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Spatial distribution and reproductive biology of western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera : Thripidae) Higgins, Charlene J.

Abstract

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), are haplodiploids. Virgin females produce sons parthenogenetically but must be mated to produce daughters. As a result, primary and secondary (adult) sex ratios can diverge from the 1:1 ratio commonly observed in diploid systems. Field studies were conducted to examine the spatial distribution of WFT on two greenhouse crops, Bell peppers, Capsicum annuum (Linn.) and Long English cucumbers, Cucumis sativus (Linn.) to determine if there was a correlation between sex ratio and density. Leaf and flower samples were taken weekly. All adult and immature WFT were counted and sex of adults determined. Yellow sticky traps were used to monitor density and sex ratio of the dispersing adult population. Lab studies were also done to investigate if male availability affected the sex ratio and number of offspring produced by individual females. Laboratory experiments were done to assess the effects of sperm availability and maternal age on sex ratio of progeny produced. On both crops, 84 to 95 % of adult WFT in flowers were females and most larvae (> 85 %) were found on leaves. Male WFT were rare on all plant parts even when caught in high numbers on traps. Approximately 75 % of females found on plants in the greenhouse were mated. Most (70 - 90 %) WFT on traps at low densities (< 200 individuals/trap) were males. This suggests that WFT populations are initiated by virgin females that likely overwintered as pseudopupae in the greenhouse. These females initially produce only sons, and may have to wait for these to emerge before they mate and produce daughters. As WFT density within a greenhouse increases, females are probably mated soon after emergence. Sex ratio of adults on traps becomes more female biased as density increases within a greenhouse. Heavily female biased (> 65 %) sex ratios were found on traps at high population densities (> 200 individuals/trap). Sex ratio of adults on traps remained male biased in the pepper greenhouse (WL) where the population density of WFT remained low. Information regarding within-plant distribution of thrips is essential for population monitoring and control. Used together, regular examination of flowers and counts of adults on sticky traps allow quick detection of potential "hot spots" of WFT density. Sex ratio and density are highly correlated. Sex ratio of the dispersing adult population is a good predictor of outbreak potential of the extant WFT population. Two WFT predators found in greenhouses were also monitored to document their effects on WFT population density and sex ratio. Mass introductions of the predatory mite, Amblysieus cucumeris, did not successfully control WFT in most greenhouses monitored. A natural infestation of pirate bugs, Orius tristicolor occurred in the only pepper house monitored. WFT density remained low throughout the growing season. O. tristicolor shows promise for future use in integrated pest management programs designed to control F. occidentalis in commercial greenhouses. Lab studies showed that sex ratio of offspring produced by mated females was influenced by sperm supply and maternal age. Mated females produced sons and daughters which suggests that females control sex of offspring produced through selective fertilization. Once mated, two-thirds of offspring produced are females. Older females produced fewer daughters than younger females. A principle conclusion from this study is that sex ratio of the WFT population within a greenhouse can be used to predict future population dynamics. Male availability may be the most important factor affecting the number of daughters produced by individual females which in turn may determine the potential of WFT populations to increase.

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