Applied Science, Faculty of
Non UBC
Engineering, School of (Okanagan)
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Insects 12 (2): 159 (2021)
Esch, Evan D.
Horner, Rachael M.
Krompetz, Dustin C.
Moses-Gonzales, Nathan
Tesche, Melissa R.
Suckling, David Maxwell
The codling moth is a serious pest of apples in most regions of the world where this fruit is produced. The sterile insect technique is one strategy used to control this pest and is employed as part of an area-wide integrated pest management program for the codling moth in British Columbia, Canada. Modified fixed wing aircraft are the most common method for the release of sterile insects in large area-wide pest management programs. However, aerial release with a full-size aircraft can be prohibitively expensive. We evaluated the use of small, uncrewed aircraft systems (UASs) for the release of sterile codling moths. Sterile codling moths released from greater altitudes were more broadly distributed and drifted more in strong winds, compared to those released from lower altitudes. Most of the released insects were recaptured in a 50 m wide swath under the release route. Recapture rates for aerially released insects were 40–70% higher compared to those released from the ground. UASs provide a promising alternative to ground release and conventional aircraft for the release of sterile codling moths.
insectsArticleOperational Parameters for the Aerial Release of SterileCodling Moths Using an Uncrewed Aircraft SystemEvan D. Esch 1,2,*, Rachael M. Horner 3 , Dustin C. Krompetz 4, Nathan Moses-Gonzales 4, Melissa R. Tesche 1and David Maxwell Suckling 3,5Citation: Esch, E.D.; Horner, R.M.;Krompetz, D.C.; Moses-Gonzales, N.;Tesche, M.R.; Suckling, D.M.Operational Parameters for the AerialRelease of Sterile Codling Moths Usingan Uncrewed Aircraft System. Insects2021, 12, 159. https://doi.org/10.3390/insects12020159Academic Editors: Kostas Bourtzisand Marc VreysenReceived: 6 January 2021Accepted: 10 February 2021Published: 13 February 2021Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affil-iations.Copyright: © 2021 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).1 Okanagan-Kootenay Sterile Insect Release Board, Kelowna, BC VIW 3Z4, Canada; Mtesche@oksir.org2 School of Engineering, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada3 The New Zealand Institute for Plant and Food Research Limited, 8140 Christchurch, New Zealand;rachael.horner@plantandfood.co.nz (R.M.H.); max.suckling@plantandfood.co.nz (D.M.S.)4 M3 Consulting Group Ltd., Dayton, OH 45068, USA; dustin.krompetz@M3cg.us (D.C.K.);nmosego@plantandfood.co.nz (N.M.-G.)5 School of Biological Sciences, University of Auckland, 1072 Auckland, New Zealand* Correspondence: Eesch@oksir.orgSimple Summary: Pest management through the release of sterile codling moths from drones oruncrewed aerial vehicles (UAVs) represents an efficient method for distribution over orchards. Inthis study, moths released from greater altitudes were more broadly distributed and drifted morein strong winds compared to those released from lower altitudes. Most of the released insects wererecaptured in a 50 m wide swath under the release route. Recapture rates from aerially releasedinsects were 40–70% higher compared to those released from the ground. We found that an uncrewedaircraft system (UAS) releasing insects from 35 m above the ground in release routes 50 m apartoutperformed standard ground release methods. UASs provide a promising alternative to groundrelease and conventional aircraft for the release of sterile codling moths.Abstract: The codling moth is a serious pest of apples in most regions of the world where this fruit isproduced. The sterile insect technique is one strategy used to control this pest and is employed aspart of an area-wide integrated pest management program for the codling moth in British Columbia,Canada. Modified fixed wing aircraft are the most common method for the release of sterile insectsin large area-wide pest management programs. However, aerial release with a full-size aircraftcan be prohibitively expensive. We evaluated the use of small, uncrewed aircraft systems (UASs)for the release of sterile codling moths. Sterile codling moths released from greater altitudes weremore broadly distributed and drifted more in strong winds, compared to those released from loweraltitudes. Most of the released insects were recaptured in a 50 m wide swath under the release route.Recapture rates for aerially released insects were 40–70% higher compared to those released from theground. UASs provide a promising alternative to ground release and conventional aircraft for therelease of sterile codling moths.Keywords: sterile insect technique; Cydia pomonella; release method; uncrewed aircraft system;codling moth; drone; apple; pest management1. IntroductionApples are among the most abundantly produced fruits all over the world, withupwards of 50 Mt produced annually [1]. The codling moth Cydia pomonella (L.) is akey pest of apples in most regions of the world where this fruit is grown [2], includingthe valleys of the Southern Interior of British Columbia (BC), Canada [3]. This pest hadestablished itself as a serious pest in the Okanagan and surrounding valleys by 1916,at which time its populations were managed with lead arsenate insecticide [4]. Leadarsenate was later replaced by organochlorine insecticides, and while the economic lossesInsects 2021, 12, 159. https://doi.org/10.3390/insects12020159 https://www.mdpi.com/journal/insectsInsects 2021, 12, 159 2 of 18from codling moths were significantly reduced, the environmental and health hazardsof these insecticides became too large to bear [4,5]. The desires to minimize the negativeimpacts of pesticide usage and slow the development of insecticide resistance promotedthe development of alternate strategies for suppressing insect pests. One such autocidalapproach to pest management, the sterile insect release method and its derivative inheritedsterility (IS) (collectively referred to here as the sterile insect technique (SIT)), becamewidely known through its successful application in the eradication of the New Worldscrew worm Cochliomyia hominivorax (Coquerel) [6]. While numerous researchers workedtowards the development of SIT to control the codling moth, it was ultimately a 15-yearresearch program by Proverbs and colleagues that led to the establishment of the Okanagan–Kootenay Sterile Insect Release (OKSIR) program [7].The OKSIR program was established in 1992 with the stated goal of eradicating thecodling moth from the Okanagan and surrounding valleys [7,8]. While eradication was neverachieved, this program has conducted an effective, area-wide integrated pest managementprogram (AW IPM), with SIT as the key component for more than 25 years [7,9]. During thistime, research was conducted to advance the field of the codling moth SIT and promoteits uptake globally [2]. Most of this research focused on optimizing the radiation dose toleverage IS [10–13], improving rearing methods [14–16], and integrating SIT with other pestmanagement strategies [17]. Other research has served to identify limitations in the OKSIRprogram’s operations [18–20] or evaluate the global expansion of codling moth SIT [21–23].Until a recent cooperative research program hosted by the International Atomic EnergyAgency, there has been a comparative dearth of research evaluating release methods forsterile codling moth SIT [23]. Since the establishment of the cooperative program in 2016,uncrewed, fixed, and rotary-winged aircraft have been used operationally for five years ina test of over several hundred hectares of apple orchards in New Zealand using insectssupplied by OKSIR [23]. Similarly, uncrewed rotary-wing aircraft (Figure 1) have been usedto release sterile codling moths over an area of 1000 ha in experimental and commercialapplications in Washington State, USA.Insects 2021, 12, x FOR PEER REVIEW 2 of 18 populations were managed with lead arsenate insecticide [4]. Lead arsenate was later re-placed by organochlorine insecticides, and while the economic losses from codling moths were significantly reduced, the environmental and health hazards of these insecticides became too large to bear [4,5]. The desires to minimize the negative i pacts of pesticide usage and slow the development of insecticide resistance promoted the develop ent of alternate strat gies for suppressing insect pests. One such autocidal approach to pest man-agement, the sterile ins ct releas method and its derivative inherited sterility (IS) (collec-tively referred to here as the sterile insect technique (SIT)), b ame widely known through its successful application in the eradication of the New World screw worm Cochliomyia hominiv ax (Coquerel) [6]. While numerous researchers worked towards the develop-ment of SIT to control the codling m th, it was ultimately a 15-ye r research program by Proverbs and colleagues that le t the establishment of the Okanagan–Kootenay Sterile Insect Release (OKSIR) program [7]. The OKSIR program was established in 1 92 with the stated goal of eradicating the codling moth from the Ok nag and surrounding valleys [7,8]. While eradicatio was nev r achieved, this program has conducted an effective, area-wide integrated pest man-agement program (AW IPM), with SIT as the key c mp nent for more than 25 years [7,9]. During this time, research was conducted to advance t field of the codling moth SIT and promote its uptake globally [2]. Most of this research focused on optimizing the radiation dose to leverage IS [10–13], improving rearing methods [14–16], and integrating SIT with other pest management strategies [17]. Other research has served to identify limitations in the OKSIR program’s operations [18–20] or evaluate the global expansion of codling moth SIT [21–23]. Until a recent cooperative research program hosted by the International Atomic Energy Agency, there has been a comparative dearth of research evaluating re-lease methods for sterile codling moth SIT [23]. Since the establishment of the cooperative program in 2016, uncrewed, fixed, and rotary-winged aircraft have been used operation-ally for five years in a test of over several hundred hectares of apple orchards in New Zealand using insects supplied by OKSIR [23]. Similarly, uncrewed rotary-wing aircraft (Figure 1) have been used to release sterile codling moths over an area of 1000 ha in ex-perimental and commercial applications in Washington State, USA. Figure 1. Photograph of the Hermes uncrewed aircraft system (UAS) used to release sterile cod-ling moths in the Okanagan in 2016 and 2017. The UAS is shown in flight. The moth storage and release device is mounted under the airframe, between the landing gear. The sterile insect technique works by reducing the frequency of mating amongst the wild pest population and generally achieves this by outnumbering wild males with sterile ones [6]. Sexual competition between male codling moths can be described as a form of scramble competition [24,25]. However, the underlying codling moth populations are ag-gregated across different spatial and temporal scales [26,27], and the locations of these aggregations may be unknown to pest managers, making the goal of outnumbering the Figure 1. Photograph of the Hermes uncrewed aircraft system (UAS) used to release sterile codlingmoths in the Okanagan in 2016 and 2017. The UAS is shown in flight. The moth storage and releasedevice is mounted under the airframe, between the landing gear.The sterile insect technique works by reducing the frequency of mating amongst thewild pest population and generally achieves this by outnumbering wild males with sterileones [6]. Sexual co petition bet een ale codling oths can be described as a form ofscra ble co petition [24,25]. However, the underlying codling moth populations areaggregated across different spatial and te poral scales [26,27], and the locations of theseaggregations ay be unkno n to pest managers, making the goal of outnumbering thewild pest more challenging. As such, the OKSIR program has employed a strategy ofuniformly distributing sterile insects within a given orchard [28] with varying release ratesInsects 2021, 12, 159 3 of 18used between orchards according to their underlying wild populations [29]. This strategyis a means of “hedging bets” against the imperfect understanding of aggregations of wildcodling moths. While the quality and competitiveness of sterile insects has been the focusof most of the research on the codling moth SIT, it is evident that the release methods andstrategy also play an important role [30].Aerial release with crewed, fixed-wing aircraft is the preferred method for most largeAW IPM programs utilizing SIT [30]. Aerial release from crewed, fixed-wing aircraft rapidlydelivers insects with excellent fitness in a uniform distribution. However, high fuel, pilot,and hangar costs make this a very expensive method, often amounting to more than aquarter of the operating budget of SIT programs [30]. Manned aircraft are also not suitablefor release in a fragmented landscape where a more targeted release is required [30]. TheOKSIR program explored the use of a crewed, fixed-wing aircraft (Cessna 205) to releasesterile insects in 2003, but this method was fraught with logistical challenges (OKSIR,unpublished data). Rotary-wing aircraft were used for the aerial release of sterile codlingmoths early in the development of the OKSIR program, but this approach was ultimatelydeemed too expensive [8,31]. Instead, the OKSIR program has used modified all-terrainvehicles (ATVs) (Figure 2) to conduct moving, ground releases of sterile insects since itstarted releases in 1994 [28]. However, this procedure exposes the chilled sterile insects tosignificant stress during the transport and release process [18]. It has been argued [18] thatthe negative impacts of transportation and cold storage needed for ground release outweighthe numerous improvements to the codling moth SIT methods that have been developedin the last 25 years. Ground release methods also require a substantial amount of time andstaff, and release vehicles are often restricted to roads or orchard rows, resulting in a lessuniform distribution of insects [30]. The growing use of uncrewed aircraft systems (UASs)in agricultural applications present opportunities to deploy aerial release technology insituations in which the costs were previously prohibitive [32]. As part of the InternationalAtomic Energy Agency Cooperative Research Program on “Improved Field Performance ofSterile Male Lepidoptera to Ensure Success in SIT Programs”, our Canadian, US, and NewZealand team conducted experiments to explore basic operational parameters to determinehow a UAS might be used to release sterile codling moths in the context of the OKSIRprogram and to establish more universally useful guidelines for other programs. Aerialreleases with UASs were evaluated against the standard ground release methods used bythe OKSIR program. Given the widely accepted superiority of aerial release methods andthe increasing use of UASs in agriculture, we hypothesized that aerial release from a UASwould be more effective than the ground release methods currently employed by OKSIR.Insects 2021, 12, x FOR PEER REVIEW 3 of 18 wild pest more challenging. As such, the OKSIR program has employed a strategy of uni-formly distributing sterile insects within a given orchard [28] with varying release rates used between orchards according to their underlying wild populations [29]. This strategy is a means of “hedging bets” against the imperfect understanding of aggregations of wild codling moths. While the quality and competitiveness of sterile insects has been the focus of most of the research on the codling moth SIT, it is evident that the release methods and strategy also play an important role [30]. Aerial release with crewed, fixed-wing aircraft is the preferred method for most large AW IPM programs utilizing SIT [30]. Aerial release from crewed, fixed-wing aircraft rap-idly delivers insects with excellent fitness in a uniform distribution. However, high fuel, pilot, and hangar costs make this a very expensive method, often amounting to more than a quarter of the operating budget of SIT programs [30]. Manned aircraft are also not suit-able for release in a fragmented landscape where a more targeted release is required [30]. The OKSIR progra explored the use of a crewed, fixed-wing aircraft (Cessna 205) to re-lease sterile insects in 2003, but this method was fraught with logistical challenges (OKSIR, unpublished data). Rotary-wing aircraft ere used for the aerial release of sterile codling moths early in the development of the OKSIR program, but this approach was ultimately deemed too expensi e [8,31]. Instead, the OKSIR program as used modified all-terrain vehicles (ATVs) (Figure 2) to conduct moving, ground rele ses of sterile insects since it started releases in 1994 [ 8]. H wever, this procedure exposes the chilled sterile inse ts to significant stress during the transport and release process [18]. It has b en argued [18] that the nega ive impacts of transportatio and cold storage needed for ground release ou -weigh the numerous improvements to the codling moth SIT meth ds that have been de-v loped in the last 25 years. Ground release methods also require a substantial amount of time and staff, nd release v hicl s are often rest icted to roads or orchard rows, resulting in a less uniform distribution insects [30]. The growing use of uncrewed aircraft syst ms (UASs) in agricultural applications pr sent opportunities to deploy aerial release technol-ogy in situations in which the costs were previously prohibitive [32]. As part of the Inter-national Atomic Energy Agency Cooperative Research Program on “Improved Field Per-for ance of Sterile Male Lepidoptera to Ensure Success in SIT Programs”, our Canadian, US, and New Zealand team conducted experiments to explore basic operational parame-ters to determine how a UAS might be used to release sterile codling oths in the context of the OKSIR program and to establish more universally useful guidelines for other pro-grams. Aerial releases with UASs were evaluated against the standard ground release methods used by the OKSIR program. Given the widely accepted superiority of aerial release methods and the increasing use of UASs in agriculture, we hypothesized that aer-ial release from a UAS would be more effective than the ground release methods currently employed by OKSIR. Figure 2. Photograph of a modified Honda Fourtrax all-terrain vehicle (ATV) used by the Okanagan–Kootenay Sterile Insect Release Program to release sterile codling moths (Cydia pomonella) in appleorchards. Moths are stored in a cooler on the rear of the ATV and released from the release device onthe front of the vehicle (photograph by Tom Walker, 2015).Insects 2021, 12, 159 4 of 182. Materials and Methods2.1. Rearing and ReleaseMoths used in these experiments were reared at the Okanagan–Kootenay SterileInsect Release Program’s mass-rearing facility in Osoyoos, British Columbia, Canada, andprepared to the same standards as those used in program operations. Moths developed intrays (45 cm × 29 cm × 2.5 cm) on a modified Brinton diet under environmental controlconditions [33]. Upon emergence, adult moths of both sexes were collected, immobilizedby chilling to 2–4 ◦C, and packaged in 10 cm Petri dishes, with each dish containingca. 800 moths in a 50:50 (M:F) sex ratio [14]. Petri dishes of moths were irradiated witha dose of 200 Gy in a Co60 irradiator (JL Shepherd and Associates, San Fernando, CA,USA), transferred to 2 L plastic bowls for marking, and then returned to Petri dishes forstorage and transportation. Fluorescent powders from Dayglo Corp. (Cleveland, OH,USA) were used to mark the moths. Different colors of Dayglo powder were used fordifferent release methods and replicates, allowing the identification of recaptured mothsto be assigned to a given release. Marking was accomplished by sprinkling a half scoop(Hayman-Style Microspatulas, FisherbrandTM) of fluorescent powder (ca. 0.05–0.07 g)over ca. 800 moths. Bowls were gently turned to ensure thorough coverage of mothswith fluorescent powder. Efficacy of marking was evaluated under ultraviolet light atthe start of each experiment (Figure S1). Moths were kept chilled at 2–4 ◦C at the rearingfacility (Osoyoos, BC, Canada) or at satellite field offices (Kelowna, BC, Canada) untiltransportation to the field in a portable cooler. Moths were transferred from Petri dishes tothe release devices immediately before release.The Hermes UAS was used for the aerial release experiments outlined below (Figure 1).This UAS has a combination of commercial and custom components, designed and builtby M3 Consulting Group Inc. (Dayton, OH, USA). The UAS can be described as an“octocopter”, capable of vertical take-off and landing, with a moth storage and releasedevice mounted under the airframe. The release of moths was monitored during eachflight with a fish-eye lens and a digital transmitter in 2016. A significant redesign of therelease mechanism occurred between 2016 and 2017 to improve the uniformity of mothsreleased per unit of time. In 2016 and 2017, numerous bench-tests and flight-tests wereconducted to calibrate the release rate, so that the release rate of moths per unit of time, theflight speed of UAS, and the spacing of flight paths could be manipulated to achieve thedesired release rate of moths per area. Flight plans for the test releases were created basedon geo-referenced orchard maps created by ArcMap and programmed in Mission Planner,ArduPilot [34].The ground release of moths was performed using a Honda Fourtrax all-terrain vehicle(ATV) equipped with a custom release device and cooler for moth storage (Figure S2). Therelease device used a constant stream of air to gently propel moths from the device tothe side of the direction of ATV travel, thereby releasing them onto the ground and thelower canopy of trees (after Proverbs) [28]. The speed of the travel (4–6 m/s on average)and release rate of moths were manually controlled by the ATV operator to achieve thedesired number of moths per ha. Release routes were laid with flagging tape prior to allexperiments and were followed by the ATV operator during releases.2.2. Experiment 1: Effect of Release Height on Moth Distribution, Drift, and RecaptureTo better understand how aerial release distributes sterile moths in an orchard, amark–release–recapture experiment was conducted on 17–19 August 2016. The test orchardselected for this experiment was a flat, 275 m × 250 m section of a larger apple orchardlocated in the Similkameen Valley, BC, Canada (49.145925◦, −119.731873◦). PredominantlyGala and Mcintosh varieties were planted in trellised, 3 m-wide rows, with 2.5–3 m-talltrees spaced 0.3–1 m apart in each row, creating a relatively uniform canopy. The testorchard was bordered by similar orchards to the north and south, with a small amountof similar orchards to the west, with a semi-natural area beyond, and a road to the east,with similar orchards beyond. The test orchard was divided into two equal size replicates,Insects 2021, 12, 159 5 of 18referred to as east and west plots hereafter, each 250 m × 125 m. There was a 25 × 250 stripbetween the edges of the east and west plots.Multiple releases were conducted over the study orchard. Release routes were des-ignated 50 m from the west edge of each plot, parallel to the long edge of the plot andproceeded from the south to the north end of the test orchard. A “release” consisted ofthe UAS taking off from a staging area near the test orchard, proceeding to the south endof the plot where the release route began, making a single pass along the release routefrom the south to north end of the plot while releasing moths, and then returning to thestaging area. The UAS travelled at 3 m/s (ground speed). One released at 25 m aboveground and one release at 50 m above ground were made in each plot, along the samerelease route, each day. A total of four releases were conducted each day. Releases wererepeated on 3 consecutive days for a total of 12 releases or six replicates at each altitude.Flights commenced at 3:00 p.m., after all orchard workers left the area. Moths released fromdifferent altitudes and on different days were marked with different colors of fluorescentpowder, for a total of 3 different colors. Temperature, relative humidity, wind speed, anddirection were recorded before each flight with an AR816 handheld anemometer (Kestrel).The release device was calibrated to release 2000 insects/ha both on a lab bench and inflight. However, these calibrations did not account for the strong winds encounteredduring the test releases, and the device used in 2016 did not perform as expected, resultingin some moths left in the release device after flights. All of the unreleased moths werecollected from the release device and weighed to estimate the actual number of mothsreleased (OKSIR, unpublished data). A determination of the number of released mothswas necessary to calculate the percent recapture.White Delta II traps (Trécé Inc., Adair, OK, USA.) were used to recapture marked malemoths in the test orchard. Traps were hung in the top third of the canopy in a 25 m × 25 mgrid of 132 traps (ca. 7 traps/ha). Traps were baited with 1 mg (1×) codlemone sexpheromone lures (Trécé Inc., Adair, OK, USA). For Experiment 1, sticky trap liners werecollected and replaced every day for 7 days, starting 1 day after release. Liners wereexamined under UV light to determine how many marked moths from each release werecollected on each day at each grid location.2.3. Experiment 2: Comparing Recapture Rates between Aerial and Ground Release MethodsTo evaluate the effectiveness of a UAS for sterile moth release, test flights simulatingthe operation requirements of the OKSIR program on 16–17 August 2017 were conducted.In a single “mission”, the UAS released moths into one orchard block each of small (ca.0.5 ha), medium (ca. 2–3 ha), and large (ca. 5–8 ha) sizes during a single flight (FiguresS3–S6). Four different “missions” were flown, each treating a different small, medium andlarge orchard block. Two missions were flown each day. The UAS flew at 8 m/s (groundspeed) 35 m above ground releasing ca. 2000 moths/ha along release routes spaced 50 mapart. During a test flight, the UAS would launch, travel to and from test orchards, andrelease insects along designated release routes above the test orchards. Each block in a testflight was 30–200 m from the next test block and situated amongst mixed-fruit orchardsand residential dwellings. Test orchards were flat and had diverse apple varieties as wellas varied tree spacings ranging from 3 m × 0.3–6 m apart. All test blocks were locatedwithin a 350 ha area of East Kelowna, BC.Ground releases were conducted using an ATV, one to two hours following the aerialreleases. The release rate (2000 moths/ha) was controlled manually by E.D.E. for allexperiments. The ATV travelled at 4–6 m/s along release routes ca. 30 m apart. Releaseroutes differed between aerial and ground release according to tree-row orientation, fences,buildings, and other obstacles which differently affected navigation in air or ground.Instead, the release routes are examples of how the two different methods are/could beutilized operationally. Moths released from the air or ground were marked with differentcolors of fluorescent powder. White delta traps with 1x lures were used to recapturemarked, male moths in the test blocks. Traps were spaced on a 50 m × 50 m grid, or asInsects 2021, 12, 159 6 of 18close to this configuration as possible, given the irregular shape of some blocks. Traps werechecked one, four, and seven days after release.A similar, unreplicated comparison of aerial and ground release was conducted inthe Similkameen Valley on 16 August 2016. Around 2000 insects/ha were released in arectangular, 3.5 ha orchard. The UAS released moths along a central release route runningthe length of the orchard at 25 m above ground, while the ATV released the same number ofinsects along 3 evenly spaced routes running the length of the orchard. Moths from aerialand ground releases were marked with different colored fluorescent powders. Moths werecollected in white delta traps placed on a 25 m × 25 m grid. In this experiment, traps werechecked every day for seven days, and moth coloring was identified as described above.2.4. Experiment 3: Comparing Release StrategiesDifferent aerial release strategies were compared. Metered releases of moths alongevenly spaced release routes (swaths) were compared to the release of moths from a singlestationary point 35 m over the center of the orchard (burst) (Figures S7–S10). This wasaccomplished by adding burst releases over the medium-sized plots in Experiment 2 in2017. Burst releases were conducted at a rate of 2000 insects/ha immediately followingmetered releases. Moths from burst releases were marked differently than other aerial andground releases. The traps were the same as those used for Experiment 2. Comparisonsbetween burst and metered releases were replicated four times.2.5. Data AnalysisRecapture rates for each trap were calculated, based on the estimated number of malemoths released, for each day after release and the total trapping period since pheromone-baited traps only attract males. For Experiment 1, the recaptures in each row of 11 trapsat given distances perpendicular to the release route (−50, −25, 0, 25, 50, and 75 m) weretabulated and totaled for each test flight. The influence of release height and the absolutevalue of trap distance (0, 25, 50, and 75 m) from the release route on total recaptures wasexamined by two-way analysis of variance (ANOVA). Total recaptures from all traps inswath and burst releases were compared with a paired t-test. The pattern of recapture(swath) was described by heat maps of percent recapture for each release altitude and dayafter release. Spatial autocorrelation of recaptured moths was described using Moran’s Ispatial statistic, calculated using the spdep package in R (R Core Team, Vienna, Austria),using k = 8 nearest neighbors, except for Experiment 3, in which k = 4 neighbors wasused to accommodate the smaller plot sizes. Moran’s I has been used widely in the studyof insect spatial patterns [35,36]. Moran’s I index compares the abundance of moths inneighboring traps (k) to the mean deviation in moth abundance in all traps and producesan index value between −1 and 1, describing the overall spatial autocorrelation for anarray of points. Values of I equal to −1 indicate a perfectly even distribution, 0 indicates arandom distribution, and values of 1 indicate a perfectly aggregated distribution. A MonteCarlo procedure with 99 permutations was used to test whether the observed distributionof moth captures was significantly more clustered than random. Recapture rates fromaerial and ground releases in each orchard size (Experiment 2) were compared using two-way ANOVA. Only large orchard blocks from Experiment 2 had enough trap locations tocalculate robust values of Moran’s I. This index was calculated using the same method asdescribed above (k = 8) for each release method in the large blocks.All experimental orchards relied on sterile insect releases as the main tactic for con-trolling wild codling moth populations. Pesticide applications may have been used tosupplement SIR, however, no pesticides were applied during the course of these experi-ments. Mating disruption was not used in any experimental orchards.Insects 2021, 12, 159 7 of 183. Results3.1. Effect of Release Height on Moth Distribution, Drift, and RecaptureOverall, 6542 marked moths were recaptured from the 12 test releases examining theeffect of release height on moth distribution. Release height did not have a significanteffect on the total proportion of recaptured moths (F1,68 = 0.014, p = 0.905). Total recapturesaveraged 23 ± 2.1% (mean ±1 standard error) and 21 ± 2.0% when released from 25and 50 m above ground, respectively. Most of the moths were recaptured directly underthe release route our immediately next to it, with the proportion of recaptured mothsdecreasing as the distance from the release route increased (F3,68 = 21.880, p < 0.005). Afterseven days, 80% of the moths were recaptured in a 50 m wide swath under the releaseroute when released from 25 m above ground. When released from 50 m above ground,67% of the moths were recaptured in the same 50 m wide swath. While more moths tendedto be caught farther away from the release route when released from a greater altitude(Figures 3 and 4), the proportion of moths recaptured at each distance between the releaseroutes did not significantly differ between release heights (F3,68 = 3.04, p = 0.0858).Insects 2021, 12, x FOR PEER REVIEW 7 of 18 All experimental orchards relied on sterile insect releases as the main tactic for con-trolling wild codling moth populations. Pesticide applications may have been used to sup-plement SIR, however, no pesticides were applied during the course of these experiments. Mating disruption was not used in any experimental orchards. 3. Results .1. Effec of Release Height on Moth Distribution, Drift, and Recapture Overall, 6542 marked moths were recaptured from the 12 test releases examining the effect of release height on oth distribution. Release height did not hav a significant ef-fect on th total proportion of recaptured moths (F1,68 = 0.014, p = 0.905). Tot l recaptures av raged 23 ± 2.1% (mean ±1 standard error) and 21 ± 2.0% when released from 25 and 50 m above ground, respectively. Most of the m ths were recaptured directly un er the re-lease route our immediately n x to it, with he proportion of captured moths decreasing as th distance from the rel ase route increased (F3,68 = 21.880, p < 0.005). After seven days, 80% of the moths were recaptured in 50 m w de swath under the release route wh n r lease from 25 m above ground. When rel ased from 50 m above ground, 67% of th moths w re rec ptured in the s me 50 m wide swath. While m re moths tended to be caught farther away from the r lease route when releas d from a greater altitude (Figures 3 and 4), the propo tion of moths r captur d at ach distance between the release ro tes did not significantly differ between release heights (F3,68 = 3.04, p = 0.0858). Figure 3. Mean percentage of recaptured moths vs. distance from the aerial release route (m). Each distance from the release route represents the sum of 11 traps in the grid at the specified distance from the release route for each replicate. Points represent week-long total recaptures averaged across replicates ±1 standard error for each distance (n = 6). Black and white points represent re-captures of moths that were release from 25 or 50 m above ground, respectively. Figure 3. Mean percentage of recaptured moths vs. distance from the aerial release route (m). Eachdistance from the release route represents the sum of 11 traps in the grid at the specified distancefro the release route for each replicate. Points represent week-long total recaptures averaged acrossreplicat s ±1 standard error for each distance (n = 6). Black and white points represent recaptur s ofmoths that were rele se from 25 or 50 m above ground, respectively.Insects 2021, 12, 159 8 of 18Insects 2021, 12, x FOR PEER REVIEW 8 of 18 Figure 4. Spatial pattern of daily male codling moth recaptures when released from 25 m (top row) or 50 m (bottom row) above ground. Each day and altitude combination was replicated 6 times. Each colored cell represents the mean percentage of moths recaptured at a given location on a given day on a 25 m × 25 m grid of pheromone-baited traps. The intensity of shading in the cell corresponds to percentage recapture (see legend). The dashed line indicates the location of the release route over the plot. Locations of each trap along the plot and the distance perpendicular to the release route in m are indicated on the axes. Recaptures are scaled such that the sum of all cells for all seven days for a given release height equals 100%. The arrow in the upper left panel indicates plot orientation relative to north. Prevailing winds were out of the north. The shaded gray arrow in the top right panel indicates the location and direction of the release route. The proportion of recaptured moths and their distribution in the plot varied between days after release. Of the moths that were recaptured, the greatest proportion was recap-tured on the second day after release (40.4%). Recaptures were relatively high on the first (20.1%), third (10.0%), and fourth (15.5%) days after release and trailed off on the fifth (7.2%), sixth (5.2%), and seventh (1.2%) days. Moth recaptures were concentrated directly under the release route and decreased with distance from the release route. This pattern was generally similar between days after release (Figure 4), with some variation. Values of Moran’s I for each day after release in each plot ranged from −0.7 to 0.45. Average Mo-ran’s I values for each release height and day after release were similar between most days and release heights (Table 1). Moth recaptures tended to be more significantly clustered than random (Monte Carlo test, p < 0.05) earlier in the week of trapping, compared to later in the week. Table 1. Summary of Moran’s I values for each release height and day after release (n = 6). I values shown are the mean from all replicates, and instances of significant clustering indicate the number of replicates out of 6 where a Monte Carlo procedure identified clustering to be more significant than random (p < 0.05). Day After Release Release Height Mean Moran’s I Instances of Significant Clustering 1 25 0.11 6 50 0.17 6 2 25 0.18 6 50 0.17 6 3 25 0.12 3 Figure 4. Spatial pattern of daily male codling moth recaptures when released from 25 m (top row) or 50 m (bottom row)above ground. Each day and altitude combination was replicated 6 times. Each colored cell represents the mean percentagef t s rec t re t i e l c ti i e 25 ri f er e- ite tr s. e i te sit fi i t ll . li i i t s t l ti f t lt t l t. Locations of each trap along the plot and t e ista ce er endicular to the release r te iindicated on the axes. Recaptures are scaled such that the sum of all cells for all seven days for a given release height equals100%. The arrow in the upper left panel indicates plot orientation relative to north. Prevailing winds were out of the north.The shaded gray arrow in the top right panel indicates the location and direction of the release route.The proportion of recaptured oths and their distribution in the plot varied betweendays after release. f the oths that ere recaptured, the greatest proportion as recap-tured on the second day after release (40.4 ). ecaptures ere relatively high on the first(20.1 ), thir (10.0 ), and fourth (15.5 ) days after release a traile off o t e fift(7.2 ), si t (5.2 ), a se e t (1.2 ) a s. oth recaptures ere co ce trate irectlnder t e r lease r t r it i t fr t r l r t . i tt rll i il t e days after release (Figure 4), with some variation. Values ofMoran’s I for each day fter releas in each plot ranged from −0.7 to 0.45. Average Moran’sI values for each releas heig t and day after release were similar between most days ndrelease heig ts (Table 1). Moth recaptures tended to be more significantly clustered thanrandom (Monte Carlo test, p < 0.05) earlie n the week of trapping, compared to later inthe week.Wind speeds were high during the experimental releases, typical for the region andseason. On the first d y of rel ase, winds were 11.9 km/h out of the north, gustingto 35.7 km/h. On the second day, winds were 20.9 km/h out of the north, gusting to51.8 km/h. On the third day, winds were 8.9 km/h out of the south, gusting to 11.9 km/h.It was difficult to isolate the effects of wind during release flights (and the subsequentrecapture period) on moth distribution, dispersal, and recapture. However, a substantialproportion of marked moths was recaptured along the southern edge of the plots on thefifth and sixth days after release (Figure 4). This pattern was more pronounced when mothswere released from greater altitudes. This observation is consistent with the hypothesisthat moths were blown out of the experimental plots by strong northerly winds duringreleases and were then returning to the nearest edge of the plot later in the week.Insects 2021, 12, 159 9 of 18The east and west plots were separated by a 25 m-wide strip of orchard. Relatively fewmoths were recaptured in the rows of traps on either side of the border between the eastand west plots. On the first day after release, 10 moths, 0.15% of all recaptured moths, werecollected in the rows of traps along the border. After 7 days, this number increased to 274,4.2% of all recaptured moths. This suggests that it is unlikely that moths released over oneplot drifted into the neighboring plot. While some moths may have moved between plotson subsequent days after release, the low numbers of moths captured along the borders ofthe plots suggest that the movement of moths between plots was limited.Table 1. Summary of Moran’s I values for each release height and day after release (n = 6). I valuesshown are the mean from all replicates, and instances of significant clustering indicate the number ofreplicates out of 6 where a Monte Carlo procedure identified clustering to be more significant thanrandom (p < 0.05).Day After Release Release Height Mean Moran’s I Instances ofSignificant Clustering125 0.11 650 0.17 6225 0.18 650 0.17 6325 0.12 350 0.18 4425 0.15 250 0.16 3525 0.13 350 0.14 4625 0.13 250 0.14 3725 0 250 0 17 Day Total 25 0.26 650 0.31 63.2. Comparing Recapture Rates between Aerial and Ground Release MethodsOverall, 10,711 marked moths were recaptured during the 2017 tests when comparingaerial and ground release methods. Total recaptures (mean ±1 standard error) tended tobe higher from aerial (18.6 ± 2.50%) than ground (10.8 ± 2.44%) releases; however, thistrend was only marginally different (F1,16 = 4.209, p = 0.0551). The proportion of recapturedmoths did not differ between orchard sizes (F2,15 = 0.674, p = 0.534) (Figure 5). Whenlooking at the percentage of moths that were recaptured on a given trap interval ratherthan the percentage of released moths that were recaptured, we found the same patternin daily recaptures as in Experiment 1. On the first day after release, 16.3 and 27.1% of allmoths were recaptured from the ground and aerial release methods, respectively. On thefourth day after release, 60% of moths from ground releases and 51% of moths from aerialreleases were recaptured. On the seventh day after release, 23.5 and 23.6% of moths wererecaptured from the ground and aerial releases, respectively.Insects 2021, 12, 159 10 of 18Insects 2021, 12, x FOR PEER REVIEW 10 of 18 Figure 5. Mean percentage of moths recaptured in small, medium, and large orchards when re-leased from an all-terrain vehicle (ATV) or uncrewed aerial system (UAS). Height of bars indicates the average percentage recapture (+1 standard error) from four replicated releases. UAS and ATV performed similarly well at distributing sterile moths in orchards. Mean values of Moran’s I were 0.09 when released from the ground and 0.08 when re-leased by air (Figures S3-S6). In only one instance did recaptured moths exhibit significant clustering. In this orchard, moths released from UAS (I = 0.238, p = 0.01) and ATV (I = 0.283, p = 0.01) showed similar clustering (Table 1). This orchard was L-shaped with pre-dominantly more moths recaptured in one arm of the “L”, presumably driving this pat-tern (Figure S3). A total of 1193 marked moths was recaptured in the unreplicated comparison of aer-ial and ground release conducted in 2016: 25.3% of moths were recaptured when aerially released, while only 17.4% of moths were recaptured when released from the ground. On the first day after release, 45.8% of all recaptured moths were collected from aerial release while only 35.4% of ground recaptured moths were collected from ground release. Subse-quently, 29.0 and 46.1% were collected on the second day, 16.0 and 12.7% on the third day, 5.2 and 1.4% on the fourth day, and 3.1 and 3.9% on the fifth day from the aerial and ground releases, respectively. Less than 1% of all recaptured moths were collected on the sixth and seventh days after release. 3.3. Comparing Recaptures between Swath and Burst Release Overall, 1415 marked moths were collected from swath releases and 1431 marked moths were collected from burst releases in the test blocks. The total percentage of recap-tured moths did not differ (t = 0.043, df = 3, p = 0.968) between swath (16.6 ± 3.57%) and burst (14.1 ± 2.78%) releases. Moths released from bursts tended to be more clustered un-der the point of release, while insects released in swaths were more evenly distributed across the block (Figures S7–S10). After 7 days, mean values of Moran’s I were 0.26 from Figure 5. Mean percentage of moths recaptured in small, medium, and large orchards when released from an all-terrainvehicle (ATV) or unc wed aerial system (UAS). Height of bars dicates the average percentage recapture (+1 standarderror) from four r plicated releases.UAS and ATV performed similarly well at distributing sterile moths in orchards. Meanvalues of Mo an’s I were 0.09 when released from the g ound and 0.08 when released byair (Figures S3–S6). In only one i stance did ecaptured moths exhibit significant clustering.In th s o chard, moths released from UAS (I = 0.238, p = 0.01) and ATV (I = 0.283, p = 0.01)showed simil clustering (Table 1). This orchard was L-shaped with predominantly moremoths recaptured in one arm of the “L”, presumably riving this attern (Figure S3).A total of 1193 marked m ths was recaptured in the unreplicated comparison of aerialand ground release conducted in 2016: 25.3% of moths were recaptured when aeriallyreleased, while only 17.4% of moths were recaptured when released from the ground.On the first day after release, 45.8% of all recaptured moths were collected from aerialrelease while only 35.4% of ground recaptured moths were collected from ground release.Subsequently, 29.0 and 46.1% were collected on the second day, 16.0 and 12.7% on the thirdday, 5.2 and 1.4% on the fourth day, and 3.1 and 3.9% on the fifth day from the aerial andground releases, respectively. Less than 1% of all recaptured moths were collected on thesixth and seventh days after release.3.3. Comparing Recaptures between Swath and Burst ReleaseOverall, 1415 marked moths were collected from swath releases and 1431 markedmoths were collected from burst releases in the test blocks. The total percentage of recap-tured moths did not differ (t = 0.043, df = 3, p = 0.968) between swath (16.6 ± 3.57%) andburst (14.1 ± 2.78%) releases. Moths released from bursts tended to be more clusteredunder the point of release, while insects released in swaths were more evenly distributedacross the block (Figures S7–S10). After 7 days, mean values of Moran’s I were 0.26 fromburst releases and 0.08 from swath releases, with more instances of significant clustering(p < 0.05) from burst releases (Table 2).Insects 2021, 12, 159 11 of 18Table 2. Summary of Moran’s I values for each release strategy (burst or swath) collection afterrelease (n = 4). I values Scheme 4 where a Monte Carlo procedure identified clustering to be moresignificant than random (p < 0.05).Days After Release Release Method Mean Moran’s I Instances of SignificantClustering1Burst 0.10 1Swath 0.06 24Burst 0.23 3Swath −0.06 07Burst 0.19 1Swath 0.02 07 Day Total Burst 0.26 4Swath 0.08 24. DiscussionAerial release has quickly become widely accepted as the preferred method for dis-tributing sterile insects in area-wide pest management/eradication programs [30] and wasthe first choice for sterile codling moth release at the inception of the OKSIR program [31].The success of SIT programs is dependent upon delivering fit, sexually competitive sterileinsects to the field. The competitiveness and fitness of a sterile insect are influenced bynumerous biological and operational attributes, including but not limited to transport andrelease technologies [37]. Evaluating moth quality in open field trials is essential to under-standing how operational changes, such as using a UAS to release sterile insects, mightimprove or limit a program’s effectiveness [38]. As such, it was essential to (1) conductresearch on how UAS technology might be used to deliver sterile insects to an orchardand (2) compare the effectiveness of UAS technology for releasing sterile insects relative tocurrent release methods, namely ground release with ATV, to ensure that the hard-wonsuccesses of the OKSIR program are not compromised by altering release methods.4.1. Comparing Release MethodsRelease methods are typically divided into three types: static ground release, movingground release, and aerial release. Depending on the specifics of a given SIT program,different methods may be employed to release different life stages of sterile insects, usuallyeither pupae or adults [30]. This study is not the first comparison of aerial and movingground release methods, but it contributes to our understanding of aerial and movingground release for sterile, adult Lepidoptera. Other studies have described aerial releasemethods [23,32,39–43] and ground release methods [18,44] and have compared the releasesof different life stages concurrently with different release methods [45,46]. Static groundrelease, moving ground release, and aerial release have all been examined to release sterileadult codling moths in BC, Canada [28,31,47], though moving ground release has been themethod employed by the OKSIR program since 1994 [8]. In this study, we found that therecapture rates of sterile codling moths were 40–70% higher when moths were releasedfrom air compared to when released from the ground (25.3 vs. 17.4% and 18.6 vs. 10.8%recaptures from aerial and ground release methods, respectively, in 2016 and 2017).We used pheromone-baited traps and recapture rates to measure the effectivenessof different release methods. Pheromone-baited traps are frequently used to measuremoth quality. Measures from this method are related to other measures of moth qualityperformed in the field and laboratory [18,48,49]. It should be noted, however, that capturein pheromone-baited traps is not a direct measure of sexual competitiveness and thatthis method can be influenced by environmental [50], biological [19], and operationalfactors [51] that are independent of moth quality and competitiveness. In this experiment,we assumed that aerial and ground release methods would be similarly influenced byInsects 2021, 12, 159 12 of 18environmental, biological, and operational factors and that greater recapture rates areindicative of some underlying superiority of one method.We report marginally higher recapture rates from aerial release compared to groundrelease. One plausible explanation for this is that the survival of aerially released mothswas greater than that of ground-released moths. It is likely that some aerially releasedmoths were intercepted by the orchard canopy as they fell. There are accounts of greaterrecapture rates for codling moths when released into the canopy rather than on the orchardfloor [18,47], with these authors hypothesizing that increased predation and/or drowningoccurs on orchard floors. Our own observation is that both the codling moth and pinkbollworm (Pectinophora gossypiella (Saunders)) experience significant mortality when chilledmoths are released onto bare ground. This explanation is consistent with differences inmoth recapture on the first day of release, with similar recapture percentages betweenmethods in the following days. Presumably, moths that escaped predation on the groundon the first day of release would have migrated to the orchard canopy [52] and would haveexperienced similar mortality rates as those from aerial releases on subsequent days. Itshould be noted that daily recaptures are reported as the percentage of total recaptureson a given day. A lower percentage of recaptured moths on a given day necessitates acorresponding higher percentage of recapture on the following days as the total recapturesmust add up to 100%. Thus, while the percentage of recaptured moths tended to be higherlater in the week for ground releases, the total number of recaptured moths in the traps wassimilar to that from aerially released moths because the overall percentage of recapturedmoths tended to be higher when released from a UAS.There were a number of limitations with the experiment comparing aerial and groundrelease. The replication of release in this experiment was low (n = 4). Greater replicationwould have likely shown statistically significant differences between release methods.However, the marginal significance of the p-value (p = 0.0551), the large effect size (40–70%greater recapture), and the agreement in results from two separate experiments conductedin different years support our conclusion that the differences between aerial and groundreleases methods are relevant. This experiment could have been further improved hadthere been a simultaneous release of insects from both the air and ground. Insects destinedfor ground release spent 1–2 h more in cold storage than those released by a UAS. However,cold storage for 24 h was shown to have no effect on codling moth flight ability [20], so it isunlikely that this additional chilling had any significant effect on our results. Lastly, therelease routes travelled by the ATV and UAV were not the same, since by their very naturethese two vehicles must move differently. We attempted to overcome this by placing trapson an evenly spaced grid. Codling moth capture in pheromone traps is strongly influencedby proximity to a trap [53,54]. While a given release route may have been closer to a rowof traps in one method compared to another, that same release route would have beencorrespondingly further from an adjacent row of traps. Presumably, these confoundingdifferences would have been averaged out over the entire array of traps in the orchard,resulting in unbiased comparisons between methods.4.2. Describing the Pattern of Recaptured MothsExperiment 1 attempted to define some of the operational parameters for the use of aUAS by the OKSIR program, namely the altitude at which sterile codling moths should bereleased and how widely release routes should be spaced. These parameters have beenpreviously explored for other SIT applications [30,32,40]; however, the release technologyand release strategy must be tailored to the specific challenges of a given SIT programand the target pest [30]. The OKSIR program services a relatively small area compared toother SIT programs (ca. 3000 ha), with many small 1 ha orchards situated amongst a matrixof non-host crops and urban areas. The program releases chilled adults into pome fruitorchards and relies primarily on cultural tools to control codling moths outside commercialplantings [29]. As such, we endeavored to deploy a UAS in such a way as to meet theseInsects 2021, 12, 159 13 of 18needs, which necessarily deviated from the established methods of other, large-scale, aerialrelease programs.Previously reported altitudes of aerial release range from 30 to 750 m above ground [32,40].It is generally accepted that insects released from a greater altitude will be more likely todrift out of the targeted release zone [32]. The swath widths reported here were comparableto another report of low level (30 m) sterile insect release [40]. There was a trend for insectsto be more widely distributed when released at a greater altitude; however, the treatmentgroups were not significantly different. A lower release would likely produce a narrowerswath; however, 25 m was the lowest altitude at which the UAS could operate and safelyavoid obstacles such as powerlines, wind machines, and ornamental trees. Had we releasedinsects from a greater altitude, we might have seen a broader distribution of insects. Themeasurements of swath width were also limited by the size of the test plots. Had largertest plots been used, a wider swath may have been detected. In this experiment, plot sizewas sacrificed to increase replication.The recapture rate did not differ between release heights. One might expect thatinsects released from high altitudes would experience greater damage when they impactthe ground. Though the terminal velocity of C. pomonella is unknown, terminal velocities ofsmaller (Aedes aegypti = 2.5 m/s) and larger (Ceratitis capitata = 3–4 m/s) sterile insects havebeen reported [41,55]. At the range of fall speeds reported, a terminal velocity of fallingcodling moths would be reached in the first 10 m of descent from aerial release [46], andit is unlikely that release from any altitude above 10 m would have an increased effecton moth quality when they reached the ground. The results presented here support thisfinding. Bouyer et al. [41] found decreased recapture when insects were released at 100 mcompared to 50 m. The difference between Bouyer et al.’s [41] findings and those presentedhere may be because a smaller insect, with a lower terminal velocity, was released from agreater altitude and, consequently, may have experienced greater influence due to windand/or drifting outside the study area.Overall, the recapture rates in these experiments (11−25%) were many times higherthan those typically reported elsewhere (<1–5%) in examinations of aerial release meth-ods [23,40,41,46,56]. However, these findings are well within the ranges of recapturereported for sterile codling moths released under different operational and environmentalconditions [18]. The recapture rates in these trials are likely higher than other aerial releasetrials due to the small plot sizes and high trap densities.The pattern of sterile insect distribution under the release route was similar (Figure 3)to the pattern of exponential decay reported elsewhere [40]. However, we expected sterileinsects to be symmetrically distributed along the release route. More moths tended to becaught on the east side of the release route. This could have been due to environmentalor operational factors. As is recommended elsewhere, releases were made either with oragainst the prevailing wind [55]. Unfortunately, the precision with which we measured thewind direction was low, and it is possible that releases were not made completely parallelto the prevailing wind. Flights were approximately 1.5 min long, so it is also possiblethat wind direction changed during releases, or that wind direction differed with altitude.The asymmetrical release pattern could also have been affected by discrepancies betweendifferent GPS programs. Orchard maps and trap locations were processed in ArcGISwhile flight plans were programmed in ArduPilot. Disagreements in coordinate locationsbetween these systems could have created a discrepancy between the planned release routeover the center of the plot and the actual release route, skewing moth distribution to oneside of the plot by several meters.The pattern of recaptured insects, in both experiments, was influenced not only by howthe different release methods distributed the insects but also by how the insects dispersedthemselves after release. Codling moth dispersal, both within and between orchards, hasbeen studied extensively [51,53,54,57]. Sterile male codling moths are believed to travel200–300 m within an orchard on a given night [53] and may travel up to 10 km betweenorchards over their lifetime [28]. In Experiment 1, moths became less significantly clusteredInsects 2021, 12, 159 14 of 18as the experiment progressed (Table 1). This is consistent with the notion of moth dispersalwithin a plot. However, trap captures tended to be clustered in the center of the plots inExperiment 1. This may be for several reasons. Firstly, this experiment used “samplingwithout replacement”, meaning moths that were captured close to their point of releasedid not have the opportunity to disperse further. Secondly, the influence of calling femalesmoths, which were also released in the thousands, is often overlooked [51]. It is likely thatthe addition of thousands of calling females released into a relatively small area wouldinfluence how sterile male moths dispersed and interacted with synthetic pheromone-baited traps. By the end of the experiment, the number of moths at the edges of the plotsincreased. However, moths were never evenly distributed in the plots, but were randomlydistributed at best according to indices of clustering. This may have consequences for thescramble competition between sterile and wild insects for wild females, and the overalleffectiveness of the SIT.4.3. Release StrategyThe release strategy used, burst vs. swath, had lasting effects on the distributionof recaptured moths. While data reported here and elsewhere [53] suggest that sterilecodling moths disperse themselves throughout an orchard as time passes, it has also beenshown that the sexual competitiveness of codling moths changes with age [58]. If thedistribution of wild codling moth population is known, a targeted (burst) release mayproduce the best sterile/wild ratio of the youngest, most sexually competitive moths in agiven area. However, if the distribution of the wild population is unknown, then a uniformrelease strategy (swath) may produce better sterile/wild ratios. It should be re-iteratedthat pheromone-baited trap captures are not a measure of direct sexual competitiveness,and that these assertions warrant further investigation. Furthermore, the number of trapsin these plots was relatively small (15–25) and a smaller number of neighbors (k = 4) hadto be used in this analysis of spatial autocorrelation. The effectiveness of Moran’s I indescribing spatial autocorrelation for small sample sizes is limited, and these results shouldbe interpreted with caution [59]. Burst releases may be suitable in treating orchards lessthan 2–3 ha in size or for targeting known problem areas; however, further research iswarranted on this topic before making operational changes.4.4. Future ResearchWhile these experiments show that the Hermes UAS is at least as effective, if not40–70% more effective, than current ground release methods employed by the OKSIRprogram, other dimensions of UAS use may need to be investigated prior to the adoptionof this technology, which may vary between jurisdictions. Of particular interest would bean analysis comparing the operating costs of ATVs vs. UASs. A study of the opinions ofvarious user groups that would be affected by the use of this technology would also bevaluable before the widespread use of UAS technology is implemented. A recent studyfound peri-urban householders in Hastings in New Zealand were generally supportive ofsterile insect release and UAS if cameras were not deployed [60].Other interesting avenues of research could investigate how this technology might beused to increase the competitiveness of sterile insect release through the optimization ofrelease time and temperature. Further investigations of how wind and rain might affectrelease from UAS are warranted. Finally, further characterization of any moth qualityimprovement might also be warranted, especially if there are increased costs associatedwith aerial release.5. ConclusionsThese experiments demonstrated that UAS can be used to deliver sterile codling mothsto an orchard. When moths were released from 25 m above ground, 80% of the recapturedmoths were collected in a 50 m wide swath beneath the release route. When moths werereleased into the same orchard from a greater altitude (50 m), the moths were more broadlyInsects 2021, 12, 159 15 of 18distributed, with 67% of the recaptured moths collected in a 50 m-wide swath beneaththe release route. Some evidence suggests that moths released from greater altitudes driftbeyond the target area more compared to moths released at a lower altitude. Recapturerates were 40–70% higher when moths were released from the air as opposed to the ground.The differences in moth recaptures between aerial and ground releases were greatest withinthe first day after release, suggesting that moths from the air were intercepted by treecanopies and experienced lower mortality than those released on the ground.Supplementary Materials: The following are available online at https://www.mdpi.com/2075-4450/12/2/159/s1. Figure S1: Photograph of codling moths (Cydia pomonella) marked with fluorescentpowder and illuminated with ultraviolet light. Both green and orange fluorescent powder can beseen in this photograph. Figure S2: Photograph showing a modified all-terrain vehicle (ATV) usedto release sterile codling moths (Cydia pomonella) by the Okanagan–Kootenay Sterile Insect ReleaseProgram. Yellow arrow indicates location and direction of moth release. Figures S3–S6: Aerial viewand schema of each replicate for experiments 2 and 3. The image shows satellite imagery of eachsmall, medium, and large orchard block (outlined in white) in each replicate. The red and bluelines indicate the routes travelled by the ATV and UAS, respectively. The shaded circles indicatethe location of pheromone traps and the percentage of moths recaptured after 7 days in each trap.The size of the circle corresponds with the percentage of moths captured. Pink circles denote trapswhere more moths were recaptured when released by the ATV and the light blue circles indicate trapswhere more moths were recaptured when released from UAS. Gray circles indicate the percentageof moths recaptured that were equal between the two release methods. Traps are spaced as close toa 50 × 50 m as possible, given each block unique dimensions. Figures S7–S10. Aerial view of theorchard blocks used in experiment 3. The shaded circles indicate the location of pheromone trapsand the total numbers of moths recaptured from each UAS release strategy. The size of the circle isproportional to the total number of moths recaptured. Pink circles denote traps where more mothswere recaptured when released in a burst and light blue circles indicate traps where more moths wererecaptured when released in a swath. Gray circles indicate that the total number of moths recapturedwere equal between the two release strategies. Traps are spaced as close to a 50 × 50 m as possible,given each block unique dimensions.Author Contributions: Conceptualization, all; methodology, D.M.S., D.C.K., E.D.E., and R.M.H.;formal analysis, E.D.E.; investigation, E.D.E., R.M.H., and D.C.K.; resources, M.R.T., N.M.-G., andD.M.S.; data curation, R.M.H. and E.D.E.; writing—original draft preparation, E.D.E.; writing—review and editing, all; visualization, E.D.E.; supervision, D.M.S.; project administration, M.R.T.;funding acquisition, M.R.T., D.M.S., and N.M.-G. All authors have read and agreed to the publishedversion of the manuscript.Funding: This research was partially funded by the International Atomic Energy Agency CooperativeResearch Program on “Improved Field Performance of Sterile Male Lepidoptera to Ensure Success inSIT Programs” and supported in New Zealand through Better Border Biosecurity (www.b3nz.org).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.Data Availability Statement: “Experiment 1 Raw Data.xlsx” includes raw data for experiment 1,“Experiment 2 Raw Data. xlsx” and “2016 UAS and ATV Comparison Raw Data.xlsx” include rawdata for experiments 2 and 3. All are available online at https://www.mdpi.com/2075-4450/12/2/159/s1.Acknowledgments: Thanks to Tom Sulivan, Christie Krompetz, Scott Arthur, Caitlin Miller, ElysiaZimmer, and others from M3 and OKSIR for their help with this work. Thanks to Michelle Cookand Shelby Legebokoff for help reviewing this manuscript, contacting growers, and helping to keepall involved safe. Thanks to Matthieu Bourbonnais for sharing his expertise in spatial statistics.Thanks to Okanagan and Similkameen apple producers for supporting the OKSIR program andvolunteering their orchards for this research. A special thank you to Marc Vreysen and colleaguesfrom the International Atomic Energy Agency Cooperative Research Program for their supportand input.Insects 2021, 12, 159 16 of 18Conflicts of Interest: N.M.-G. and D.M.C. declare a commercial interest in positive results but werenot involved in the analysis of data. The other authors declare no conflicts of interest.References1. Food and Agriculture Organization of the United Nations, FAOSTAT. Available online: http://www.fao.org/faostat/en/#rankings/countries_by_commodity (accessed on 13 May 2020).2. Vreysen, M.J.B.; Carpenter, J.E.; Marec, F. 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Operational Parameters for the Aerial Release of Sterile Codling Moths Using an Uncrewed Aircraft System
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