l e f t exposed for 12 days. Six were hung outside onAiJuly. .5,-and f i v e were hung, outside on August 7. Insects between 1mm and 5mm long, captured on the sticky traps, Were.identified to taxonomic order. Homopterans and Hymenopterans were further categorized i n the same way as prey captured i n webs. Prey larger than 5mm were seldom captured on these traps and assumed to be beyond the s i z e range s p i d e r l i n g s were capable of handling. 9 Because the census only provided a snapshot view of daily catches, an experiment was conducted to determine whether or not this was a good estimate of the number of prey captured i n a day. Fourteen webs, over f i v e days, were examined hourly from the time of spinning u n t i l dismantling, and the number of prey captured by each was recorded. The spiderlings chosen for this detailed^ study were found i n webs, observed u n t i l they dismantled the old web and spun a new one, and records of prey catches were taken from the time the new web was spun u n t i l i t too was dismantled. RESULTS A t o t a l of 1429 webs were observed over the 11-week study. Of these, 1282, or 89.7%, were empty (Fig. 2.1). Although most spiderlings with any prey captured less than f i v e prey per web, there were examples of much higher prey catches. One individual captured 68 prey items and another 46. Aphids clearly represented the prey group most frequently found i n webs, since 76% of the prey captured were aphids. The capture rates for webs that contained one or more prey are summarized i n Table 2.1. The same table also summarizes the number of each prey type captured when these were the only types of prey i n webs. The data were broken down i n this fashion because most webs that contained any prey contained only one type of prey. In contrast to the prey found i n webs, Diptera were the most commonly found prey on the sticky traps (Table 2.2). Although Homoptera were the second most common group of insects found on the traps, aphids s p e c i f i c a l l y were not common. The hymenopterous parasitoids were as common on sticky traps as aphids. 10 FIGURE 2.1 NUMBER OF PREY CAPTURED PER WEB. Note that 89.7% of the webs were empty.and,of those that contained prey most had less than 5. A few individual webs contained very high numbers of prey. F R E Q U E N C Y o o O o NO o to oo 3 c o — bo o o o O o ^ 1 © TABLE 2.1 SUMMARY OF PREY CAPTURED IN WEBS ALL PREY HOMOPTERA DIPTERA .. HYMENOPTERA. COLEOPTERA UNIDENTIFIABLE (Aphididae) Parasitoids Other Number of Observations 495 Number of Webs Exclusively Containing 147 Each Prey Type Mean Number of Prey Per Web 3.4 Standard error 0.6 Percent of Total Capture 7 " . 377 51 7 65 30 3 5.8 1.7 2.3 1.3 0.3 0.9. 76.2 10.3 1.4 2 2 56 1 2 46 2.0 1.0 1.22 0.2 0.4 0.4 11.3 TABLE 2.2 SUMMARY OF ARTHROPODS CAPTURED ON STICKY TRAPS HOMOPTERA DIPTERA HYMENOPTERA COLEOPTERA ARANEAE LEPIDOPTERA (Aphididae) Parasitoids Number of Observations 54 (29) 91 29 54 9 1 Percent of Total 22.7 (12.2) 38.2 12.2 22.7 3.8 0.4 ro 13 Spiderlings kept each web exposed for 8.0 + 1.5 hours. Ten of these webs (71.4%) captured no prey. The relationship between the time a web was exposed and the number of prey captured (Fig. 2.2) appears to be b e l l shaped. DISCUSSION The majority of juvenile cross-spiders captured no prey a result which agrees with the findings of several other researchers for other species of orb-weavers (Kajak 1965, Janetos 1982, Nyffeler 1982). Aphids also constituted a large portion of the diet i n these other studies, although Diptera usually represented the most common prey and aphids the second most common prey. Hymenopterous parasitoids were captured at rates far below rates expected from their abundance, a promising result considering that one of the major concerns about using orb-weaving spiders i n bi o l o g i c a l control programs i s that they w i l l capture b e n e f i c i a l insects p r e f e r e n t i a l l y . In contrast, aphids were captured at rates well above those expected from their abundance—also a promising result from the perspective of bi o l o g i c a l control. The laboratory study described i n Chapter 4 used a hymenopterous parasitoid and an aphid as sample prey types i n order to determine i f spiderlings were p a r t i c u l a r l y adept at capturing aphids and, conversely, p a r t i c u l a r l y incompetent at capturing parasitoids. The comparison between prey captured on the sticky traps and that captured i n the webs has l i m i t a t i o n s . I t would have been better to place traps immediately beside webs and monitor them only for the time that webs were exposed. An attempt was made to conduct such a 14 FIGURE 2.2 NUMBER OF PREY CAPTURED PER WEB AS A FUNCTION OF THE PERIOD OF EXPOSURE. Hourly observations were made on the number of prey captured for these 14 webs. 14a 6 H P L , 4 H 0 -0 r 8 12 16 H O U R S W E B S U P T 2 0 2 4 15 study, but the results were inconclusive. Traps were placed beside webs when a spider was seen spinning, with the intent of comparing the prey captured in a web with the available prey in the same micro-habitat. However, after monitoring 25 webs in this fashion the pro-ject was abandoned. In no case did any of the spiders monitored capture any prey, nor did any of the traps catch insects. Laboratory experiments, which will be discussed in Chapter 4, provided some in-sight into why this occurred. The frequency of web-spinning increased with low prey availability. As a result, the likelihood of spotting a poor forager in a poor habitat spinning was greater than the likeli-hood of spotting a good forager in a good habitat spinning. The development of a better monitoring scheme to compare prey captures and prey availability would be a prerequisite for future studies. The estimates of prey capture over an entire day, taken from only one observation per day, proved to supply adequate estimates of daily prey captures. Most of the 14 webs (i.e. 10) which were ob-served hourly over their entire exposure period captured no prey. Those that did succeed in capturing prey caught between 1 and 5 in-sects, a result which compares well with average number of prey captured when webs were observed only once a day. The lack of a linear relationship between prey captured and the period of web ex-posure indicates that a cue other than the amount eaten must play some role in determining when a web is dismantled. However, there were so few webs that captured any prey that it is difficult to determine whether or not these results are a function of a small sample size. Again, further field experiments are warranted. 16 CHAPTER 3 FUNGUS AND POLLEN CONSUMPTION IN A^ DIADEMATUS SPIDERLINGS INTRODUCTION Many orb-weaving spiders c o l l e c t and eat their old webs i n f a i r l y regular i n t e r v a l s — a well documented behaviour (Breed et a l . 1964, Gertsch 1978) which i s usually explained as a mechanism for recovering some of the costs of producing s i l k (Peakall 1971). The boom and bust si t u a t i o n i n which orb-weavers are generally found (Janetos 1982, Chapter 2) renders such a mechanism important to indivi d u a l s u r v i v a l . However, the energetic costs of web building exceed returns from recycling old webs (Peakall and Witt 1976, Prestwich 1977). I t would, therefore, not be possible for spiderlings to spin and dismantle several webs without consuming some nutrients. Yet, i n the f i e l d , they were observed to spin several webs without apparently capturing any insects. This chapter examines the p o s s i b i l i t y that web eating provides spiderlings with the opportunity to gain nutrients from the a e r i a l plankton, and p a r t i c u l a r l y the fungus and pollen, captured on their sticky s p i r a l s of the web. Benefits from eating microorganisms could be p a r t i c u l a r l y important for second instar spiderlings ( i . e . the f i r s t web building stage) since this stage has already survived one molt and a period of about one to 2 weeks with no n u t r i t i o n a l input other than yolk reserves (Witt et a l . 1972, Burch 1979). Accounting for the fact that spiders can s a c r i f i c e 50% of their body weight to produce s i l k 17 (Witt 1963) spiderlings would s t i l l soon exhaust their silk-producing a b i l i l t y i f they were starving. A variety of microorganisms could he seen when webs of second instar spiderlings were examined under a l i g h t microscope. Some were microscopic animals ( i . e . insects and nematodes) but most were plant pollen and fungal spores. Lubin (1978) remarked that webs can sometimes be so densely coated with pollen and dust as to reduce the i r effectiveness for capturing prey. I f juvenile spiderlings obtain nutrients from the microorganisms on their webs two well established dogmas about orb-weaving spiders would be shattered: that spiders are exclusively carnivorous (Turnbull 1973, Foelix 1982), and that orb-weavers consume thei r webs s t r i c t l y for the benefits of recycling the s i l k protein (Peakall 1971). It i s advantageous for animals used i n b i o l o g i c a l control programs to be able to endure periods of food shortage, either by withstanding starvation or eating alternative foods (Luff 1983). I f spiderlings can survive on microorganisms, there would be obvious implications about their a b i l i t y to wait out periods of low prey a v a i l a b i l i t y . 18 MATERIALS AND METHODS THE MICROORGANISMS Webs of juvenile kj_ diadematus were c o l l e c t e d on glass microscope s l i d e s i n May and June, 1982. Examination of these s l i d e s under a l i g h t microscope determined that fungus and pollen were the most abundant microorganisms on webs. Therefore, one fungus and one pollen were chosen as representatives of a e r i a l plankton to use i n the experiments. The Fungus Cladosporium herbarum (Pers) Link et S.F.Gray was c o l l e c t e d by pressing spider webs against a plate of potato dextrose agar and incubating at room temperature. Several species of fungus grew from these c o l l e c t i o n s , but C^ _ herbarum was the most abundant. C.  herbarum i s a common mould of decaying vegetation and constitutes about 33% of a l l spores caught i n i n e r t i a l traps i n the a i r of Western North America (Lacey 1981) so i t was not sur p r i s i n g that i t was also common on spider webs. The C. herbarum used i n these experiments was obtained from subcultures of the o r i g i n a l colonies. 19 The Pollen Birch, Betula papyrifera Marsh, catkins were collected l o c a l l y between late March and early A p r i l , a i r dried on fine copper screens, and the pollen sieved. The pure pollen was stored in glass jars at room temperature. In the f i e l d , the f i r s t spiderlings were already i n webs when the birch was i n flower. THE SPIDERS Communal groups of A_^_ diadematus spiderlings were collected on May 28 and 29, 1983, from Westham Island, B r i t i s h Columbia, Canada. These second i n s t a r (I ) spiderlings were maintained communally u n t i l required for the experiment. Spiderlings were then transferred into i n d i v i d u a l cages. On May 30, 60 spiderlings were separated from the i r s i b l i n g s , put into i n d i v i d u a l glass cages, and fed one aphid, Macrosiphum euphorbiae (Thomas), per day u n t i l they molted. The th i r d instars obtained were used i n experiments immediately after they had molted so that comparisons between I„'s and I~'s could be made. 20 CAGES AND LABORATORY SETTING Two types of cages were used. The f i r s t type was a 20 cm. square by 9 cm deep plywood frame with a removable glass window on one side and clear acetate on the opposite side. A 10 cm square was cut i n the acetate and a piece of cotton mesh inserted to provide v e n t i l a t i o n . These were used for a l l experimental t r i a l s using the microorganisms, as well as a starved control group. Glass dishes 10 cm i n diameter and 8 cm high were used to house the 60 aphid-fed s p i d e r l i n g s . An 8 cm square cardboard frame placed i n each dish provided support for webs. A l l the experiments were conducted i n the Plant Science greenhouse, Un i v e r s i t y of B r i t i s h Columbia, between May 31 and July 15, 1983. Greenhouse temperatures fluctuated between highs of 30-33 C and lows of 15-18 C. The animals were under a natural photoperiod of 16 hours l i g h t , 8 hours dark. EXPERIMENTAL DESIGN Second i n s t a r s p i d e r l i n g s were taken from the communal groups and put into i n d i v i d u a l plywood cages arranged on a table. Once s p i d e r l i n g s had spun t h e i r f i r s t web 69 were randomly assigned to one of three treatment groups, i n which they were given spores from the fungus, grains of bir c h p o l l e n , or starved. Twenty-seven I^'s were transferred into plywood cages the day a f t e r molting and randomly assigned to a treatment group. Some of the I^'s were s t i l l a l i v e and being treated when the 1.,'s were assigned to treatments. A l l cages were sprayed with d i s t i l l e d water twice a day to protect the spiderlings from dehydration. Twice a day, at 11 am and 11 pm, observations were made and any new webs were treated. Records were kept of the l i f e span of each spider, the number of webs spun, the duration each web was up, and the time i n t e r v a l between webs. Fungous spores and pollen were applied to webs by brushing the fungus colonies or pollen l i g h t l y with a fine sable brush and then tapping the brush over a web. The densities of black spores, and the bright yellow birch pollen were rea d i l y v i s i b l e on the webs, so that new webs were e a s i l y i d e n t i f i e d and subsequently treated. New webs of the starving group were marked with a fine thread of cotton. The aphid-fed sp i d e r l i n g s were taken from the communal group, put into i n d i v i d u a l glass cages, and fed one aphid per day u n t i l they molted. Their web spinning behaviour was not recorded. They were used only to obtain a measure of the time to the f i r s t molt, outside of the communal web, to be expected i n spiderlings on an insect d i e t . 22 RESULTS Two-way analysis of variance (anova) was used as a method of separating treatment e f f e c t s from time spent i n the communal group, for second i n s t a r s , and time to the molt, for t h i r d i n s t a r s . Data that did no meet the assumptions of normality or equal variances were transformed with either a log or an angular transformation. The angular transformation was used for proportional data ( G i l b e r t 1973, Sokal and Rohlf 1981). For log transformed data the geometric means ( i . e . the back-transformed means) are presented; otherwise arithmetic means are presented ( G i l b e r t 1973). Where the anova test indicated a s i g n i f i c a n t treatment e f f e c t Student-Newman-Keuls (SNK) a_ p o s t e r i o r i t e s t s , at the 0.05 l e v e l of s i g n i f i c a n c e , were calculated i n order to determine where the difference lay (Sokal and Rohlf 1969). The second i n s t a r data were segregated into two groups according to whether i n d i v i d u a l s had spent le s s than one week, or one to two weeks, i n a communal group. In order to have an equal number of s p i d e r l i n g s i n each of these two groups records for one s p i d e r l i n g from each treatment were randomly removed from the an a l y s i s . This l e f t a t o t a l of 66 1^ s p i d e r l i n g s , 22 i n each treatment group and 33 i n each age group. Third i n s t a r s were s i m i l a r l y divided into 3 groups of 9 according to when they molted. The period i n a communal group had a s i g n i f c a n t e f f e c t on l i f e s p a n for the ^ ' s (Table 3 . 1 ) — l e s s time i n a communal group resulted i n a longer l i f e s p a n . Treatment e f f e c t s were also s i g n i f i c a n t . However, the frequency of web spinning was s i g n i f i c a n t l y d i f f e r e n t 23 TABLE 3.1 TWO-WAY ANALYSIS OF VARIANCE SUMMARY TABLE FOR LIFESPAN BY TREATMENT AND TIME IN THE COMMUNAL GROUP. This analysis was computed f o r second i n s t a r s p i d e r l i n g s . A log transformation was performed on the data to meet the assumption of equal variances. Source of Variation Sum of DF Mean F Significance Squares Square of F Subgroups 19.818 5 3.964 10.895 <0.001 Treatment 6.524 2 3.262 8.966 <0.001 Time in Connunal Web 12.594 1 12.594 34.719 <D.001 2-Way Interactions 0.700 2 0.350 0.962 0.388 within groups (error) 21.828 60 0.364 . Total 41.645 65 0.641 24 for treatments only (Table 3.2). For t h i r d i n s t a r s both l i f e s p a n and web spinning were s i g n i f i c a n t l y d i f f e r e n t across treatments but not molting groups (Tables 3.3 and 3.4). Spiderlings s u b s t a n t i a l l y increased t h e i r chances of s u r v i v a l by feeding on p o l l e n . Compared with t h e i r starving s i b l i n g s , s p i d e r l i n g s fed pollen doubled t h e i r mean l i f e span, and spun s i g n i f i c a n t l y more webs per 12 hour i n t e r v a l . In contrast, the group receiving fungus did not d i f f e r i n any of these respects from the starving group. However, there was also no s i g n i f i c a n t difference between the fungus and pollen-fed groups i n the frequency of web-spinning (Table 3.5). The r e s u l t s were s i m i l a r for t h i r d i n s t a r s , i n that s p i d e r l i n g s fed pollen again increased t h e i r l i f e span by about 5 days when compared to the starving and the fungus-fed groups. However, with the Ig's the fungus-fed group spun webs s i g n i f i c a n t l y l e s s often than both the pollen-fed and starving groups and there was no s i g n i f i c a n t difference between the pollen-fed and starving groups (Table 3.5). Lifespan and frequency of web spinning combine to give the t o t a l web exposure. For a s p i d e r l i n g the t o t a l web exposure i s equivalent to searching time for other predators and i s , therefore, an important factor i n ensuring i n d i v i d u a l s u r v i v a l . Total web exposure was s i g n i f i c a n t l y greater i n the pollen-fed group than either the fungus-fed or starving groups for both i n s t a r s (Table 3.6). No s i g n i f i c a n t differences across treatments were detected i n the period i n d i v i d u a l webs remained i n t a c t for either l ^ ' s or Io' s . There was, however, a marked difference across treatments 25 TABLE 3.2 TWO-WAY ANALYSIS OF VARIANCE SUMMARY TABLE FOR FREQUENCY OF WEB SPINNING BY TREATMENTS AND TIME IN THE COMMUNAL GROUP. This analysis was ca l c u l a t e d f o r second i n s t a r s p i d e r l i n g s . Frequency of web spinning = T o t a l number of webs spun Number of 12-Hour Intervals A l i v e . An angular transformation (Sokal and Rohlf 1981). was computed to meet the assumption of equal variances. An o u t l i e r , which was > 3 standard deviations from the mean, was deleted from the s t a r v i n g group. The datum removed represented the frequency of web-spinning f o r a s p i d e r l i n g that had l i v e d one day and spun one web., Therefore, the angular transformation of the frequency of web-spinning was 90, d i s t o r t i n g the actual values for the starving group. In order to maintain the orthogonal design one datum was removed, at random, from each of the pollen-fed and fungus-fed groups. Source of Variation Sum of DF Mean F Significar Squares Square Subgroups 846.066 5 169.213 2.306 0.056 Treatment 623.377 2 311.688 4.248 0.019 Time in Connunal Web 26.675 1 26.675 0.364 0.546 2-Way Interaction 196.013 2 98.006 1.336 0.271 Within groups (error) 4181.820 57 73.365 Total 5027.887 62 81.095 26 TABLE 3.3 TWO-WAY ANALYSIS OF VARIANCE SUMMARY TABLE FOR LIFESPAN BY TREATMENTS AND MOLTING GROUP. This analysis was computed for t h i r d i n s t a r s p i d e r l i n g s . Source of Variation Sum of DF Mean F Signifies Squares Square Subgroups 1278.000 8 159.750 2.101 0.091 Treatment 580.222 2 290.111 3.815 0.042 Molting group 76.222 2 38.111 0.501 0.614 2-Way Interactions 621.556 4 155.389 2.044 0.131 Within groups (error) 1368.665 18 76.037 Total 2646.665 26 101.795 27 TABLE 3.4 ANALYSIS OF VARIANCE SUMMARY TABLE FOR FREQUENCY OF WEB SPINNING BY TREATMENTS AND MOLTING GROUP. This analysis was calculated f o r t h i r d i n s t a r s p i d e r l i n g s . Frequency of web-spinning = T o t a l number of webs spun Number of 12 hr. i n t e r v a l s a l i v e . Source of variation Sum of DF Mean F Significanc Squares Square Subgroups 0.083 8 0.010 3.442 0.014 Treatment 0.059 2 0.030 9.877 0.001 Molting Group 0.020 2 0.010 3.315 0.059 2-Way Interactions 0.003 4 0.001 0.288 0.882 Within groups (error) 0.054 18 0.003 Total 0.137 26 0.005 28 TABLE 3.5- EFFECTS OF POLLEN AND FUNGUS FEEDING ON LIFESPAN AND FREQUENCY OF WEB-SPINNIG. Means and 95% confidence i n t e r v a l s f o r l i f e s p a n and the frequency of web-spinning are presented. The confidence i n t e r v a l s are given rather than standard errors because s t a t i s t i c a l tests were ca l c u l a t e d on log transformed data, for the ^ ' s , i n order to meet the assumption of equal variances. The geometric means are given f o r I~'s and arithmetic means are given for I_'s. The arrows j o i n means that were not s i g n i f i c a n t l y d i f f e r e n t at the 0.05 l e v e l of s i g n i f i c a n c e , as determined by Student-Newman-Keuls M u l t i p l e Range Test (Sokal and Rohlf 1969). INSTAR TRT MEAN 95% C.I. LIFESPAN (12-hour Intervals) 2 »Fungus-Fed Pollen-Fed »Starving 1.0.96 20.17 9.90 8.10 -13.86 -7.25 -14.82 29.34 13.51 3 •> Fungus-Fed Pollen-Fed • Starving 17.22 27.11 17.33 13.07 -18.88 -9.11 -21.38 35.34 25.56 FREQUENCY of SPINNING (# of webs/ lifespan) 2 i Fungus-Fed <i Pollen-Fed «j * Starving 0.25 0.30 0.19 0.19 -0.25 -0.13 -0.30 0.36 0.25 3 Fungus-Fed +Pollen-Fed •» Starving 0.15 0.26 0.21 0.10 -0.22 -0.17 -0.19 0.30 0.25 29 TABLE 3.6 EFFECTS OF POLLEN AND FUNGUS FEEDING ON TOTAL WEB EXPOSURE. The means and confidence i n t e r v a l s f o r t o t a l web exposure are presented. The p r o b a b i l i t y was determined by a one-way ana l y s i s of variance t e s t . T o t a l Web Exposure = T o t a l number of 12 hr. i n t e r v a l s s p i d e r l i n g s were i n webs over t h e i r e n t i r e l i f e s p a n . The arrows j o i n means that were not s i g n i f i c a n t l y d i f f e r e n t at the 0.05 l e v e l of s i g n i f i c a n c e as determined by Student-Newman-Keul's M u l t i p l e Range Test (Sokal and Rohlf 1969). INSTAR Trt Mean 95% C.I. P 2 »Fungus-Fed 6.55 5.00 - 8.59 <0.001 Pollen-Fed 14.22 9.25 -21.85 • Starving 5.11 3.77 - 6.93 3 »Fungus-Fed 9.11 4.84 -13.38 0.004 Pollenf-Fed 20.44 12.87 -28.02 * Starving 9.33 4.79 -13.88 30 i n the i n t e r v a l between webs. This period was s i g n i f i c a n t l y shorter for the pollen-fed s p i d e r l i n g s (Table 3.7). F i f t y percent of the fungus-fed and starving ^ ' s were dead a f t e r 7 days and 100% were dead a f t e r 14 days and 12 days r e s p e c t i v e l y . In contrast, i n the pollen-fed group 50% of a l l i n d i v i d u a l s were dead by the 12th day and 100% by the 36th day. The l i f e span of the pollen-fed group compared well with the aphid-fed group, i n which 50% of the i n d i v i d u a l s had either molted or died by Day 12 and 100% had molted or died by Day 22. The t h i r d i n s t a r pollen-fed group also outlived the other two treatment groups. However, some i n the starved group l i v e d longer than the fungus-fed group (see Table 3.8 for summary). The percent s u r v i v a l of each group i s i l l u s t r a t e l d i n F i g . 3.1. None of the s p i d e r l i n g s i n the three treatment groups molted. Nineteen of the 60 (32%) insect fed s p i d e r l i n g s died before molting. The re s t molted a f t e r 8 and before 19 days and had either molted or died by Day 21 ( F i g . 3.1a). Some of the pollen-fed s p i d e r l i n g s l i v e d for 36 days, but without molting. 31 TABLE 3.7 EFFECTS OF POLLEN AND FUNGUS FEEDING ON WEB-SPINNING BEHAVIOUR. The means and confidence i n t e r v a l s f o r the number of 12-hour i n t e r v a l s i n d i v i d u a l webs remained intact and the number of 12-hour i n t e r v a l s between webs are presented. The data did no meet the assumption of equal variances even with a transformation. Therefore Kruskal-Wallis One-Way Anova was cal c u l a t e d to determine the p r o b a b i l i t i e s . The Student-Newman-Keuls Multiple Range Test, used i n the other analyses, i s only appropriate when the variances are equal, so i t was not c a l c u l a t e l d i n t h i s case. INSTAR TRT MEAN 95% ( ".I. P # 12 HR. 2 Fungus-Fed 2.94 2.63 - 3.25 0.547 INTERVALS Pollen-Fed 3.10 2.89 - 3.30 WEBS REMAINED Starving 3.22 2.58 - 3.85 INTACT 3 Fungus-Fed 3.57 2.46 - 4.67 0.306 Pollen-Fed 3.13 2.72 - 3.55 Starving 3.32 2.86 - 3.78 # 12 HR. 2 Fungus-Fed 3.29 2.38 - 4.20 0.001 INTERVALS Pollen-Fed 1.98 1.71 - 2.24 BETWEEN WEBS Starving 4.11 2.78 - 5.44 3 Fungus-Fed 5.39 3.47'- 7.32 0.003 Pollen-Fed 2.72 2.04 - 3.39 Starving 4.57 3.27 - 5.87 32 TABLE 3 . 8 PROBABILITY OF SURVIVING UNDER DIFFERENT TREATMENTS. L T ^ Q = Day on which 50% of the spiderlings were dead. L T 1 Q 0 = D A Y ° N W 1 L I C N ^ 0 % of the sp i d e r l i n g s were dead. For the aphid fed group the LT's represent the day on which s p i d e r l i n g s were no longer i n the second i n s t a r stage because they had e i t h e r molted or died. L T 5 0 L T 1 0 0 Fungus-Fed 7-8 14 SECOND Pollen-Fed 12 36 INSTARS Starving 7 12 Aphid-Fed. 12 22 Fungus-Fed 9-10 14 THIRD Pollen-Fed 15-16 28 INSTARS Starving 8-9 21 33 FIGURE 3.1 SURVIVAL RATES FOR EACH TREATMENT GROUP. A. Second Instars. This figure includes the aphid-fed group. A — — —A=fungus-fed. O O =pollen-fed. A — A =starving. • •• - • =aphid-fed. B. Third Instars. There were no aphid-fed spiderlings that started the experiment as I _'s. 33a L I P K t P A M ( d a y s ) 34 DISCUSSION Birch pollen proved to be a valuable source of n u t r i t i o n to second and t h i r d i n s t a r cross-spiders. The fungus, C^ herbarum provided no apparent n u t r i t i o n a l value and was perhaps deleterious. As pollen i s an important nutrient for many t e r r e s t r i a l arthropods (Stanley and Linskens 1974) i t i s not su r p r i s i n g that i t may, on occasion, be a source of food for spiders. However, there i s l i t t l e d i fference i n the food value of fungous spores and pollen grains (Stanley and Linskens 1974, G r i f f i n 1981). The negative r e s u l t s obtained with fungus are therefore puzzling and warrant further study. Fungous spores (diameters 3-30 micrometers) and pollen grains (diameters 20-50 micrometers, H i r s t and H i r s t 1967) are too large to pass through the c u t i c u l a r p l a t e l e t s of a spider's pharynx. P a r t i c l e s greater than 1 micrometer are passively f i l t e r e d out (Foel i x 1982). The presence of chitinase i n spiders' digestive f l u i d (Mommsen 1978, 1980), however, suggests that they should have no d i f f i c u l t y i n digesting the chitinous outer coat of fungous spores (Aronson 1981), through e x t r a i n t e s t i n a l digestion, when they consume an old web. Enzymes capable of degrading pollen exine ( i . e . the outer coat of a pollen grain) have not been a c t i v e l y sought i n web-building spiders or a l l i e d predators, since such enzymes reputedly occur so r a r e l y i n the animal kingdom (Stanley and Linskens 1974). Insects that feed on pollen empty the contents of pollen grains through the germination pores by mechanically crushing the grains or through chemical processes i n the gut. Spiders, 35 however, must digest pollen grains e x t r a i n t e s t i n a l l y . I t i s conceivable that s p i d e r l i n g s are able to extract the contents of pollen grains by macerating pollen with t h e i r chelicerae and thereby leaving the exine behind. In the course of t h i s study the mouthparts sp i d e r l i n g s that had just fed on pollen were examined by scanning electron microscopy, for the remnants of pollen grains. No such p a r t i c l e s were found. However, r e p l i c a t e s of t h i s type of study would be necessary to d e f i n i t i v e l y determine whether or not sp i d e r l i n g s digest pollen exine. They may be one of the few animals able to do so. Orb webs may be covered with many kinds of small animals and plant fragments i n the f i e l d . A complement of several species of microorganisms may provide a complete d i e t during times of low prey a v a i l a b i l i t y , and possibly be s u f f i c i e n t to enable s p i d e r l i n g s to molt. Spider s i l k i t s e l f provides a r i c h proteinaceous substrate that may enable some organisms to grow. The by-products of the growth of microorganisms may further increase the n u t r i t i o n a l benefits of web-eating. The small, f i n e l y meshed f i r s t webs of the second i n s t a r s (2.5 to 4.0 cm. i n diameter; Pfletschinger 1979) are better suited for f i l t e r i n g a e r i a l plankton than they are at detaining insect prey. Microorganisms may be the main food of sp i d e r l i n g s with insects providing only a dietary supplement! 36 WEB SPINNING Orb-weaving spiders are caught i n the dilemma of "no food without a web and no web without food" (Fabre c i t e d i n Bristowe 1958). Prolonged exposure of a functional web, which increases the chances of catching an insect, i s p a r t i c u l a r l y important i n springtime when prey of appropriate s i z e s for these small s p i d e r l i n g s are scarce. Pollen feeding not only resulted i n an increased frequency of web spinning for second i n s t a r s p i d e r l i n g s but also promoted more rapid spinning of subsequent webs when compared to fungus feeding and s t a r v a t i o n . The r e s u l t s were d i f f e r e n t for t h i r d i n s t a r s . Although pollen feeding did not increase the frequency of web spinning, fungus feeding decreased the frequency of web spinning. The spores of the fungus chosen for t h i s experiment may have been " d i s t a s t e f u l " so that the s p i d e r l i n g s avoided i t by not spinning. The increased i n t e r v a l between webs may have d i f f e r e n t causes for those s p i d e r l i n g s feeding on fungus and those starving. The fungus-fed animals seemed to be avoiding spinning, whereas the starving s p i d e r l i n g s were l i k e l y unable to manufacture the s i l k required to spin a web. 37 MOLTING A high percentage of spiderlngs fed on aphids died without molting, i n d i c a t i n g that natural mortality i s r e l a t i v e l y high for early i n s t a r cross-spiders. However, none of the starved s p i d e r l i n g s or those fed only on spores or pollen molted. Some of the spi d e r l i n g s fed pollen o u t l i v e d the period that the aphid-fed s p i d e r l i n g s remained as second i n s t a r s . The pollen-fed s p i d e r l i n g s may have died because they could not molt, rather than from starvation • per se. Spiderlngs fed a sing l e aphid at the beginning of an experimental period were able to molt, even when they had been kept on an insect free diet a f t e r the f i r s t meal, whereas those receiving only pollen or spores never molted. D i s t i n c t differences e x i s t between species of plants i n the n u t r i t i o n a l value of t h e i r pollen. For example, under experimental conditions pollen from anemophilous ( i . e . wind pollinated) plants i s n u t r i t i o n a l l y poorer for bees than pollen from entomophilous plants ( i . e . insect p o l l i n a t e d ) . S p e c i f i c a l l y , tyrosine, an e s s e n t i a l amino acid i n the formation of new c u t i c l e f o r both insects (Chapman 1969) and spiders ( C o l l a t z and Mommsen 1975), i s found i n trace amounts only i n several species of wind-transpored pollens (Stanley and Linskens 1974). I t would not be s u r p r i s i n g i f a tyrosine deficiency i n the anemophilous birch pollen used i n t h i s experiment was responsible for the i n a b i l i t y of s p i d e r l i n g s to molt. 38 ABILITY TO SURVIVE STARVATION The a b i l i t y of spiderlings to survive long periods of starvation was remarkable. Second instars, weighing only 0.75 + 0.02 mg were able to survive for 12 days, and t h i r d instars were able to survive for 21 days. SUMMARY Second and thi r d instar A^ diadematus spiderlings derived nutrients from birch pollen. In the absence of any other nutrient sources, pollen sprinkled on the sticky webs doubled the l i f e expectancy of spiderlings and altered their web spinning behaviour so that they spun more often. Windborne fungal spores are another potential nutrient source i n the springtime, when spiderlings f i r s t appear and insect prey i s sparse. Spores of the fungus Cj_ herbarum however, did not lengthen the l i f e expectancy or increase the web spinning a c t i v i t y of juvenile spiders. The spiderlings given birch pollen did not molt, although they l i v e d to an age that, had they been fed an insect d i e t , would have included a molting period. 39 CHAPTER 4 FUNCTIONAL RESPONSE OF JUVENILE CROSS-SPIDERS TO TWO PREY TYPES —AN APHID AND A HYMENOPTEROUS PARASITOID INTRODUCTION The experiments i n the previous chapter showed that juvenile cross-spiders are w e l l adapted to survive the long periods of low insect a v a i l a b i l i t y they experience i n the f i e l d . However, insect prey are occasionally abundant, especially i n a g r i c u l t u r a l ecosystems i n which pest populations often become dense. In this chapter the a b i l i t y of juvenile cross-spiders to respond to changing insect a v a i l a b i l i t y i s examined. Two d i s t i n c t aspects of predation are important i n rendering a par t i c u l a r predator capable of responding to changing prey densities i n a way that regulates the prey population, the predator's numerical and functional responses. The numerical response refers to the predator's a b i l i t y to increase i t s own numbers i n response to an increase i n the numbers of i t s prey. This can occur either by an increase i n fecundity or an increase i n immigration (Luff 1983). Increased fecundity, i n response to increased food a v a i l a b i l i t y , i s well documented for spiders (Riechert and Tracy 1975, Gertsch 1978, Wise 1979). However, this response could not be s u f f i c i e n t to control insect pests. Temperate spiders usually require at least one season to reach maturity. Therefore, the lag inherent i n reproductive numerical response would be too great to affect the numbers of a rapidly growing pest 40 population. Web-spinning spiders do, however, tend to aggregate where prey numbers are high (Turnbull 1964, Riechert 1974, Rypstra 1983), indicating that immigration to patches of high prey density can be expected. This aggregative numerical response may add to the usefulness of some spiders as pest control agents. The functional response of orb-weavers holds the most potential for them to be important pest control agents. The functional response refers to a predator's rate of prey consumption as prey density increases. Although the concept of functional respones was f i r s t introduced by Solomon (1949) i t was not u n t i l 10 years l a t e r that Holling (1959) popularized i t s use. Holling (1959, 1961, 1965) described four types of functional responses, one of which most predators exhibit (Fig. 4.1). Only the sigmoid (Type 3) functional response can cause density-dependent mortality i n prey and, therefore, establish a predator as pot e n t i a l l y important i n prey population regulation (Holling 1968, Berryman 1981). Density-dependent prey mortality means that an increased proportion of prey die as t h e i r density increases. Predators with a Type 1 response capture prey at a constant rate as prey density increases, u n t i l they become satiated. With a Type 2 response a predator captures prey at a decreasing rate as density increases. Neither of these two responses results i n density-dependent prey mortality. During the r i s e phase of the functional response curve of Type 3 predators, prey capture i s increasing at an accelerating rate. This results i n a greater proportion of prey dying as prey density increases, and, hence, density-dependent prey mortality. Type 4 responses have generally been ignored by textbook writers. At high 41 FIGURE 4.1 SUMMARY OF FOUR TYPES OF FUNCTIONAL RESPONSES. Most predators exhibit one of the four types of functional responses described by Holling (1959, 1961, 1965). 41a T Y P E 1 T Y P E 2 T Y P E 3 T Y P E 4 P R E Y D E N S I T Y 42 densities the Type 4 response leads to a decrease i n prey consumption, usually because the predator has learned to avoid a d i s t a s t e f u l prey or the prey have become more d i f f i c u l t to capture because of their responses to their own density. Holling (1959) hypothesized that aquatic f i l t e r feeders would display Type 1 responses, other invertebrate predators would display Type 2 responses and vertebrate predators would display Type 3 responses. This was based on the b e l i e f that the underlying mechanism behind the Type 3 response was learning, and that invertebrates could not learn. However, since Holling's o r i g i n a l papers many experiments have been conducted to derive the functional responses of a wide variety of predators, and the Type 3 response has indeed been found i n some invertebrates (Hassell et a l . 1977). It i s important i n understanding predator-prey interactions, to distinguish among Type 1, 2 and 3 predator functional responses because of their different potential to regulate prey populations. The functional response of spiders i s poorly understood (Riechert and Luczak 1982). Some models imply the existence of a Type 2 functional response (Hardman and Turnbull 1974, 1980) and some writers asssume spiders must have a functional response of some kind (Turnbull 1973, Riechert 1974). However data are scanty. Furthermore, the few studies on spider functional responses have been done on wandering spiders, not on web-spinning spiders (e.g. Haynes and Sisojevic 1966, Hardman and Turnbull 1974,1980, Greenstone 1978). There i s some indirect evidence that web-spinning spiders may have a sigmoid functional response. Kajak (1965, 1978) demonstrated 43 that a relationship exists between the density of certain ephemeral prey species and the number of prey caught i n spider webs i n the f i e l d . She found no such correlation, however, with most prey species. Chant (1956) observed an increase i n percentage of prey consumption as prey density increased for web-spinning spiders but not for wandering spiders. He used only two levels of density i n his experiments, which were done long before the concept of functional responses was widely accepted. To date, the functional response of orb-weavers has not been elucidated. The experiments described i n this chapter were undertaken to f i l l t h i s gap and determine the functional response for juvenile cross-spiders. Prey characteristics can exert a considerable effect on predation (Holling 1959, Hassell 1978). For orb-weaving spiders, the prey characteristics of importance are: the response of the prey to i t s own density, p a r t i c u l a r l y i f overcrowding leads to dispersal and, hence, an increased chance of contact with spider webs, the a b i l i t y of a prey species to create a s u f f i c i e n t vibration to indicate i t s presence i n the web, and the a b i l i t y of a prey species to escape from the web before the spider can attack (Nentwig 1982). Different prey types may, indeed, e l i c i t a different type of functional response i n a polyphagous predator. Therefore, two prey species were used i n this study. An aphid, Macrosiphum euphorbiae (Thomas), i s representative of a group of animals that are major a g r i c u l t u r a l pests as we l l a prey group often found i n spider webs (Kajak 1965, Nentwig 1983, Nyffeler 1982, Chapter 2 this thesis); Aphidius nigripes Ashmead, a hymenopterous parasitoid of that aphid, i s representative of a major group of b e n e f i c i a l insects often used 44 in b i o l o g i c a l control programs, and seldom found i n spider webs (see Chapter 2). Ultimately, with generalist predators, functional responses to several prey species presented simultaneously must be determined (Murdoch 1973, Murdoch and Oaten 1975), The f i r s t step, however, i s to examine the functional response to individual prey species (Akre and Johnson 1979, Mace 1983). A further study would have to examine the cross-spider's response to these two prey offered together. The choice of these two different prey species was also made to examine a controversy. Nentwig (1982) demonstrated that orb webs are able to retain slow f l y i n g insects, p a r t i c u l a r l y aphids, and unable to retain more active f l i e r s , p a r t i c u l a r l y hymenopterous parasitoids. The experimental evidence i s i n direct c o n f l i c t with older writings that claim orb-weavers are best at capturing beneficial insects (Gertsch 1949) and sometimes reject aphids and mites (Chant 1956, Bristowe 1958). 45 CHAPTER 4A FUNCTIONAL RESPONSE WITH A BENEFICIAL INSECT AS PREY MATERIALS AND METHODS THE PREY The "beneficial prey" was the parasitoid A^ nigripes (Rymenoptera:Aphidiidae). This parasitoid i s indigenous to North America and parasitizes several species of aphids (Pare et a l . 1979). Like a l l members of this family the females lay their eggs inside th e i r hosts. Under laboratory conditions the eggs hatch within about 3 days. I t takes 5-6 days for the larvae to molt through 4 instars and pupate (Pare et a l . 1979). The hosts become hardened into t y p i c a l aphid "mummies" while the parasitoid develops. Ten days from pupation the adults emerge from the aphid mummies, mate, feed on nectar and lay eggs i n their hosts. There i s evidence that another parasitoid i n the same genus, Aphidius uzbeckistanicus Luzhetzki, i t s e l f exhibits a Type 3 functional response (Hassell et a l . 1977). Colonies were started from an o r i g i n a l 1000 mummified aphids sent from Laval University by C. Cloutier. The parasitoid was reared on one of i t s hosts, the potato aphid M. eurphorbiae . 46 THE SPIDERS Predators were second instar A^ diadematus spiderlings with no previous feeding or web-spinning experience. Animals were taken d i r e c t l y from t h e i r communal groups i n order to control for predatory experience as this has been shown to be important i n functional response experiments (Holling 1968). Five communal groups were collected on 17, 19, 28 and 29 May, 1983, from Westham Island, B r i t i s h Columbia, Canada. Communal groups were kept together i n glass jars for one day to four weeks. As no corpses were found i n the jars i t was assumed that no cannibalism had occurred and, therefore, a l l spiderlings were existing solely on yolk reserves. MATERIALS The experimental cages were 20 cm X 20 cm X 10 cm wooden frames with glass windows on opposite sides. A potato shoot ( Solanum tuberosum L.) with four leaves was placed i n a v i a l of water i n each cage. The parasitoids could then search the plants for hosts as they normally would. A strand of nylon thread dipped i n honey was hung from each cage to provide food for the parasitoids. A l l experiments were conducted i n the Plant Science greenhouse, University of B r i t i s h Columbia, as described i n Chapter 3, between 19 May and 10 July 1983. 47 THE EXPERIMENTAL DESIGN An inexperienced spiderling was placed i n a cage with 0, 1, 10, 20, 40 or 60 parasitoids and observed for 5 days. Unfortunately, because of an inconsistent a v a i l a b i l i t y of parasitoids and a limited number of cages i t was not possible to test an equal number of spiderlings at each density. The numbers i n each group were as presented below, with the density followed by N i n brackets: 0 (12), 1 (20), 10 (23), 20 (19), 40 (25), 60 (18). When a parasitoid landed i n a web the inhabiting spiderling either wrapped i t i n s i l k or i t escaped. Silk-wrapped parasitoids were discarded onto the cage bottom after they were consumed or when webs were dismantled. I t was, therefore, easy to distinguish between parasitoids k i l l e d by spiderlings and those that died for other reasons. The number of prey captured was subsequently determined by the numbers of silk-wrapped parasitoids found on cage bottoms. A l l prey, whether wrapped i n s i l k , dead for other reasons, or a l i v e , were removed from cages once a day and freshly emerged parasitoids were reintroduced. The replacement of prey each day was done for two reasons: to ensure that densities remained constant over the experimental period, and to remove any p o s s i b i l i t y that parasitoids l e f t i n cages would become experienced at avoiding webs or escaping from them. Cages were examined twice a day, at noon and at midnight, for the presence of new webs because f i e l d observations indicated that spiderlings sometimes spin two webs per day. Each new web was 48 marked with a fine thread taken from a b a l l of cotton batting. A l l cages were sprayed with d i s t i l l e d water once a day to keep the humidity i n cages high. Spiderlings sometimes molted during the experiment. If t h i s happened, any days during which webs had not been spun prior to the molt were discounted and the experiment was continued from the f i r s t post-molt web u n t i l f i v e days had passed. For example, i f a spiderling had not spun from Day 2 to Day 5, molted, and then had spun again on Day 6, i t was observed for another 4 days. S i m i l a r i l y , spiderlings that stopped spinning i n the l a s t three days of the experiment, but had not molted, were observed for an extra two days to determine i f they were i n the pre-molt state. A preliminary experiment showed that A^ diadematus spiderlings stop spinning for 1 to 4 days prior to a molt, so the above procedure was adopted to d i f f e r e n t i a t e between those spiderlings not spinning because they were i n the pre-molt state and those that were not spinning because of treatment effects. A t o t a l of 9 spiderlings molted during or immediately after the experiment. They were represented i n a l l density groups, except 0, and the number of prey eaten prior to the molt ranged from 1 to 34 (see F i g . 4A.01) 4 9 FIGURE 4A.01 NUMBER OF PREY EATEN PRIOR TO A MOLT. TJ JJ m -< m > —i m FREQUENCY -o do o J3 50 RESULTS ANALYTICAL METHODS Two-way analysis of variance (anova) was used as a method of testing differences i n the number of prey captured and web-spinning behaviour as a function of prey density and experimental day. Data that did not meet the assumptions of normality and equal variances were transformed with a log transformation (Gilbert 1973, Sokal and Rohlf 1981). For log transformed data the geometric means ( i . e . the back-transformed means) are presented with the i r corresponding 95% confidence intervals (Gilbert 1973). Student-Newman-Keuls (SNK) a_ p o s t e r i o r i tests, at the 0.05 l e v e l of significance, were calculated where the anova tests indicated a s i g n i f i c a n t difference i n order to determine where the difference lay (Sokal and Rohlf 1969). Sometimes even a transformation did not make the data suitable for anova. In these cases Kruskal-Wallis One-Way Anova was used because i t i s a ranked order test not requiring the assumptions of normality and equal variances (Siegel 1956). F i n a l l y , least square regression analysis was used to i d e n t i f y s i g n i f i c a n t linear relationships between a variable and both parasitoid density and experimental day. Regression analysis i s based on the same assumptions as anova. If transformations did not make the data suitable for regression analysis Spearman's correlation coeffiecients were calculated to determine the presence of s i g n i f i c a n t relationships. 51 NUMBER OF PREY CAPTURED The unusual functional response curve of A. diadematus spiderlings, when A. nigripes was the prey species i s i l l u s t r a t e d i n Fi g . 4A.02. There was a s i g n i f i c a n t difference between the densities employed (Table 4A.01). From densities of 0 prey to 40 prey the curve seems to describe a t y p i c a l Type 2 response with prey capture increasing at a decreasing rate, u n t i l a plateau i s reached. At the plateau mean prey capture ranges from 3-3.9 prey per day. This compares exceptionally w e l l with f i e l d captures of 3.37 + 0.61 prey per web for webs with 1 or more prey. Usually this plateau represents the prey consumption l e v e l at which the predator i s satiated. However, at a density of 60 A^ _ nigripes per day the curve takes a sharp swing upwards. (See Table 4A.02 for summary of SNK a po s t e r i o r i t e s t ) . Although the linear regression of the number of parasitoids captured i n 5 days on parasitoid density was s i g n i f i c a n t i t only accounted for 42% of the observed va r i a t i o n so was not assumed to provide a good description of the actual functional response: r2=0.42, log^Q (number of parasitoids captured + 1) = 0.62 + 0.016 X (parasitoid density), p<0.001. There was also a s i g n i f i c a n t difference i n capture rates due to experimental day (Table 4A.01). More parasitoids were captured on the f i r s t day than on any other day, regardless of density (Table 4A.03). A detailed breakdown of the data by density (Fig. 4A.03) suggested that the upswing i n the functional response 52 FIGURE 4A.02 FUNCTIONAL RESPONSE OF CROSS-SPIDERLINGS WITH A^ _ NIGRIPES AS THE PREY. A log transformation was performed on the data i n order to meet the assumption of equal variance required for both anova and regression. The geometric means are given with t h e i r corresponding 95% confidence i n t e r v a l s . The curve i s best described as a Type 2 curve up u n t i l a density of 40. The ri s e i n the curve after a density of 40 i s unusual because the plateau i n a Type 2 curve usually represents a satia t i o n l e v e l . NO. OF PARASITOIDS CAPTURED IN 5 DAYS ( g e o m e t r i c m e a n s ) L-Ol 01 ro o ro CO o CO cn J 53 TABLE 4A.01 ANALYSIS OF VARIANCE SUMMARY TABLE FOR NUMBER OF PARASITOIDS CAPTURED BY PARASITOID DENSITY AND DAY. A log transformation was performed on the data i n order to meet the assumption of equal variances. Source of Variation Sum of DF Mean F Significance Squares Square of F Main Effects 22.739 9 2.527 18.216 0.001 Parasitoid Density 20.099 5 4.020 28.982 0.001 Day 2.640 4 0.660 4.759 0.001 2-Way Interactions 5.486 20 0.274 1.978 0.007 Explained 28.225 29 0.973 7.017 0.001 Residual 76.978 555 0.139 Total 105.203 584 0.180 54 TABLE 4A.02 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR DENSITY EFFECTS. Differences i n the number of parasitoids captured over 5 days due to each density were calculated on the log transformed data. Geometric means are presented. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y d ifferent from each other are joined by a bar. Parasitoid Density 0 1 20 40 10 60 Number of Parasitoids 0 2.2 12.1 16.1 16.4 25.5 Captured 55 TABLE 4A.03 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR DAY EFFECTS. Differences i n the number of parasitoids captured over 5 days for each day were calculated on the log transformed data. Geometric means are presented and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. Experimental Day 3 4 2 5 1 Number of Parasitoids 1.0 1.1 1.3 1.4 2.2 Captured 56 FIGURE 4A.03 NUMBER OF PARASITOIDS CAPTURED EACH DAY AND SEGREGATED BY DENSITY. The geometric means and corresponding 95% confidence intervals are presented. 56a 1 5 J >-< Q cr LU Q_ i 10 ID ~ i _ » 0-< s ° E CO 9 u O o £ E CO o 5 5 5. L L O 6 z 0_l 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 DAY 0 10 20 40 60 PARASITOID DENSITY 57 curve at a density of 60 prey per day was a result of s i g n i f i c a n t l y higher prey catches on the f i r s t day of the experiment at the high density (F=4.646, df=4,85, p=0.002, Table 4A.04). Note also that capture rates on Day 1 were not s i g n i f i c a n t l y different from Day 5, but different from those on Days 2 to 4. Si m i l a r l y at 40 prey per day more prey were captured on Day 1 (F=3.8, df=4,110, p=0.006, Table 4A.04), although the difference was not s i g n i f i c a n t l y different for Day 2. At densities of 1,10 and 20 there was no s i g n i f i c a n t difference i n the number of prey captured on any day. A breakdown of the data by day further i l l u s t r a t e d the difference between Day 1 and the other experimental days (Fig. 4A.04, Table 4A.05). No s i g n i f i c a n t differences were found between prey catches at densities of 10 and 20 and a density of 60 on any day except Day 1. However, for a l l days there was no difference betweeen densities of 40 and 60, or between densities of 0 and 1. Most, but not a l l , spiderlings captured their f i r s t parasitoid on Day 1 (Fig.4A.05). In order to determine i f the day effects were actually effects of predatory experience, the data were divided into 3 groups: the days before any prey were captured (BPC), the day on which the f i r s t prey was captured (FDPC), and a l l days after the f i r s t prey was captured (AFPC). The BPC group was discarded from this part of the analysis because prey capture was always 0. On the f i r s t day of prey capture there was a si g n i f i c a n t difference i n the number of prey captured at a l l densities except 10 and 20 (Table 4A.06 and 4A.07). There were, however, no s i g n i f i c a n t effects due to the day on which the f i r s t prey was captured (Table 4A.06). The functional response for the f i r s t day any prey were 58 TABLE 4A.04 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR THE NUMBER OF PARASITOIDS CAPTURED ON EACH DAY AT EACH DENSITY. Differences i n the number of parasitoids captured on each day were calculated on the log transformed data. Geometric means are presented. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. 58a TABLE 4A.04 STUDFOT-^ EWMAN-4<EULS MULTIPLE RANGE TEST SUMMARY DENSITY n 1 100 Day 2 1 3 4 5 Capture Rates 0.3 0.3 0.3 0.5 0.6 10 95 Day 1 3 4 2 5 Capture Rates 1.9 1.9 2.3 2.4 3.0 20 125 Day 3 5 2 4 1 Capture Rates 1.3 1.3 1.6 1.9 2.3 40 115 Day 4 3 5 2 1 Capture Rates 1.1 1.2 1.4 2.2 4.5 60 90 Day 4 2 3 5 1 Capture Rates 1.1 1.8 2.0 3.5 8.2 59 FIGURE 4A.04 NUMBER OF PARASITOIDS CAPTURED AT EACH DENSITY S E G R E G A T E D BY DAY. Geometric means and th e i r corresponding 95% confidence intervals are presented. v 59a 15-^ > < Q CO 111 Q_ Q 5*10-r- <Z Q. < o o E CO « Q i -— 4^  O a> r— E CO © < Q. o 6 5H 0-1 J 0 1 10 204060 0 1 10* 0 1 10 2040 W 0 1 • 10 2040 66 i 0 1 10 2040 60 PARASITOID DENSITY 1 3 DAY 60 TABLE 4A.05 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR THE NUMBER OF PREY CAPTURED AT EACH DENSITY BROKEN DOWN BY DAY. The geometric means are presented. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. 60a TABLE 4A.05 STuTOOT-NEWMAN-KFMS MULTIPLE RANGE TEST SUMMARY DAY n 1 117 Parasitoid Density 0 1 : 10 20 40 60 Capture Rates 0 0.4 1.9 2.3 4.5 8.2 2 117 Parasitoid Density 0 1 20 60 40 10 Capture Rates 0 0.3 1.6 1.8 2.2 2.4 3 117 Parasitoid Density 0 1 40 20 10 60 Capture Rates 0 0.3 1.2 1.3 1.9 2.0 4 117 Parasitoid Density 0 1 40 60 20 10 Capture Rates 0 0.5 1.1 1.1 1.9 2.3 5 117 Parasitoid Density 0 1 20 40 10 f>0 Capture Rates 0 0.6 1.3 1.4 3.0 3.5 61 FIGURE 4A.05 DAYS ON WHICH THE FIRST PARASITOID WAS CAPTURED. F r e q u e n c y o f c a p t u r i n g t h e f i r s t p a r a s i t o i d o n e a c h d a y O ON O to 3 I 62 TABLE 4A.06 ANALYSIS OF VARIANCE SUMMARY TABLE FOR THE FIRST DAY OF PREY CAPTURE. Differences i n the number of parasitoids captured across parasitoid density and experimental day were determined. The analysis was performed on the log transformed data. Source of Variation Main Effects Parasitoid Density Day 2-Way Interactions Explained Residual Total Sum of DF Mean Squares Square 7.559 8 0.945 5.656 4 1.414 0.374 4 0.093 0.633 11 0.058 8.192 19 0.431 3.367 85 0.040 11.559 104 0.111 F Significance of F 23.854 0.001 35.700 0.001 2.360 0.060 1.453 0.165 10.885 0.001 63 TABLE 4A.07 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR THE FIRST DAY OF PREY CAPTURE. Differences i n the number of parasitoids captured at each density were calculated. The analysis was performed on the log transformed data and geometric means are presented. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y d i f f e r e n t from each other are joined by a bar. Parasitoid Density 1 20 10 40 60 Number of Parasitoids Captured 1.0 4.1 4.5 7.3 12.2 64 captured i s presented i n F i g . 4A.06. In this case the curve i s a better f i t to the linear model because the plateau between densities of 10 and 40 parasitoids, found i n the functional response over 5 days, has been narrowed to between 10 and 20 parasitoids. This exponentially increasing functional response resembles the r i s i n g phase of a Type 3 response curve; capture rates increased with parasitoid density. Unlike the functional, response curve for the f i r s t day of prey capture, after the f i r s t prey had been captured a Type 2 response was obtained (Fig. 4A.07). Although a s i g n i f i c a n t difference existed between densities (Table 4A.08) only prey captured at a density of 1 were s i g n i f i c a n t l y d i f f e r e n t . A plateau was reached between 10 and 60 available parasitoids (Table 4A.09). Spearman's correlation indicated that no s i g n i f i c a n t relationship could be found between prey density and prey capture (r=0.04; p=0.251). For comparison, Spearman's correlation coe f f i c i e n t between prey capture and density for the f i r s t day of prey capture was: r=0.74, p=0.001. There was also no s i g n i f i c a n t day effect after the f i r s t day of prey capture (Table 4A.10). The mean number of prey captured for these experienced spiderlings was lower than for the f i r s t day of prey capture ( F i g . 4A.08). The differences i n the number of prey captured between Day 1 and the rest of the experimental period were actually attributable to the fact that most spiderlings captured the i r f i r s t prey on Day 1. The important differences were not due to experimental day, per se ,but to the predatory experience of the spiderling. The percent predation curve also suggests that experienced 65 FIGURE 4A.06 FUNCTIONAL RESPONSE ON THE FIRST DAY OF PREY CAPTURE. The regression analysis was calculated on log transformed data. The geometric means with corresponding 95% confidence intervals are presented. Arithmetic means are presented for comparison. 65a 15 H Q w <* o H < U 10 c/3 Q O H «: < Ph O O* FIRST DAY OF PREY C A P T U R E r2=0.57 P< 0.001 log10(y + l) - 0.56+ 0.01 x • ARITHMETIC MEANS • GEOMETRIC MEANS WITH 95% C.I. 1 0-! 10 20 40 I 60 PARASITOID DENSITY 66 FIGURE 4A.07 FUNCTIONAL RESPONSE FOR DAYS AFTER THE FIRST PREY WAS CAPTURED. The data did not meet the assumption of equal variances, even with a log transformation. Therefore, regression analysis could not be calculated. The arithmetic means with standard errors are presented. 66a I P4 Q w P4 p H < u O •—I C O P H O D A Y S A F T E R F I R S T P R E Y C A P T U R E D O 0 J 10 2 0 4 0 P A R A S I T O I D D E N S I T Y 60 67 TABLE 4A.08 KRUSKAL-WALLIS 1-WAY ANOVA SUMMARY FOR DAYS AFTER THE FIRST PREY WAS CAPTURED. There was a s i g n i f i c a n t difference i n the number of parasitoids captured as a function of parasitoid density. Parasitoid Density 1 10 20 40 Number of Observations 53 62 86 82 Mean Ranks 130.43 212.84 177.08 168.30 185.85 Corrected for Ties Chi-Square p Chi-Square p 19.981 0.001 21.655 0.001 68 TABLE 4A.09 KRUSKAL-WALLIS 1-WAY ANOVA SUMMARY FOR DAYS AFTER THE FIRST PREY WAS CAPTURED. By removing the data for density = 1 from the analysis there was no longer a s i g n i f i c a n t difference i n the number of prey captured across parasitoid density. Parasitoid Density 10 20 40 60 Number of Observations 62 86 82 68 Mean Rank 171.68 143.71 137.02 151.65 Corrected for Ties Chi-square p Chi-Square p 6.256 0.10 6.756 0.80 69 TABLE 4A.10 KRUSKAL-WALLIS 1-WAY ANOVA SUMMARY FOR THE EFFECTS OF DAY ON THE NUMBER OF PARASITOIDS CAPTURED. This analysis was calculated for days after the f i r s t prey was captured. Day Number of Observations 0 " 7 0 87 93 102 Mean Rank 0 187.10 172.10 166.61 182.00 Corrected for Ties Chi-Square p Chi-Square p 2.098 0.55 2.273 0.52 70 FIGURE 4A.08 COMPARISON OF THE NUMBER OF PREY CAPTURED ON FIRST DAY OF PREY CAPTURE AND DAYS AFTER THE FIRST PREY WAS CAPTURED. Means with standard errors are presented. Student's t-tests were calculated to determine differences between prey captures at each density. They were: Density=l p<0.005 df=72, Density=10 p>0.1 df=80, Density=20 p<0.005 df=110, Density=40 p<0.005 df=104 Density=60 p<0.005 df=85. 70a 71 spiderlings ( i . e . AFPC) behaved l i k e predators with a Type 2 response curve (Fig. 4A.09). When the curve levels off experienced spiderlings captured means of 7 to 15 percent of the available prey. The percent predation curve for the f i r s t day of prey capture, although si m i l a r i n form to that for days after the f i r s t prey capture (Fig. 4A.09), contains several important differences. At the plateau the inexperienced spiderlings captured means of 22 to 25 percent of the available prey. At the low prey density of one, inexperienced spiderlings captured 100 percent of the available prey, whereas the experienced spiderlings captured captured only 58 percent of the available prey. Percent predation was correlated with prey density for experienced spiderlings (rs=0.33;p<0.001), although not as strongly as for inexperienced spiderlings (r g=0.63; p<0.001). FREQUENCY OF WEB SPINNING The number of new webs spun i n 5 days decreased as parasitoid density increased (Fig. 4A.10a and Table 4A.11) At a density of 1 parasitoid juvenile cross-spiders spun more webs than at any other density. The difference was not s i g n i f i c a n t for 10 prey, although i t was for a l l other densities (Table 4A.12). There was, however, no s i g n i f i c a n t linear trend to web spinning as a function of parasitoid density. In order to determine whether spiderlings at the higher densities were s t i l l searching for prey, but using old webs rather 72 FIGURE 4A.09 PERCENT PREDATION COMPARED FOR FIRST DAY OF PREY CAPTURE AND DAYS AFTER THE FIRST DAY OF PREY CAPTURE. Note that on the f i r s t day of prey capture spiderlings were capturing 22 to 25% of the available parasitoids between densities of 20 and 60. In contrast, after the f i r s t prey was captured only 7 to 15% of the available prey were captured at the same densities. Also, spiderlings are better at capturing parasitoids at the lowest density of 1 on the f i r s t day of prey capture. 72a F i r s t D a y o f P r e y C a p t u r e O — — O D a y s A f t e r F i r s t P r e y C a p t u r e d • 10CH 8CH o I—I o w PH H PH 60-J 40 H 20- I 0-i 10 20 40 P A R A S I T O I D D E N S I T Y 73 FIGURE 4A.10 WEB SPINNING AS A FUNCTION OF PARASITOID DENSITY. (a) More new webs were spun at the low densities than the high densities. (b) Total web exposure = new webs + old webs Total web exposure was also greater at the low densities than the high densities. 73a PARASITOID DENSITY 74 TABLE 4A.11 ANALYSIS OF VARIANCE SUMMARY TABLE FOR NUMBER OF NEW WEBS SPUN BY PARASITOID DENSITY AND DAY. Source of Variation Sura of DF Mean F Significance Squares Square of F Main Effects 12.228 9 1.359 5.030 . 0.001 Parasitoid Density- 6.187 5 1.237 4.581 0.001 Day 6.041 4 1.510 5.591 0.001 2-Way Interactions 6.723 20 0.336 1.245 0.212 Explained 18.951 29 0.653 2.419 0.001 Residual 149.911 555 0.270 Total 168.862 584 0.289 75 TABLE 4A.12 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR THE NUMBER OF NEW WEBS SPUN BY PARASITOID DENSITY. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. Parasitoid Density 60 40 0 20 10 1 Number of Webs Spun 0.58 0.59 0.60 0.66 0.80 0.84 Per Day 76 than dismantling and spinning new webs, the frequency of web spinning was further analyzed using the sum of old webs plus new webs to obtain the t o t a l number of days i n which webs were exposed. In t h i s case the curve was s i m i l a r (Fig. 4A.10b) but a s i g n i f i c a n t difference existed only between densities of 1 and both 40 and 60 parasitoids (Tables 4A.13 and 4A.14). Web spinning behaviour was also affected by the experimental day (Table 4 A . 1 1 ) — s i g n i f i c a n t l y more webs were spun on Day 1 (Table 4A.15). On Day 1 at a density of 60 parasitoids juvenile cross-spiders spun more webs than on the other 4 days (F=3.948, df=4,84, p=0.005, F i g . 4A.11, Table 4A.16). No s i g n i f i c a n t difference was found i n the number of prey captured as a result of day alone at any other density. However, the increased prey capture on Day 1 cannot be attributed solely to the fact that more spiderlings were i n webs. When old and new webs were added together, no s i g n i f i c a n t difference could be detected between days at any density (Fig. 4A.12). It i s possible that an old web i s less effective at holding prey than a new one, but t h i s was not tested for . The data were segregated by day (Fig. 4A.13). On Day 4 there was a s i g n i f i c a n t difference i n the number of webs spun at each density (F=5.155, d f = 5 , l l l , p<0.001). More webs were spun at densities of 1, 10 and 20 than 60. For Day 5 the number of new webs spun was not s i g n i f i c a n t l y different across densities (F=2.268, d f = 5 , l l l , p=0.053, F i g . 4A.13) The same trend existed for the t o t a l web exposure on Day 4 (F=3.92, p=0.003, d . f . = 5 , l l l , F i g . 4A.14, Table 4A.17). However, when the t o t a l web exposure was examined for Day 5 s i g n i f i c a n t l y more webs were exposed at densities of 0 and 1 77 TABLE 4A.13 ANALYSIS OF VARIANCE SUMMARY TABLE FOR TOTAL WEB EXPOSURE BY PARASITOID DENSITY AND DAY. Total web exposure = new webs + old webs. Source of Variation Main Effects Parasitoid Densit Day 2-Way Interactions Explained Residual Total Sum of DF Mean F Signifies Squares Square of F 4.408 9 0.490 2.103 0.028 3.138 5 0.627 2.694 0.020 1.272 4 0.318 1.365 0.245 6.072 20 0.304 1.304 0.169 10.480 29 0.381 1.552 0.034 129.238 555 0.233 139.716 584 0.239 78 TABLE 4A.14 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR TOTAL WEB EXPOSURE. Total web exposure = new webs + old webs. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. Parasitoid Density 60 40 0 20 10 1 Total Web Exposure 0.70 0.71 0.78 0.80 0.84 0.91 79 TABLE 4A.15 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR NEW WEBS SPUN BY DAY. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y d i f f e r e n t from each other are joined by a bar. Day 2 3 4 5 1 No. New Webs 0.56 0.63 0.64 0.7 1 0.86 80 FIGURE 4A.11 NEW WEBS SPUN ON EACH DAY SEGREGATED BY DENSITY. The means and standard errors are presented. A sig n i f i c a n t difference was detected i n the number of new webs spun on each day only at a density of 60 parasitoids (p = 0.005). 80a < 1.2 Q 0 . 8 H LU Q. CO m LU £ 0.4 L U Hi 0-> 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 ' D A Y  0 1 1 0 20 40 60 P A R A S I T O I D D E N S I T Y 81 TABLE 4A.16 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR NEW WEBS SPUN ON EACH DAY AT DENSITY = 60. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y d i f f e r e n t from each other are joined by a bar. Density=60 Day 4 2 3 5 1 No. New Webs 0.33 0.44 0.50 0.67 0.94 82 FIGURE 4A.12 TOTAL WEB EXPOSURE PER DAY SEGREGATED BY DENSITY. Total web exposure = new webs + old webs. Means and standard errors are presented. There were no sig n i f i c a n t differences i n the t o t a l web exposure on each day at any density. 82a 5 1.2 Q DC 111 0_ L U cc CO o Q-X LU CO LU 0 . 8 -0 . 4 -0 . 2 -m o 0 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1'2 3 4 5 1 2 3 4 5 1 2 3 4 5 DAY 0 1 10 20 40 PARASITOID DENSITY 60 83 FIGURE 4A.13 NEW WEBS SPUN AT EACH DENSITY SEGREGATED BY DAY. There were s i g n i f i c a n t differences i n the number of new webs spun on Day 4 at densities of 1, 10 and 20 than 60 (p < 0.001). There were no si g n i f i c a n t differences on any other day. Means with standard errors are presented. 83a $ 1-2 e 0.8 G O PQ W ^ 0.4 Z 0.0 rh 0 1 10204060 0 1 10 204060 0 1 10204060 0 1 10204060 0 1 10204060 P A R A S I T O I D D E N S I T Y  2 3 4 5 D A Y 84 FIGURE 4A.14 TOTAL WER EXPOSURE AT EACH DENSITY SEGREGATED BY DAY. Total web exposure = new webs + old webs. Means with standard errors are presented. There are si g n i f i c a n t differences i n the t o t a l web exposure on Day 4 (p = 0.003) and Day 5 (p = 0.018). 84a 5 S 1.2' w O 0.8 PQ 0.4 Hi. < 0.0 H O H PARAS ITOID DENSITY 1 ~ 2 3 4 5 D A Y 85 TABLE 4A.17 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR TOTAL WEB EXPOSURE BROKEN ON DAYS 4 AND 5. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. A. Day=4 Parasitoid Density 60 0 40 20 10 1 New Webs 0.33 0.42 0.44 0.76 0.89 0.90 Total Webs Exposure 0.50 0.58 0,61 0.84 0.95 1.00 B. Day=5 Parasitoid Density 40 20 60 10 1 0 New Webs 0.52 0.56 0.67 0.84 0.90 0.92 Total Web Exposure 0.65 0.68 0.78 0.84 0.95 1.20 86 than 40 and 20 (F=2.860, d.f.=5,111, p=0.018, F i g . 4A.14, Table 4A.17), indicating that some spiderlings did not take their webs down on Day 4. On the f i r s t day of prey capture a l l spiderlings were i n webs so i t was inappropriate to examine their web spinning behaviour. Web spinning data from the days after the f i r s t prey was captured were segregated and, i n this case, the frequency of web spinning was s i g n i f i c a n t l y different across densities but not experimental day (Table 4A.18). S i g n i f i c a n t l y more webs were spun at the densities of 1 and 10 than 40 and 60 (Table 4A.19). However, web spinning could not be described as a lin e a r function of parasitoid density (Fig. 4A.15). These data were further examined to determine the effects of the number of prey captured on subsequent web spinning, rather than prey density. The number of prey captured on the previous day had a s i g n i f i c a n t effect on the frequency of web spinning, while experimental day did not (Table 4A.20). The relationship was partly explained by a linear model—the fewer prey captured, the greater the l i k e l i h o o d of a web being spun on the next day (Fig. 4A.16). On the days before any prey were captured (for densities >^ 1) web spinning was s i g n i f i c a n t l y different across densities but not day (Table 4A.21). Generally, more webs were spun at the lower densities. However, the difference was s i g n i f i c a n t only for densities of 20 and 1 (Table 4A.22) and web spinning was not a strong l i n e a r function of parasitoid density (Fig. 4A.17) or experimental day for the days before prey capture. Experimental day was responsible for only 1% of the va r i a t i o n . The web spinning 87 TABLE 4A.18 ANALYSIS OF VARIANCE SUMMARY TABLE FOR NEW WEBS SPUN ON DAYS AFTER THE FIRST PREY WAS CAPTURED. Source of Variation Sum of DF Mean F Signif icai Squares Square of F Main Effects 7.316 7 1.045 4.008 <0.001 Parasitoid Density 5.512 4 1.378 5.285 <0.001 Day 1.474 3 0.491 1.885 0.132 2-Way Interactions 4.225 12 0.352 1.350 0.189 Explained 11.541 19 0.607 2.329 0.001 Residual 86.568 332 0.261 Total 98.109 351 0.280 88 TABLE 4A.19 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR NEW WEBS SPUN ON DAYS AFTER THE FIRST PREY WAS, CAPTURED. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. Parasitoid Density 60 40 20 10 1 New Webs Per Day 0.46 0.50 0.59 0.76 0.79 89 FIGURE 4A.15 NEW WEBS AS A FUNCTION OF PARASITOID DENSITY FOR DAYS AFTER THE FIRST PREY WAS CAPTURED. Means and standard errors are presented. Although the slope of the regression l i n e was s i g n i f i c a n t l y different from 0 i t did not account for very much of the observation variation i n the data. 89a A f t e r F i r s t P r e y C a p t u r e d 1.0 0.8 < O 0.6 rt oo cq 0.4 ^ 0.2 r 2 =0.05 P < 0.001 y -0 .77-0 .006 x 0 . 0 - 1 10 20 40 6 0 P A R A S I T O I D D E N S I T Y 90 TABLE 4A.20 ANALYSIS OF VARIANCE SUMMARY TABLE FOR NEW WEBS BY PREY CAPTURED ON THE PREVIOUS DAY AND EXPERIMENTAL DAY. This analysis was done on the data a f t e r the f i r s t prey was captured. Source of Sum of DF Mean F Significanc Variation Squares Square of F Main Effects 6.098 9 0.678 2.643 0.006 No. Parasitoids Captured Previous Day 4.294 6 0.716 2.792 0.012 Day 0.491 3 0.164 0.638 0.591 2-Way Interactions 8.945 18 0.497 1.938 0.013 Explained 15.043 27 0.557 2.173 0.001 Residual 83.065 324 0.256 Total 98.109 351 0.280 91 FIGURE 4A.16 PROBABILITY OF SPINNING A NEW WEB AS A FUNCTION OF THE NUMBER OF PARASITOIDS CAPTURED ON THE PREVIOUS DAY. This analysis was calculated for the days after the f i r s t prey was captured only. r2=0.41 p =0.03 y=Q71-Q03x I 1 1 ! 1 1 1 1 1 1 1 0 5 >10 NO. P A R A S I T O I D S CAPTURED PREVIOUS DAY 92 TABLE 4A.21 ANALYSIS OF VARIANCE SUMMARY TABLE FOR NEW WEBS BY PARASITOID DENSITY AND DAY, ON THE DAYS BEFORE ANY PARASITOIDS WERE CAPTURED. Source of Sum of DF Mean F Signif icar Variation Squares Squre of F Main Effects 4.884 8 0.611 2.344 0.032 Parasitoid Density 3.563 4 0.891 3.418 0.015 Day 1.330 4 0.333 1.277 0.292 2-Way Interactions 2.797 9 0.311 1.193 0.321 Explained 7.682 17 0.452 1.734 0.068 Residual 12.766 49 0.261 Total 20.448 66 0.310 9 3 TABLE 4A.22 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR NEW WEBS SPUN BY DENSITY. The analysis was calculated for the days before any parasitoids were captured. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. Parasitoid Density 20 60 40 10 1 New Webs Per Day 0.21 0.25 0.40 0.69 0.78 94 FIGURE 4A.17 NEW WEBS SPUN AS A FUNCTION OF PARASITOID DENSITY FOR DAYS BEFORE ANY PREY WERE CAPTURED. Means with standard errors are presented. 94a B e f o r e F i r s t P r e y C a p t u r e d P A R A S I T O I D D E N S I T Y 95 behaviour of spiderlings at 0 density was examined separately to determine whether web spinning would be different i f no prey were present at a l l . Spiderlings s t i l l spun regularly with no si g n i f i c a n t difference due to experimental day (Table 4A.23). None of the unfed spiderlings died and most were i n a new web on Day 5 (Fig. 4A.18). A summary of web spinning on days before prey were captured, after prey were captured and at a density of 0 i s presented i n F i g . 4A.19. There was no sig n i f i c a n t difference at any density, except 20 (p < 0.01 for a density of 20, and p > 0.1 for a l l other densities, as determined by Student's t - t e s t ) . PREY CAPTURED PER WEB On the f i r s t day a parasitoid was captured a l l spiderlings were i n a web. Therefore, the capture rates per web were equivalent to the capture rates already described (refer to F i g . 4A.06). After the f i r s t prey was captured spiderlings were not always i n a web, and capture rates per web were were not the same as the capture rates previously described. Removing the days on which there were no webs enabled the calculation of capture rates per web. These were signicantly different across densities but not day (Table 4A.24), although i n this case there was a plateau at densities of 10, 20, and 40 prey and captures per web at a density of 60 parasitoids were not s i g n i f i c a n t l y greater than at a density of 40 parasitoids (Table 4A.25). This relationship was not strongly l i n e a r i n nature (Fig. 4A.20). More prey per web were captured on the f i r s t day of prey capture than after the f i r s t day of prey 96 TABLE 4A.23 ANALYSIS OF VARIANCE SUMMARY TABLE FOR WEB SPINNING BY DAY AT DENSITY = 0. Source of Sum of DF Mean F Significance Variation Squares Square of F Between Groups 1.90 4 0.475 1.274 0.291 Within Groups 20.50 55 0.373 Total 22.40 59 97 i FIGURE 4A.18 NEW WEBS SPUN AS A FUNCTION OF DAY FOR DENSITY = 0. Means with standard errors are presented. ( 97a D e n s i t y = 0 rt 1.2-1 .0-0 . 8 -0.6 S 0.4H ^ 0.2-0.0 ^ l — i — i — i — r 1 2 3 4 5 D A Y 98 FIGURE 4A.19 COMPARISON OF NEW WEBS SPUN FOR DAYS BEFORE THE FIRST PREY WAS CAPTURED, DAYS AFTER THE FIRST PREY WAS CAPTURED AND DENSITY=0. Means with standard errors are presented. Student's t-tests were calculated to determine s i g n i f i c a n t differences at each density. They were: Density=l p>0.1 df=79 Density=10 p>0.1 df=75 Density=20 p>0.1 df=99 Density=40 p<0.01 df=91 .. . Density=60 p>0.2 df=71. 98a 1 . 0 - •NSTARVING r{ 0 . 8 -w C O PQ 0 . 6 -0.4 0 . 2 -0.0- 1 1 ! I ii I I 0 1 10 20 40 6 P A R A S I T O I D D E N S I T Y • JBEFORE 1st DAY j PREY CAPTURE AFTER 1st DAY PREY CAPTURE I 99 TABLE 4A.24 ANALYSIS OF VARIANCE SUMMARY TABLE FOR PARASITOIDS CAPTURED PER WEB BY PARASITOID DENSITY AND DAY. The analysis was done on the log transformed data f o r days after the f i r s t parasitoid was captured. Source of Sum of DF Mean F Signifies Variation Squares Square of F Main Effects 9.089 8 1.136 9.317 <0.001 Parasitoid Density 9.008 4 2.252 18.468 <0.001 Day 0.063 4 0.016 0.128 0.972 2-Way Interactions 0.672 13 0.052 0.424 0.960 Explained 9.761 21 0.465 3.812 <0.001 Residual 22.925 188 0.122 Total 32.686 209 0.156 100 TABLE 4A.25 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR PARASITOIDS CAPTURED PER WEB BY PARASITOID DENSITY. The analysis for done on log transformed data for the days after the f i r s t parasitoids was captured only. Geometric means are presented. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different are joined by a bar. Parasitoid Density 1 20 10 40 60 Parasitoid Captured 0.57 3.18 3.42 3.90 5.92 Per Web 101 FIGURE 4A.20 PARASITOIDS CAPTURED PER WEB AS A FUNCTION OF PARASITOID DENSITY. The geometric means with corresponding 95% confidence intervals are presented. Arithmetic means are also presented for comparison. w 8H 0-' r2=0.17 P<0.001 logio(y+1)=0.39+0.01x • A R I T H M E T I C M E A N S • G E O M E T R I C M E A N S W W I T H 95% C . I . 10 20 40 PARASITOID DENSITY 102 capture, although the differences were s i g n i f i c a n t for densities of 1 and 40 parasitoids only (Fig. 4A.21). 103 FIGURE 4A.21 COMPARISON OF PARASITOIDS CAPTURED PER WEB FOR THE FIRST DAY OF PREY CAPTURE AND DAYS AFTER THE FIRST PREY WAS CAPTURED. Means with standard errors are presented. Student's t-tests were calculated to determine s i g n i f i c a n t differences between pairs. The results were: Density=l p<0.005 df=65 Density=10 p>0.1 df=67 Density=20 p>0.1 df=75 Density=40 p<0.005 df=62 Density=60 p>0.05 df=48. 103a 1 6 -rt W PH Q 12 w rt H C O Q 8 o H PH § 4 0 — ' 1-1 1 I 1 1 1 1 10 20 P A R A S I T O I D « I I P i i 1 40 60 D E N S I T Y 1st E^PREY CAPTURED •DAYS AFTER 1st PREY CAPTURED 104 DISCUSSION There are two d i s t i n c t components to the foraging tactics of web-spinning spiders—web-building and wrapping prey that land i n the web. Both of these behavioural responses, measured i n terms of the number of webs spun and the t o t a l number of prey wrapped, can change as available prey changes. The results just described demonstrated that the responses to changing densities of A.  nigripes d i f f e r r e d for spiderlings with previous prey capture experience and those without. A l l of the effects due to experimental day were removed by considering separately the days before any prey were captured, the f i r s t day of prey capture and the days after prey were captured, indicating that predatory experience i s far more important than the time of exposure to prey. The functional response curve obtained before the pa r t i t i o n i n g of the data ( i . e . F i g . 4A.02) was shaped by the combination of two d i s t i n c t types of functional responses—one for inexperienced spiderlings on their f i r s t day of prey capture, and one for experienced spiderlings, after they had captured the i r f i r s t prey. INEXPERIENCED SPIDERLINGS The functional response curve of inexperienced spiderlings f i t s an exponential curve (Fig. 4A.06). It appears that these unusual predators have an unlimited a b i l i t y to capture an increasing percentage of a prey population as density r i s e s . A sat i a t i o n l e v e l 105 was not reached i n the experiments described here, nor did the high numbers of prey items sometimes found wrapped i n webs in the f i e l d suggest that a s a t i a t i o n l e v e l i s normally attained i n the f i e l d . These spiderlings did not seem to be limited by handling time i n their capacity to respond to sudden flushes of a prey species. Available space i n webs was not l i m i t i n g at even the highest densities. If i t had been, the functional response curve would have begun to l e v e l o f f . Presumably i t would be possible to find a prey density i n the laboratory at which the functional response curve for inexperienced spiderlings reaches s a t i a t i o n . However, i t i s l i k e l y that such a high density would be at supraphysiological l e v e l s . Remarkably, inexperienced spiderlings behaved more l i k e aquatic f i l t e r feeders than other t e r r e s t r i a l predators! The v i s c i d webs of cross-spiders act as passive f i l t e r s of a e r i a l plankton. Once the spider has invested an i n i t i a l energy expenditure i n the building of a web, no further searching energy i s required to temporarily detain prey items. However, webs do not hold a l l insect types. The hymenopterous parasitoids are p a r t i c u l a r l y adept at freeing themselves from the sticky s p i r a l s (Nentwig 1982). Therefore, the spider must also run out from i t s retreat or from the hub and wrap any parasitoids that have been detained by the web. The necessity of orb-weavers to wrap some prey types i n order to keep them i n their webs creates the d i s t i n c t difference between these spiders and most other f i l t e r feeders and probably accounts for the functional response curve being exponential rather than li n e a r . R ecall that f i l t e r feeders usually exhibit a Type 1 ( i . e . linear) functional response. Only some of the web-building 106 Trichoptera have the same problem. Although these do not wrap their prey i n s i l k they must also run out and capture the larger prey items that are temporarily halted by their webs ( J . Richardson, i n prep.). Interestingly, the Trichopterans feed mainly on the aquatic plankton trapped on thei r underwater webs. They do not consume thei r webs but do periodically clean them of trapped plankton (Edington 1968). I t i s surprising that the comparision between orb-weavers, which are generally a e r i a l plankton feeders, and web-spinning Trichopterans, which are aquatic plankton feeders, has not been exploited. The functional response of Trichopterans i s unknown. However, i t would not be surprising i f they behaved i n the same way as cross-spiderlings. The percent predation curve (Fig. 4A.09) was not as one would expect from a predator with a sigmoid functional response. Although cross-spiderlings were able to capture disproportionately more of the available prey as prey density rose, the increased predation pressure was not enough to result i n an increased percent mortality i n the prey. However, these spiderlings should not be excluded as potential regulators of prey populations. The high percentage of parasitoids captured ( i . e . 22-25%) at the high densities indicates a potential to maintain the equilibrium density of a prey population at muted l e v e l s . The prey under examination i n these experiments was a b e n e f i c i a l insect. I t seems as i f the low parasitoid numbers observed i n webs i n the f i e l d could not have been a result of the i n a b i l i t y of spiderlings to retain them once they landed i n a web, as Nentwig (1982) implied. I t was l i k e l y more a result of the parasitoids' a b i l i t y to avoid webs—an hypothesis 107 worthy of further study. One reason that inexperienced spiderlings were able to capture such large numbers of prey i s that they k i l l e d ( i . e . wrapped i n s i l k and probably injected with digestive enzymes) more than they consumed. If a parasitoid landed i n a web while an inexperienced spiderling was engaged i n wrapping another parasitoid, the spiderling stopped what i t was doing and rushed to locate and wrap the new a r r i v a l . Apparently, inexperienced spiderlings did not make decisions about l i m i t i n g the number of prey items wrapped. They attempted to wrap a l l parasitoids that landed i n t h e i r webs and were r e l a t i v e l y successful. This behaviour resulted i n a considerable amount of "wasteful k i l l i n g " . The tendency to k i l l more prey than i s consumed i s well documented for orb-weaving spiders (Kajak 1978, Riechert and Luczak 1982). As outlined i n the description of experimental design, I counted the number of parasitoids wrapped i n s i l k and thereby k i l l e d by the spiderlings i n order to determine prey capture rates. I did not measure the extent to which spiderlings actually consumed the prey they k i l l e d . This i s because I was interested i n death rates of the prey. A half-eaten parasitoid i s just as dead as a f u l l y eaten one! However, at the high densities silk-wrapped parasitoids, whose contents had not been consumed, were found on cage bottoms. Such wasteful k i l l i n g i n spiders i s attributed to the lag period between wrapping a prey item and in j e c t i n g i t with digestive enzymes and returning to consume the pre-digested prey (Riechert and Lockley 1984). Wasteful k i l l i n g i s also known i n other predators. Damselfly 108 naiads, Ischnura ramburii (Hagen) (Johnson et a l . 1975) preying on Daphnia magna Straus, the predatory mites, Amblyseius largoenisis (Muma) and Aj_ concordis (Chant), preying on phytophagous mites (Sandness and McMurtry 1970), and the crab spider Philodromus rufus Walckenaer preying on spruce budworm (Haynes and Sisojevic 1966) are a l l known to k i l l more than they consume. These three predators a l l display functional response curves similar to the one found for juvenile cross-spiders before inexperienced and experienced spiderlings were separated (Fig. 4A.02); they a l l show the unusual r i s e i n prey capture at the highest densities. None of these authors accounted for the experience acquired by the predator over the course of the experiment. Johnson et a l . (1975) found a day effect s i m i l a r to the one found i n this experiment before the effects of experience were separated. Had they made the d i s t i n c t i o n due to experience they too might have discoverd two different responses. The functional response found on the f i r s t day of prey capture, i n this experiment, may be t y p i c a l of predators with a tendency towards wasteful k i l l i n g . Holling (1968) did consider wasteful k i l l i n g and found that i f i t increased with prey density i t could exert a s t a b i l i z i n g influence i n his simulations. Unfortunately I was unable to c l a r i f y whether the exponential functional response of inexperienced spiderlings was due solely to the fact that they had never captured any prey or whether i t was actually due to the fact that they were hungry. In order to control for predatory experience at the onset of the experiment a l l of the animals were taken from communal webs. They were, therefore, both inexperienced and hungry up to this point. They had not fed on 109 anything other than yolk reserves. If the unlimited capture rates observed at the high prey densities were only a function of predatory naiv i t e then these results would not indicate any potential for the regulation of prey populations by cross-spiderlings. This i s because the effect would be removed by experience. However, i f the exponential increase i n capture rates with prey density was mainly a function of the fact that the inexperienced spiderlings were hungry then the tendency to k i l l large numbers of prey could indeed have an impact on a target prey population. Indirect evidence did indicate that the exponential functional response was due to the fact that the spiderlings were hungry and, therefore, would also occur with hungry spiderlings that had previous predatory experience. For example, hunger levels are important i n i n i t i a t i n g prey searching ( i . e . web-spinning) for experienced spiderlings. The less prey an experienced spiderling captured on one day, the more l i k e l y i t was to spin a web on the next day (Fig. 4A.18). No such measure could be d i r e c t l y calculated for inexperienced spiderlings because, as pointed out i n the resu l t s , they had a l l captured 0 prey on a l l previous days. Further evidence that the 'inexperienced' response i s , i n fact, the same as a hungry response l i e s i n the results of some preliminary experiments. There was less probabilty of a parasitoid escaping from the web of a previously starving spiderling than a recently fed one. Starving spiderlings, l i k e inexperienced spiderlings, were less l i k e l y to allow a parasitoid to escape. Future experiments would have to c l a r i f y t his point. A group 110 of in d i v i d u a l s which were experienced at capturing prey but also starving would have to be considered. I f , as expected, such a group responded to increasing prey densities with the same exponential trend as inexperienced spiderlings then the capacity of these animals to dampen the population levels of prey species which arrive i n a sudden flush would be remarkable. Sudden increases i n population numbers are not an uncommon phenomenon for insects with density-dependent dispersal mechanisms. EXPERIENCED SPIDERLINGS Once spiderlings had captured t h e i r f i r s t parasitoid they ceased o v e r k i l l i n g and, under the given experimental regime, exhibited a t y p i c a l invertebrate Type 2 functional response (Fig. 4A.07). They no longer realized the f u l l p o t e ntial of their prey capturing c a p a b i l i t i e s , as evidenced by the fact that the numbeis of parasitoids captured were lower for the experienced spiderlings than the inexperienced ones at a l l densities except 10 (Fig. 4A.08). This result was not t o t a l l y a function of more prey being captured per web by inexperienced spiderlings because s i g n i f i c a n t l y more prey were captured per web only at densities of 1 and 40 (Fig. 4A.23). A combination of a reduction in web spinning and fewer prey per web resulted i n the difference i n prey capture. At any rate, spiderlings had to be making decisions about how much prey to capture. This i s not surprising as most predators have' mechanisms to prevent o v e r k i l l i n g t h e i r prey to the extent that the prey become extinct. As already mentioned, the number of prey captured on any day I l l had a direct effect on web-spinning behaviour for experienced spiderlings. The more prey captured the less l i k e l y a spiderling was to spin a web the following day. Besides providing a second mechanism to prevent prey extinction, this behaviour could serve another function. Considerable predation from a variety of other arthropods occurs on cross-spiderlings. Because cross-spiderlings usually s i t at the hub of the web they are p a r t i c u l a r i l y vulnerable to predation by the v i s u a l l y searching arthropods such as jumping spiders (Salticidae) and wasps (Vespidae and Sphecidae) as well as vertebrates such as birds. The cost of exposing themselves to predation may be so high for these spiderlings that searching i s extremely unprofitable when hunger levels are not high. Several predator avoidance mechanisms have been described for orb-weavers (Hoffmaster 1982). Not exposing oneself i n a web should be further studied and possibly added to the l i s t . EFFECTS UNEXPLAINED BY PREDATORY EXPERIENCE Although no s i g n i f i c a n t day effects could be detected once the data were partitioned, some of the o r i g i n a l day effects cannot be e n t i r e l y explained by predatory experience. P a r t i c u l a r l y , at the highest density there was no s i g n i f i c a n t difference i n capture rates between Day 1 and Day 5 (Table 4A.04), even though on Day 1 a l l spiderlings were inexperienced and by Day 5 most spiderlings were experienced. Two possible explanations e x i s t . The dampening of predation at high densities might be only a temporary behavioural response. After continual exposure to high prey densities ( i . e . for 112 5 days here) spiderlings may somehow make the 'assessment' that available prey w i l l remain high and conservation of the food supply i s unnecessary. They might begin to again r e a l i z e their f u l l capturing potential, as they did as inexperienced spiderlings. Some indirect evidence supports t h i s hypothesis. I t i s well known that spiders can increase both the i r rate of development and their fecundity by eating more (Riechert and Luczak 1982). Some species, including the cross-spider, can go through an extra molt when food i s unlimited (Gertsch 1978). Therefore, i t i s reasonable to assume that after a threshold exposure to high prey densities spiderlings might switch into a behavioural mode whereby developmental rate and reproductive output are enhanced. The long term benefits of such a strategy would be the ensuing increase i n f i t n e s s . The second possible hypothesis i s that prey capture rates are c y c l i c . In th i s case If the experiment had been continued over a longer period spiderlings would have captured large catches every 5 to 6 days and dampened their capture rates i n between. This type of behaviour might conceivably occur as a response to physiological state such as the stage i n the molting cycle. Perhaps the starvation period d i r e c t l y before molting i s counterbalanced by high feeding rates i n the middle of the cycle. Only a future experiment, conducted over a longer period, could distinguish between these hypotheses. 113 CHAPTER 4B FUNCTIONAL RESPONSE WITH A PEST INSECT AS PREY MATERIALS AND METHODS THE PREY The "pest prey species" was the potato aphid, M^_ euphorbiae. It i s a large and conspicuous aphid, common on potatoes but possessing many other hosts including several garden plants (Blackman 1974). Alates (winged morphs) were obtained from the Agriculture Canada Research Station, Vancouver and colonies were maintained on potatoes i n the Plant Science greenhouse. Only winged aphids were used i n the experiments. An aphid was chosen as the pest prey because aphids were the most commonly found insect group i n the f i e l d study and one of the most common prey groups of orb-weavers found by other researchers (Kajak 1965, Nentwig 1983). THE SPIDERS Spiderlings used i n this experiment were from the same communal groups described i n the preceding section. THE MATERIALS The same plywood cages as described i n the previous section were used. A potato shoot i n a v i a l of water was supplied as both 114 support for the webs and to encourage normal a c t i v i t y patterns i n the aphids. THE EXPERIMENTAL DESIGN The experimental design using aphids as prey was generally the same as with the parasitoids, with the few exceptions described below. Alate aphids were introduced into cages at densities of 1, 10, 20 and 40 per cage. The t o t a l number of spiderlings tested at each density was 10, 8, 14 and 10, respectively. No group with 60 aphids was established because of the d i f f i c u l t y i n obtaining enough alates to maintain cages at such a high density. Cages at 0 density were not repeated—the ones from the previous parasitoid experiment were used here because the aphid t r i a l s were conducted simultaneously with the parasitoid t r i a l s . Aphids were not removed every day, as the parasitoids were. Rather, counts were made of the number of alates a l i v e i n the cage and numbers were replenished to keep the densities constant. Alate aphids were not as active as the parasitoids, therefore they were unlikely to gain experience escaping from webs. Again, aphids k i l l e d by the spiderlings were easily distinguished from those that died for other reasons because they were wrapped i n s i l k . 115 RESULTS NUMBER OF PREY CAPTURED Overall prey catches were lower with aphids as the prey than with parasitoids (Fig. 4B.01). For example, at a density of 40 parasitoids spiderlings were capturing 19.4 +2.4 ( + S.E.) prey over 5 days. In contrast, at a density of 40 aphids spiderlings were capturing only 2.6+0.8 ( + S.E.) prey over 5 days. Inspite of the low number of aphids captured a si g n i f i c a n t difference was noted i n the numbers of aphids captured due to both aphid density and experimental day (Table 4B.01). A Type 2 functional response curve with a s a t i a t i o n l e v e l from 10 to 40 prey was the best f i t to the data, using linear regression. However, 77% of the variation was not accounted for by the regression model (Fig. 4B.02). Only mean capture rates at 0 density were s i g n i f i c a n t l y less than those at other densities (Table 4B.02). Percent predation i s also a gauge of the type of functional respose exhibited. The data for percent predation did not meet the assumption of equal variances so linear regression could not be determined. However, percent predation was correlated only weakly with density (r s=0.18, p=0.002) and day (r s=0.21, p=0.001). A s i g n i f i c a n t difference i n predation rate due to density was noted, but again only 0 density was s i g n i f i c a n t l y different (Fig.4B.03, Table 4B.03). The difference i n capture rates due to day (Table 4B.01) was a result of s i g n i f i c a n t l y greater catches on Day 5 (Fig. 4B.04, Table 4B.04). The number of aphids captured on Day 5 was 116 FIGURE 4B.01 COMPARISON OF THE NUMBER OF APHIDS CAPTURED AND THE NUMBER OF PARASITOIDS CAPTURED. The mean number of prey captured + S.E. over 5 days, with aphids and parasitoids as the prey were compared. Spiderlings consistently captured more paraistoids than aphids. 116a 24 " I GO a to 16 2; i—( P w rt PH 8 <J c j rt PH 0- " PI 1 i 1 1 10 20 4 0 I I 1 ^ P A R A S I T O I D S | [ A P H I D S PREY DENSITY 117 TABLE 4B.01 TWO-WAY ANALYSIS OF VARIANCE SUMMARY TABLE. Differences i n the number of aphids captured due to aphid density and experimental day were calculated. The prey capture transformation. data was transformed using a log Source of Variation Sum of Squares DF Mean Square Significance of F Main Effects 1.484 8 0.186 6.915 0.001 Aphid Density 0.483 4 0.121 4.496 0.002 Day 1.002 4 0.250 9.335 0.001 2-Way Interactions 0.625 16 0.039 1.456 0.117 Explained 2.110 24 0.088 3.276 0.001 Residual 6.574 245 0.027 Total 8.684 269 0.032 118 FIGURE 4B.02 FUNCTIONAL RESPONSE OF CROSS-SPIDERLINGS WITH APHIDS AS PREY. The functional response curve was drawn using the linear regression model presented i n the figure. Data for both the numbers of aphids captured and aphid density were transformed using a log transformation i n order to obtain the best f i t to a linea r regression model. The means and confidence intervals presented are back-transformed. Arithmetic means are presented along with the geometric means for comparison. 118a r2=0.23 P=0.006 log10(y+1)=0.06+0.24(log10x) • A R I T H M E T I C M E A N S • G E O M E T R I C M E A N S WITH 95 0/0 C.I . 10 20 40 APHID DENSITY 119 TABLE 4B.02 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y d i f f e r e n t from each other are joined by a bar. Data for the number of aphids captured was log transformed i n order to meet the assumption of equal variances and the back-transformed means ( i . e . geometric means) are presented. Aphid Density 0 1 10 40 20 Aphids Captured 0 0.28 1.19 1.25 2.75 120 FIGURE 4B.03 PERCENT PREDATION AS A FUNCTION OF APHID DENSITY. Percent predation = Number of aphids captured aphid density Linear regression was not calculated because the data did not meet the assumption of equal variances, even with a transformation. Means + standard errors are presented. 121 TABLE 4B.03 KRUSKAL-WALLIS 1-WAY ANOVA SUMMARY. Aphid density did not have a s i g n i f i c a n t effect on percent predation. Percent predation was only s i g n i f i c a n t l y different at a density of 0, as indicated by the fact that the mean ranks for densities of 1 to 40 were very close. Aphid Density 0 1 10 20 40 Number of Observations 60 50 40 70 50 Mean Ranks 118.50 130.50 146.15 145.07 138.98 Corrected for Ties Chi-Square Significance Chi-Square Significance 4.94 0.29 14.88 0.005 122 FIGURE 4B.04 NUMBER OF APHIDS CAPTURED AS A FUNCTION OF DAY. The mean number of aphids captured + standard errors are presented. More aphids were captured on Day 5 than the other 4 days. » 122a 1.5H 1 . 0 -Q W H PH <i U GO Q K 0.5 PH < 6 o.o- 1 t 2 3 4 DAY 123 TABLE 4B.04 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY. The data for the number of prey captured was log transformed i n order to meet the assumption of equal variances. Geometric means are presented. Means are arranged i n ascending order and means that were not si g n i f i c a n t l y different from each other are joined by a bar. Day 4 1 3 2 5 Aphids Captured 0.01 0.03 0.14 0.16 0.5 124 s i g n i f i c a n t l y greater than on the other days at a l l densities except 1 (Fig. 4B.05, Table 4B.05). Because the f i r s t prey was captured on Day 2 as often as on Day 5 (Fig. 4B.06) the f i r s t day of prey capture did not account for as much of the day effect as i t did with the parasitoids. As expected from the prey catches, percent predation was higher on Day 5 than the other 4 days (Fig. 4B.07, Table 4B.06). The f i r s t day of prey capture was s t i l l analyzed separately. No s i g n i f i c a n t differences i n the numbers of aphids captured were found as a result of either density or day (Table 4B.07) and a si g n i f i c a n t linear relationship between prey capture and either density or day did not e x i s t . However, percent predation was a function of aphid density and the resulting curve was as expected for a predator with a Type 2 response curve (Fig. 4B.08). The mean percent predation was s i g n i f i c a n t l y different across density but not day (Table 4B.08). However, i t was only mean capture rates at a density of 1 which were different than the other densities (Table 4B.09). After the f i r s t prey had been captured no significant differences were apparent i n capture rates due to density, but a day effect was noticeable (Table 4B.10). More prey were captured on Day 5 than any other day (Fig. 4B.09). Again no line a r relationship between capture rates and aphid density was discernable. The percent predation data did not have equal variances so regression could not be calculated. However, no correlation existed between percent predation and density (r =-0.06, p=0.363) and no si g n i f i c a n t s differences i n percent predation due to aphid density (Table 4 B . l l a ) , for days after the f i r s t prey was captured were found. Prey mortality was correlated with day (r =0.47, p=0.001) and, 125 FIGURE 4B.05 NUMBER OF APHIDS CAPTURED PER DAY SEGREGATED BY DENSITY. The means and standard errors are given for the number of aphids captured on each day, at each density. Note that capture rates on Day 5 are s i g n i f i c a n t l y : g r e a t e r for a l l densities except 1. The si g n i f i c a n t tests are presented i n TABLE4B.05. 125a DEL A i 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 DAY 1 2 3 4 5 0 1 10 20 APHID DENSITY 40 126 TABLE 4B.05 KRUSKAL-WALLIS 1-WAY ANOVA SUMMARY. Significance tests for the effects of experimental day on the number of aphids captured are presented. None of the data met the assumption of equal variances, even with a log transformation, so a non-parametric t e s t , Kruskal-Wallis 1-Way Anova, was performed. Note that there i s a si g n i f i c a n t effect due to experimental day at a l l densities except 1. TABLE 4B.05 KRUSKAL-WALLIS 1-WAY ANOVA SUMMARY DENSITY 1 Day 1 2 3 4 5 Mean Ranks 25.95 25.95 23.50 23.50 28.60 CORRECTED FOR TIES CHI-SQUARE SIGNIFICANCE CHI-SQUARE SIGNIFICANCE 0.85 0.93 3.83 0.43 10 Day 1 2 3 4 5 Mean Ranks 16.50 19.12 23.88 16.50 26.50 CORRECTED FOR TIES CHI-SQUARE SIGNIFICANCE CHI-SQUARE SIGNIFICANCE 4.76 0.31 9.77 0.05 20 Day 1 2 3 4 5 Mean Ranks 30.68 38.29 31.36 30.68 46.50 CORRECTED FOR TIES CHI-SQUARE SIGNIFICANCE CHI-SQUARE SIGNIFICANCE 6.50 0.17 13.34 0.01 40 Day 1 2 3 4 5 Mean Ranks 21.50 24.15 26.40 21.50 33.95 CORRECTED FOR TIES CHI-SQUARE SIGNIFICANCE CHI-SQUARE SIGNIFICANCE 4.99 0.29 12.26 0.02 127 FIGURE 4B.06 DAY THE FIRST APHID WAS CAPTURED. The f i r s t aphid was captured as often on Day 2 as often as on Day 5. This differed from the results when parasitoids were the prey. In that case more prey were captured on Day 1 than any other day. Frequency of capturing the first Aphid on each day O L 00 128 FIGURE 4B.07 PERCENT PREDATION AS A FUNCTION OF DAY. Percent predation = Number of aphids captured aphid density The percent predation was highest on Day 5. Means with standard errors are presented. PERCENT PREDATION 129 TABLE 4B.06 KRUSKAL-WALLIS 1-WAY ANOVA SUMMARY. The percent predation was higher on Day 5 than any other day, as determined by the Kruskal-Wallis 1-Way Anova Test. The s i g n i f i c a n t difference due to day i n (a) disappears when Day 5 i s removed from the analysis (b) . (a) Day Number of Observations Mean Ranks 1 2 3 4 5 54 54 54 54 54 123.56 136.15 133.33 120.80 163.66 CORRECTED FOR TIES CHI-SQUARE SIGNIFICANCE CHI-SQUARE SIGNIFICANCE 10.24 0.037 30.84 <0.001 (b) Day Number of Observations Mean Ranks 54 54 54 - 54 104.53 114.64 112.44 102.40 CHI-SQUARE SIGNIFICANCE 1.47 0.69 CORRECTED FOR TIES CHI-SQUARE SIGNIFICANCE 7.12 0.68 130 TABLE 4B.07 ANALYSIS OF VARIANCE SUMMARY TABLE. For the f i r s t day aphids were captured neither aphid density nor experimental day had a s i g n i f i c a n t effect on the number of aphids captured. Source of Sum of DF Mean F Significance Variation Squares Square of F Main Effects 23.439 7 3.348 0.604 0.741 Aphid Density 3.207 3 1.069 0.193 0.899 Day 14.221 ' 4 3.555 0.642 0.645 2-Way Interactions 10.462 4 2.616 0.472 0.756 Explained 33.902 11 3.082 0.556 0.825 Residual 55.417 10 5.542 Total 89.318 21 4.253 131 FIGURE 4B.08 PERCENT PREDATION AS FUNCTION OF APHID DENSITY. Percent predation for the f i r s t day of prey capture i s compared with days after the f i r s t day of prey capture. Note that the curves are very similar i n form. For the f i r s t day of prey capture percent predation i s s i g n i f i c a n t l y greater at a density of 1 than the other densities. After the f i r s t day of prey capture there i s no si g n i f i c a n t difference due to density. The data for the f i r s t day of prey capture met the assumptions of regression. The best f i t linear regression model was: r 2 = 0.77 log (y+1) = 0.86 - 0.58 X log x The data for days after the f i r s t day of prey capture did not meet the assumption of equal variances so linear regression could not be determined. Means with standard errors are presented. 131a O 80-i—i H *• PH PH PH H £ 4 0 PH U PH W PH 1 FIRST DAY OF PREY CAPTURE O O AFTER FIRST PREY CAPTURED' \ \ o-J f IT 10 20 40 APHID DENSITY 132 TABLE 4B.08 ANALYSIS OF VARIANCE SUMMARY TABLE. Percent predation, for the f i r s t day an aphid was captured, was s i g n i f i c a n t l y d i f f e r e n t across aphid densities but not day. Source of Sum of DF Mean F Significance Variation Squares Square of F Main Effects 1.412 7 0.202 9.909 0.001 Aphid Density 1.162 3 0.387 19.030 0.001 Day 0.032 4 0.008 0.389 0.812 2-Way Interactions 0.018 4 0.004 0.215 0.924 Explained 1.429 11 0.130 6.384 0.003 Residual 0.204 10 0.020 Total s 1.633 21 0.078 133 TABLE 4B.09 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY. Percent predation on the f i r s t day an aphid was captured was s i g n i f i c a n t l y greater at a denstiy of 1 than the other densities. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. Aphid Density 40 20 10 1 Percent Predation 8.5 13.5 22.0 100.0 134 TABLE 4B.10 ANALYSIS OF VARIANCE SUMMARY TABLE. The number of aphids captured, after the f i r s t day an aphid was captured, was not s i g n i f i c a n t l y different across aphid densities. A s i g n i f i c a n t difference due to experimental day was found. The data were transformed using the log transformation in order to meet the assumption of equal variances. Source of Sum of DF Mean F Significance Variation Squares Square of F Main Effects 0.858 7 0.123 2.253 0.061 Aphid Density 0.043 3 0.014 0.261 0.853 Day 0.823 4 0.206 3.785 0.014 2-Way Interactions 0.267 8 0.033 0.615 0.758 Explained 1.125 15 0.075 1.379 0.227 Residual 1.468 27 0.054 Total 2.593 42 0.62 135 FIGURE 4B.09 NUMBER OF APHIDS CAPTURED AS A FUNCTION OF DAY. After the f i r s t aphid was captured more aphids were captured on Day 5 than any other day. Means with standard errors are presented. After First Prey Captured w p< H PH < u CO Q O o—1 T I I T I 1 2 3 4 5 DAY 136 TABLE 4B.11 PERCENT PREDATION AFTER THE FIRST PREY WAS CAPTURED. (a) Percent predation was not s i g n i f i c a n t l y d i f f e r e n t across d e n s i t i e s a f t e r the f i r s t prey was captured, (b) It was s i g n i f i c a n t l y greater on Day 5 than any other day. (c) S i g n i f i c a n t e f f e c t s due to day disappeared when Day 5 was removed from the a n a l y s i s . KRUSKAL-WALLIS 1-WAY ANOVA SUMMARY A. Aphid Density Mean Ranks 1 10 20 40 23.57 23.11. 20.47 23.00 CORRECTED FOR TIES CHI-SCUARE SIGNIFICANCE CHI-SQUARE SIGNIFICANCE 0.51 0.92 0.82 0.85 B. Day Mean Ranks 1 2 3 4 5 16.00 22.50 18.67 16.00 30.30 CORRECTED FOR TIES CM-SQUARE SIGNIFICANCE CHI-SQUARE SIGNIFICANCE 10.846 0.028 17.364 0.002 C. Day Mean Ranks 1 2 3 4 13.50 18.00 15.11 13.50 CORRECTED FOR TIES CHI-SQUARE SIGNIFICANCE CHI-SQUARE SIGNIFICANCE 0.829 0.842 4.156 0.245 137 as expected, aphid mortality was highest on Day 5 (Table 4B.llb and c). These results were complicated by the fact that 20 out of 42 spiderlings that were presented with aphids captured none. The high number of spiderlings f a i l i n g to capture aphids occurred because the aphids were so sedentary that the chance of an aphid ever contacting a web was small. FREQUENCY OF WEB SPINNING No differences i n the frequency of web spinning as a result of either aphid density or experimental day could be detected (Table 4B.12, Fig.4B.10a). Nor was there a difference when old and new webs were added together (Table 4B.13, Fig.4B10b). No linear relationship could be found between the number of webs spun and aphid density. The same results held when the data were s p l i t into the f i r s t day of prey capture and days after the f i r s t day of prey capture. On days before any aphids were captured more webs were spun on Day 5 than any other day. However, this difference was only si g n i f i c a n t for Day 4 (Tables 4B.14 and 4B.15). PREY PER WEB The number of aphids captured per web did not increase with prey density as i t did when parasitoids were used as prey. The mean number of aphids captured per web was s i g n i f i c a n t l y different for density but not for day (Table 4B.16), with capture rates per web at a density of 20 being greater than a density of 1 138 TABLE 4B.12 ANALYSIS OF VARIANCE SUMMARY TABLE FOR WEB SPINNING. There were no s i g n i f i c a n t effects due to either aphid density or day for the number of new webs spun. Source of Sum of DF Mean F Significance Variation Squares Square of F Main Effects 3.028 8 0.379 1.257 0.267 Aphid Density 0.791 4 0.198 0.657 0.623 Day 2.237 4 0.559 1.856 0.119 2-Way Interactions 4.439 16 0.277 0.921 0.546 Explained 7.467 24 0.311 1.033 0.425 Residual 73.807 245 0.301 Total 81.274 269 0.302 139 FIGURE 4B.10 WEB SPINNING AS A FUNCTION OF APHID DENSITY. (a) The number of new webs spun i n 5 days did not vary with aphid density. (b) The t o t a l web exposure ( i . e . new webs + old webs) did not vary with aphid density either. 139a 140 TABLE 4B.13 ANALYSIS OF VARIANCE SUMMARY TABLE FOR TOTAL WEB EXPOSURE. The t o t a l web exposure (new webs + old webs) was not affected by aphid density or experimental day. Source of Sum of DF Mean F Significance Variation Squares Square of F Main Effects 2.744 8 0.343 1.328 0.230 Aphid Density 0.581 4 0.145 0.562 0.690 Day 2.163 4 0.541 2.093 0.082 2-Way Interactions 3.835 16 0.240 0.928 0.538 Explained 6.579 24 0.274 1.061 0.390 Residual 63.288 245 0.258 Total 69.867 269 0.260 141 TABLE 4B.14 ANALYSIS OF VARIANCE SUMMARY TABLE FOR DAYS BEFORE ANY APHIDS WERE CAPTURED. There was a s i g n i f i c a n t difference i n the number of new webs spun as a function of experimental day but not aphid density. Source of Sum of DF Mean F Significance Variation Squares Square of F Main Effects 3.471 8 0.434 1.488 0.164 Aphid Density 0.371 4 0.093 0.318 0.866 Day 3.181 4 0.795 2.727 0.031 2-Way Interactions 4.805 16 0.300 1.030 0.427 Explained 8.276 24 0.345 1.183 0.263 Residual 53.938 185 0.292 Total 62.214 209 0.298 142 TABLE 4B.15 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY. The number of new webs spun, before any prey were captured, was s i g n f i c i a n t l y less on Day 4 than Day 5. There were no s i g n i f i c a n t difference between any other days. Means are arranged i n ascending order and means that were not s i g n i f i c a n t l y different from each other are joined by a bar. Day 4 3 2 1 5 New Webs ' 1.9 4.0 4.5 4.6 6.4 143 TABLE 4B.16 ANALYSIS OF VARIANCE SUMMARY TABLE FOR APHIDS CAPTURED PER WEB. The number of aphids captured per web was s i g n i f i c a n t l y d i f f e r e n t across aphid density but not day. A log transformation was performed on the data to meet the assumption of equal variances. Source of Sum of DF Mean F Significance Variation Squares Square of F Main Effects 0.916 8 0.114 2.741 0.008 Aphid Density 0.644 4 0.161 3.855 0.005 Day 0.262 4 0.065 1.567 0.186 2-Way Interactions 0.454 16 0.028 0.679 0.811 Explained 1.369 24 0.057 1.366 0.134 Residual 5.931 142 0.042 Total 7.301 166 0.044 0 144 (Table 4B.17, F i g . 4B.11). The number of aphids captured per web was not a linear function of aphid density although i t was weakly correlated with aphid density (r g=0.23, p=0.002). Separating the data into days before and after prey catching experience did not give any new insights into these r e s u l t s . 145 a TABLE 4B.17 STUDENT-NEWMAN-KEULS MULTIPLE RANGE TEST SUMMARY FOR NUMBER OF APHIDS CAPTURED PER WEB. The geometric means are presented because a log transformation was performed on the data i n order to meet the assumption of equal variances. Means are arranged i n ascending order and means that are not s i g n i f i c a n t l y different are joined by a bar. Note that there i s a si g n i f i c a n t difference between densities of 20 and both 1 and 0 only. Aphid Density 0 1 40 10 20 Aphids Captured Per Web 0 0.08 0.31 0.40 0.44 146 FIGURE 4B.11 NUMBER OF PREY CAPTURED PER WEB. The number of prey captured per web was not a linea r function of aphid density. The geometric means and confidence intervals are presented because the data was transformed with a log transformation i n order to meet the assumption of equal variances. Arithmetic means are presented for comparison. 146a APHID DENSIT7 147 DISCUSSION The low prey catches i n the aphid experiment were more a r e s u l t of aphid behaviour than s p i d e r l i n g behaviour. Soon a f t e r introduction i n t o a cage the aphids usually walked up the potato shoot, s e t t l e d , and began feeding. Although they were a l l winged the conditions i n the cages did not seem to be appropriate to e l i c i t f l y i n g behaviour on the i n i t i a l introduction, hence the low capture rates on Day 1. Because the winged adults were not removed each day they began l a r v i p o s i t i n g . Some in d i v i d u a l s flew to a new leaf a f t e r l a r v i p o s i t i o n . It was then that they became ava i l a b l e to the s p i d e r l i n g s , and t h i s behaviour resulted i n the f i r s t aphid being captured on Day 2 as often as on Day 5. By the f i f t h day there were so many small nymphs on the potato leaves that the adult alates that were o r i g i n a l l y introduced began dispersing. During t h i s d i s p e r s a l phase most alates were captured. Even though the f i r s t aphid was often caught on Day 2 s p i d e r l i n g s that captured aphids captured more on Day 5. The s i g n i f i c a n t l y greater capture rates on Day 5 were, therefore, almost e x c l u s i v e l y a r e s u l t of aphid behaviour. R e c a l l that with the parasitoids capture rates on Day 1 were greater. This was more a re s u l t of the s p i d e r l i n g ' s behaviour as the a v a i l a b i l i t y of a ctive parasitoids was not l i m i t i n g . In s p i te of the high number of spi d e r l i n g s which captured no prey a picture does emerge of a predator with a Type 2 functional response. These r e s u l t s are, however, to be taken l i g h t l y as the capture rates of aphids i n these experiments were lower than ones observed i n the f i e l d . It i s doubtful that an actual s a t i a t i o n 148 l e v e l was attained and a different experimental set-up would have to be designed to take aphid behaviour into consideration. For example, i t would have been better to s a c r i f i c e attempts to duplicate conditions i n the parasitoid experiment exactly and omit the potato shoot for the aphid t r i a l s . By excluding a place for the aphids to s e t t l e they would l i k e l y have been encouraged to f l y . Web spinning was not affected by either aphid density or experimental day, as i t was i n the previous experiment. Likely a threshold number of prey wrapped i s required to reduce web spinning and with the aphids this threshold was not reached. 149 EPILOGUE The foraging behaviour of A. diadematus spiderlings was investigated from three different perspectives i n order to assess their potential impact on populations of a "pest" insect and a "b e n e f i c i a l " insect. F i r s t , a f i e l d study demonstrated that the webs of most spiderlings are taken down before any prey have been captured. In the few webs that contained prey items, large numbers of aphids and few of the beneficial hymenopterous parasitoids were caught, suggesting that cross-spiderlings are s p e c i a l i s t s feeding on ag r i c u l t u r a l pests . A laboratory experiment determined that during periods of very low prey a v a i l a b i l i t y both second and t h i r d instar spiderlings were able to survive on alternative foods, such as birch pollen, thereby avoiding starvation or the need to disperse when insect prey are unavailable. I t was suggested that the a b i l i t y of spiderlings to increase their survival by feeding on pollen may provide an additional explanation for the adaptive significance of web-eating. Previously t h i s behaviour was explained only as a mechanism to recycle s i l k proteins. Preference studies on various microorganisms and insect prey would now be i n order to determine whether or not the a v a i l a b i l i t y of microorganisms would indeed be a deterrent to dispersal when insect populations are low. Not a l l microorganisms captured on webs provided the same n u t r i t i o n a l value. In the study described here a fungus, herbarum provided no n u t r i t i o n a l value and there was some indication that the fungus was actually deleterious. « 150 A second laboratory study showed that during periods of increasing prey a v a i l a b i l i t y cross spiderlings were able to respond with an exponential functional response. The functional response to prey density was not consistent for a l l experimental conditions. Both the experience of the spiderling and the species of the prey had an impact on the shape of the functional response curve. When the prey was a parasitoid and the spiderlings were inexperienced an exponential functional response curve was obtained. However, experiments using an aphid or experienced spiderlings resulted i n a ty p i c a l invertebrate Type 2 functional response curve. Although a beneficial species of prey e l i c i t e d the exponential functional response, these same beneficial insects were rarely seen i n webs i n the f i e l d , indicating that cross spiderlings are unlikely to have an impact on their population l e v e l s . However, other strong f l i e r s , l i k e dipterans, which are commonly found i n webs i n the f i e l d , would l i k e l y e l i c i t the same type of functional response. The pest species used did not e l i c i t the exponential functional response, p a r t i a l l y because of problems with the laboratory environment. The contradictions between f i e l d and laboratory results raise questions about the a p p l i c a b i l i t y of laboratory experiments. On the one hand, parasitoids are rarely captured i n the f i e l d and orb webs are poor at retaining them. Yet, spiderlings are adept at capturing parasitoids i n a cage. On the other hand, aphids are very common i n webs i n the f i e l d and spiderlings seem p a r t i c u l a r l y poor at capturing them i n a cage. Unfortunately, the behaviour of the prey i s seldom considered i n functional response experiments and i t 151 obviously plays a c r u c i a l role. A l l aspects of the prey behaviour, i n r e l a t i o n to the predator, must be considered when attempting to extrapolate from the laboratory to the f i e l d . Usually only the predator's behaviour i s considered. I t i s d i f f i c u l t to simulate the f i e l d conditions which are an integral part of the complicated relationship between a predator and i t s prey. As with a l l laboratory experiments, the ones described here are somewhat limited by the inherent differences between a cage and the f i e l d . 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