size and sex of cricket prey predict capture by a sphecid wasp

Ecological Entomology (2014), 39, 195–202 DOI: 10.1111/een.12083

Size and sex of cricket prey predict captureby a sphecid waspK Y L A E R C I T Department of Ecology and Evolutionary Biology, University of Toronto at Mississauga, Mississauga,

Canada

Abstract. 1. Female-biased predation is rare in nature; however, sphecid wasps oftentake more female than male prey, including Isodontia mexicana , which hunt Oecanthustree crickets.

2. This study tests the hypothesis that wasps prefer females because they are largerthan males. This predicts a female sex bias only for sexually size-dimorphic prey.

3. Prey from artificial I. mexicana nest holes in Central Ontario were comparedwith surviving crickets sampled from the hunted population. Sex ratios of prey andsurvivors were examined and compared with the occurrence of female-biased sexualsize dimorphism. Logistic regression was used to determine whether body size, sex,species, and life stage of crickets predicted capture by wasps.

4. As predicted, wasps took a disproportionate number of adult females only ofsexually size-dimorphic prey Oecanthus nigricornis . No sex bias was found in adultprey of Oecanthus quadripunctatus or in nymphal prey of either species. However,female-biased sexual size dimorphism did not necessitate female-biased predation:even though female O. nigricornis nymphs were larger than males, female nymphswere not hunted more often. Body size was a significant predictor of predation, butthis relationship was non-linear. There was also evidence of an interaction among sex,life stage, and body size of prey in relation to predation risk.

5. These results support the size-preference hypothesis, but do not rule out alternativehypotheses. For example, sex differences in behaviour or life-history traits that developin adulthood may also contribute to differences in predation risk. Predation thatconsistently targets large adult females of a population may result in evolutionarychanges in the behaviour or life history of the prey species.

Key words. Isodontia mexicana , Oecanthus nigricornis , prey choice, sex-biasedpredation.

Introduction

The observed diet of a predator is a function of several fac-tors, including both intrinsic factors (nutritional requirements,hunting abilities, size constraints of the predator) and extrinsicfactors (conspicuousness, availability, and vulnerability ofprey). These factors often cause predators to take one sex ofprey more often than the other. Males are reported to fall vic-tim more often (Boukal et al., 2008), and this is probably dueto risks taken while attracting and securing mates (Gwynne &O’Neill, 1980; Burk, 1982; Zuk & Kolluru, 1998). Despite the

Correspondence: Kyla Ercit, Department of Biology, University ofToronto at Mississauga, 3359 Mississauga Road North, Mississauga,Ontario L5L 1C6, Canada. E-mail: [email protected]

general prevalence of male-biased predation, female prey maybe taken more often by some predators for several reasons.Females may be subject to higher predation rates because ofthe physical handicap of carrying eggs (Hairston et al., 1983),or because of risks inherent in searching for stationary mates(Sakaluk & Belwood, 1984; Heller, 1992). Females in polyan-drous or sex-role-reversed species may also take on riskyconspicuous behaviours usually assumed by males when com-peting for mates (Gwynne & Bussiere, 2002). Also, femalesmay be more valuable to the predator than male prey, espe-cially among insects, due to their generally larger body massand the nutritional contents of their egg stores (Lin, 1979).

Apoid wasps of the families Sphecidae and Crabronidaehunt and provision arthropods for their offspring, and severalspecies have been recorded hunting more female than male

© 2013 The Royal Entomological Society 195

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prey (Lin, 1979; Gwynne & Dodson, 1983; Volkova et al.,1999; O’Neill & O’Neill, 2007; Kurczewski et al., 2010).This prey bias may be due to the larger size of females;larger prey are probably at a greater risk of capture, dueto either a preference by the wasp for large prey or adifference in conspicuousness between small and large prey.This hypothesis was tested using Isodontia mexicana Saussure(Hymenoptera: Sphecidae), a sphecid wasp that consistentlyprovisions its nests with more female tree cricket (Oecanthusspp.) (Orthoptera: Gryllidae) prey than male prey (O’Neill& O’Neill, 2003, 2009) The hypothesis that female bias inprey is due to a size preference predicts that female bias inwasp prey should occur within species and life stages of preyshowing female-biased size dimorphism. There should also bea significant positive relationship between body size of preyand the probability of capture by a wasp, regardless of sex,age, or species of the prey.

Materials and methods

Study organisms

Isodontia mexicana is a common solitary wasp, foundthroughout southern Canada and the U.S.A., and is a predatorof certain small ensiferan Orthoptera (Iwata, 1971; Bohart& Menke, 1976). Females sting and paralyse their prey andcarry them back to their nest to feed their offspring (Iwata,1971). Isodontia mexicana nest in naturally formed tubes, suchas hollow stems, rolled leaves, or, frequently, the abandonedexcavations of carpenter bees and other insects in trees(Krombein, 1967). They also use artificial trap nests consistingof holes bored in wood (Krombein, 1967). A female waspsubdivides her nest into separate cells, and lays one or twoeggs in each cell (Medler, 1965). She then provisions betweenseven and 20 prey items per offspring in the cell (Medler,1965). Once the wasp has collected sufficient food for theegg, she seals off the cell with a partition of compacted grass(Medler, 1965). Isodontia mexicana nests are identified by thepresence of a terminal grass plug, the blades of which mayprotrude up to 5 cm from the end of the bore (Medler, 1965).

Nymphal and adult Oecanthus tree crickets (Gryllidae)are common prey of I. mexicana (O’Neill, 2001). Female-biased sexual size dimorphism has been observed in somebut not all Oecanthus species. In this study, I focus onO. nigricornis F. Walker, and O. quadripunctatusBeutenmuller, the two prey species that were most fre-quently observed by me in I. mexicana nests. These speciesare visually similar and share many similar habitats andlife-history and morphological traits. The total body lengthof O. nigricornis is approximately 12–14 mm, and that ofO. quadripunctatus is 11.5–14 mm (Blatchley, 1920). Bothspecies are typically found in open meadows with Solidagospp., Rubus spp., and Daucus spp. (Fulton, 1915). Matingoccurs on these plants, and females use stems as ovipositionsites (Fulton, 1915). During courtship, male Oecanthus aremostly stationary, and attract females with a conspicuous call-ing song (Walker, 1957; Bell, 1980). Thus, in pair formation,females are the more mobile sex.

Study site and equipment

The nesting behaviour of I. mexicana was monitored in2009, 2010, and 2012 at the University of Toronto KofflerScientific Reserve (KSR) in King City, Ontario. Their preywere sampled using artificial trap nests. Each set of trap nestsconsisted of a 1.7 × 7.9 × 15.2 cm pine block with four to sixlong tunnels bored with a router, 150 mm in length and visiblethrough a clear acrylic lid (see Hallett, 2001 for details). Tunneldiameters were 6.4, 8, or 9.6 mm, a range of sizes that thewasps have been observed using (Medler, 1965). Nest blockswere grouped in five stacks of seven, within boxes coveredwith wooden lids and roofing shingles, and placed on woodenplatforms 1 m off the ground.

Trap nest boxes were seeded with 10 blocks (with flexibleplastic film lids instead of hard acrylic) containing approxi-mately 20 I. mexicana prepupae (overwintering larvae) fromthe previous summer, as I. mexicana have been observed toreturn to provision in the area of their natal nest (P. Hallett,pers. comm.). Each seeded block replaced a randomly chosen,similarly sized empty block. Trap nest boxes were placed inpartial sun at the margin of large meadows and forest. Boxescontaining seeded blocks were set out in April. As nest tun-nels became occupied with I. mexicana and other hole-nestingHymenoptera, in August, a second box of empty nest blockswas added on top of the first box.

Sampling methods

To identify whether body size, sex, species, and age ofcrickets were related to prey capture by I. mexicana , Icompared prey from nests to surviving crickets collectedfrom the surrounding meadow. These samples were takenapproximately weekly. Prey samples were taken from the mostrecently provisioned cell of I. mexicana nest tunnels, and theentire contents of each recently provisioned cell were taken.Recently provisioned cells were identified as those that: (i)were new since the previous week’s survey; (ii) had most preywithin a cell still alive; and (iii) had a wasp egg or small larvapresent. Prey were removed from wasp nests with forceps.Samples of surviving crickets (‘survivor samples’) were takenon the same day as prey samples or the previous day. Toincrease the likelihood of sampling the same population inwhich the wasps were hunting, survivors were caught bysweep-net sampling from appropriate prey habitat within 300 mof the wasp nest. Sweep-net samples were taken by at leasttwo different people on each sampling day to reduce bias. In aseparate study, I tested possible size and sex bias in collectingsurvivors via net-sweeping versus malaise traps (DocumentS1), and there was no difference in size or sex ratio of cricketscollected between the two sampling methods. In 2012, toconfirm that survivor samples were indeed taken from the samepopulation the wasps were harvesting, I caught, marked withfluorescent dust (Luminous Powder Kit, BioQuip Products,Inc., Rancho Dominguez, California), and released 63 maleand 50 female tree crickets within our sampling area. Tenintact marked crickets were later found as prey in wasp trapnests. There may have been more marked crickets provisioned

© 2013 The Royal Entomological Society, Ecological Entomology, 39, 195–202

Sex-biased predation by wasps 197

in nests, but some wasp larvae ate provisioned crickets veryquickly, and often only various fluorescent-marked cricketremnants were left in a nest cell. All samples were housedin small plastic containers and then fixed in 95% ethanol.

Measurement

Size measurements of prey and survivor samples were takenfrom digital photos (from an AmScope 5 MP microscopedigital camera mounted to a Wild Heerbrugg M5A dissectingmicroscope) using imagej software. Photos were taken at 12times magnification using AmScope mt software. Prey andsurvivor samples were not weighed due to the expected massloss in paralysed, stored prey (e.g. due to dehydration anddefecation). Instead, measurements of a sclerotised body partwere used that best predicted wet body mass in both sexes. Todetermine which body part best predicted mass, 20 males and27 females were collected from the wild, weighed and thenkilled by freezing. Head width, pronotum length, femur lengthand width were measured, and their correlation to body masswas calculated. Pronotum length was most strongly correlatedwith mass (r45 = 0.66, P < 0.01); however, the other bodymetrics were also strongly correlated with mass (head width,r45 = 0.52, P < 0.01; femur length, r45 = 0.55, P < 0.01; femurwidth, r45 = 0.64, P < 0.01).

Statistics

All statistical tests were conducted with r, version 2.14.0(R Development Core Team, 2011). To test whether waspshunt both sexes of prey in proportion to their occurrence inthe surviving populations, sex ratios of prey and survivorswere compared using G-tests (Sokal & Rohlf, 1981). In caseswhere sample sizes were too small for a G-test, Fisher’sexact test was used. I compared sex ratios between prey andsurvivors among adults and fourth- and fifth-instar nymphsseparately, and among I. mexicana’s two main prey species,

O. nigricornis and O. quadripunctatus , separately. I tested forsexual size dimorphism within adults and nymphs and withinboth Oecanthus species, using ancova with sampling date asa covariate.

Logistic regression of general linear models was used to testthe effect of sex, body size, species, and life stage (adult ornymph) of crickets on the binary response variable of whetheror not a cricket would be caught by I. mexicana . All pairwiseinteractions among body size, sex, species, and life stage werealso included in the model, as well as a quadratic term to testfor a non-linear relationship between body size and predation.Sampling date was included as a covariate, and year wasincluded as a blocking factor in the model. If the model showedthat year was a significant factor, years were analysed usingseparate models. Non-significant terms were removed frommodels using backward stepwise model selection, using the rfunction ‘step’. The best models were identified as those withthe lowest values of Akaike’s information criterion (AIC). Iftwo models had similar values of AIC (i.e. difference < 2),then models were eliminated if they did not fit the data well(using Hosmer–Lemeshow goodness-of fit test, r packagerms; Harrell, 2012), or if they did not explain more deviancethan the null model. If more than one model still remained,then the model with the highest accuracy (highest proportionof correctly sorted observations determined by jackknifing)was selected as the best. Regression coefficients (β) wereobtained from the logistic regressions, as well as the posteriorprobabilities of capture of each cricket.

Results

Over the three years of sampling, 425 Oecanthus prey werecollected from 53 I. mexicana nests, and 364 survivors werecollected from the hunted population. It was not possibleto count exactly how many different wasps provisionedthese nests, as wasps that were marked with paint or com-mercial bee-tags tended to abandon their nest. However,

Table 1. Sex ratio and sexual size dimorphism of Oecanthus tree cricket prey taken by predaceous wasp Isodontia mexicana . Items in boldindicate a statistically significant result.

Sex ratios (M : F) Pronotum length (mm)

Species Age class Survivors PreyIs predationfemale-biased? M male M female

Is size dimorphismfemale-biased?

O. nigricornis Fourth-instar nymph 4 : 13 14 : 20 No 1.55 1.64 Marginally(G = 1.60, P = 0.21) (F = 3.82, P = 0.06)

Fifth-instar nymph 28 : 46 36 : 47 No 2.01 2.07 Yes(G = 0.50, P = 0.48) (F = 7.00, P = 0.01)

Adult 72 : 87 69 : 151 Yes 2.29 2.43 Yes(G = 7.63, P < 0.01) (F = 65.27, P < 0.01)

O. quadripunctatus Fourth-instar nymph 4 : 6 3 : 3 No 1.50 1.53 No(Fisher’s exact, P = 0.55) (F = 0.08, P = 0.78)

Fifth-instar nymph 18 : 14 17 : 20 No 1.84 1.89 No(G = 0.73, P = 0.39) (F = 2.19, P = 0.14)

Adult 31 : 41 17 : 28 No 2.10 2.14 No(G = 0.32, P = 0.57) (F = 2.65, P = 0.11)


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from observing the wasps’ activity, behaviour, and nestingpreferences, I estimate that I sampled the prey of between19 and 31 different wasps. I detected that predation by I.mexicana was significantly female-biased only among adultO. nigricornis prey (ratio of male : female survivors = 72 : 87,ratio of male : female prey = 69 : 151 , G = 7.63, P < 0.01).There was no significant sex-bias in prey within nymphs orwithin any age of O. quadripunctatus samples (Table 1).

There was also significant female-biased sexual sizedimorphism in O. nigricornis but not in any age classof O. quadripunctatus (Table 1). Female-biased sexualsize dimorphism is significant in adult and fifth-instar O.nigricornis (ancova F 2,371 = 65.27, P < 0.01; F 2,153 = 7.00,P = 0.01, respectively), and marginally significant in fourth-instar nymphs (ancova F 2,47 = 3.82, P = 0.06).

When testing which variables affect risk of capture bywasps, the results were significantly different between years,so 2009, 2010, and 2012 data were analysed separately(Table 2). Every year, body size significantly predicted whetherO. nigricornis would be caught by I. mexicana (Table 2).However, the relationship between body size and probabilityof capture (calculated via logistic regression) varied betweenthe years. In 2009, there was a significant non-linear concaverelationship between body size and probability of capture(β = 2.74, P = 0.02; Fig. 1a), but there was also a significanteffect of species [O. nigricornis were more likely to be caughtthan O. quadripunctatus (β = 0.63, P = 0.02) (Fig. 1b)] andlife stage (nymph or adult) (β =−1.01, P = 0.02). In 2009,nymphs had a consistently high chance of being captured,regardless of body size, but adults’ probability of capture

Table 2. Multiple logistic regression of variables affecting predation on Oecanthus tree crickets by wasp Isodontia mexicana .

Model Model evaluation

Year Term β P χ2 P of χ2 Accuracy GOF

All Constant −4.68 0.01 50.8 < 0.01 0.61 0.15Pronotum length 2.97 < 0.01Sex −2.23 0.05Life stage −1.31 < 0.01Species 5.69 < 0.01Pronotum length × sex 1.15 0.03Life stage × species 1.46 < 0.01Pronotum length × species −2.95 < 0.01Year −0.29 < 0.01Sampling date −0.02 0.01

2009 Constant 11.49 0.02 43.3 < 0.01 0.63 0.75Pronotum length −8.18 0.07Life stage −1.01 0.02Species 0.63 0.02Pronotum length2 2.74 0.02Sampling date −0.02 0.05

2010 Constant −17.52 0.01 19.7 0.01 0.63 0.69Pronotum length 16.01 0.01Sex −4.76 0.07Species 5.54 0.08Pronotum length2 −3.64 0.02Pronotum length × sex 2.52 0.04Pronotum length × species −2.37 0.12Sex × species −0.72 0.35

2012 Constant 31.40 0.42 35.8 < 0.01 0.64 0.89Pronotum length −47.55 0.01Sex 14.77 0.76Life stage 12.17 0.73Species 24.17 0.50Pronotum length2 14.42 < 0.01Sex × life stage 1.19 0.07Sex × species −15.34 0.75Life stage × species 13.04 0.70Pronotum length × life stage −11.56 0.02Pronotum length × species −9.15 0.20Sampling date −0.02 0.24

A quadratic term was added to the model to test for a non-linear relationship between body size of prey and their risk of capture. Terms in boldindicate that a variable was significant. The χ2 values represent the difference in residual deviance explained by the selected model versus the nullmodel. GOF represents the P -values of the results of Hosmer–Lemeshow goodness-of-fit tests. Non-significant values indicate that the model fitsthe data well. Accuracy represents the proportion of observations correctly classified by the model, determined by jackknifing.



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Fig. 1. Relationship between pronotum length (a proxy of total body size) of Oecanthus tree crickets and the probability of capture by a predaceouswasp, Isodontia mexicana wasps in 2009, 2010, and 2012 (a, d, and g, respectively). (b, e, h) Illustration of an interaction between body size andsex in relation to predation risk in 2009, 2010, and 2012, respectively; (c, f) illustration of an interaction between body size and life stage (adultor nymph) in relation to predation risk. (i) Illustration of a significant interaction between life stage and sex in relation to predation risk in 2012.

increased steeply as their body size increased (Fig. 1c). Sexdid not significantly predict capture in 2009. In 2010, therelationship between body size and probability of capture wassignificant and convex (β =−3.64, P = 0.02; Fig. 1d). Therewas also a significant interaction effect between body sizeand sex and its relationship to predation: above a pronotumlength of approximately 2.1 mm, females were more likely tobe caught, but below that size, males were more likely tobe caught (β = 2.52, P = 0.04; Fig. 1f). In 2010, life stagewas not a significant enough variable to be included in themodel, and species of prey did not have a significant effecton probability of capture (P = 0.08; Fig. 1e). In 2012, therelationship between body size and probability of capture wassignificant and concave (β = 14.42, P < 0.01; Fig. 1g). In thisyear, there was also a significant interaction effect of body size

and life stage (β = −11.56, P = 0.02; Fig. 1i) on probabilityof capture, but species did not have a significant effect onprobability of capture (P = 0.50; Fig. 1h).

All logistic regression results and evaluation of logisticregression models are summarized in Table 2.

Given that sex-biased predation only occurred among adultO. nigricornis , I also analysed this subset of the data in sep-arate logistic regressions to identify the relative importanceof sex and body size in a case where female-biased preda-tion occurred. This model included sex and body size, aninteraction term for sex and body size, a quadratic term totest a non-linear relationship between body size and preda-tion, and sampling date and year as covariates. Again, non-significant terms were removed and the best model was chosen,as earlier. Among adult O. nigricornis , sex was a significant


200 Kyla Ercit

predictor of capture by wasps: females were more likely to becaught by wasps, regardless of body size (β = 0.53, P = 0.03;Fig. 2) and neither the linear nor the quadratic body sizeterms were significant predictors of capture (linear, β =−5.50,P = 0.73; quadratic, β = 1.35, P = 0.69). These logistic regres-sion results and evaluation of the model are summarized inTable 3.

Discussion

Among adults and nymphs of two species of Oecanthus prey,I. mexicana take females disproportionately only in adult O.nigricornis . Adult O. nigricornis showed the largest degreeof female-biased sexual size dimorphism. Body size wasconsistently a significant predictor of the probability of capturein all three sampling years. These results offer some supportto the hypothesis that female-biased predation by I. mexicanais a result of wasps hunting larger prey.

However, there is also evidence against the above hypothe-sis. I found that female-biased sexual size dimorphism in preydid not necessitate female-biased predation by wasps. Signif-icant female-biased size dimorphism was observed in adultO. nigricornis , as well as within fifth instar nymphs, but sig-nificant sex-biased predation was not observed in these groups.Also, the relationship between body size and probability ofcapture was not linear, and body size was also not the only pre-dictor of whether or not a cricket would be caught by wasps inall three sampling years. Most notably, among adult O. nigri-cornis , the only demographic in which female-biased predationwas observed, body size was not a significant predictor of cap-ture by wasps.

Sex was a significant predictor of capture among adultO. nigricornis , even when sexual size dimorphism was con-trolled for. At the same body size, females were still at higherrisk of capture than males (Fig. 2). In 2010, there was alsoan interaction between sex and size and probability of cap-ture in Oecanthus . Large females experienced higher predation

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Fig. 2. Relationship between pronotum length (a proxy of total bodysize) of adult Oecanthus nigricornis tree crickets and the probabilityof capture by Isodontia mexicana wasps.

Table 3. Multiple logistic regression of variables affecting predationon adult Oecanthus nigricornis tree crickets by wasp Isodontiamexicana .

Model Model evaluation

Term β P χ2 P of χ2 Accuracy GOF

Constant 6.74 0.72 15.5 0.01 0.61 0.74Pronotum length −5.50 0.73Sex 0.53 0.03Pronotum length2 1.35 0.69Sampling date −0.03 0.02Year −0.26 0.07

Terms in bold indicate that a variable was significant. The χ2 valuesrepresent the difference in residual deviance explained by the selectedmodel versus the null model. GOF represents the P -values of theresults of Hosmer–Lemeshow goodness-of-fit tests. Non-significantvalues indicate that the model fits the data well. Accuracy representsthe proportion of observations correctly classified by the model,determined by jackknifing.

risk than large males. There are several possible explanationsfor higher predation risk in adult females. Mate-searching byadult female O. nigricornis may make them more vulnerable toI. mexicana predation. In Oecanthus tree crickets, females arethe mobile sex and, when sexually receptive, must locate acalling male for mating to occur (Fulton, 1915). In Orthopterasuch as O. nigricornis , where females receive nutritious nuptialgifts, and benefit directly from mating (Brown, 1997), femalesare more likely to be the mate-searching sex (McCartney et al.,2012) and more likely to assume the associated predation risks(Heller, 1992). In O. nigricornis , large females respond tomale calls more quickly (Brown, 2008) and may thus attractmore predator attention. Oecanthus mating behaviour typi-cally starts in the early afternoon, continuing into the night(Fulton, 1915), and as I. mexicana hunt from late morninguntil shortly before sundown (K. Ercit, pers. obs.), wasp andcricket activities overlap. Adult females may also experiencegreater predation because of the physical handicap of carry-ing mature eggs. When eggs develop inside the ovary, thefemale’s abdomen becomes enlarged. The added weight andbulk may make it more difficult to escape a predator, and theswelling of the abdomen may make a fertile female more con-spicuous to wasps. Similarly, Hairston et al. (1983) found thatfemale copepods suffered higher predation by fish because ofthe conspicuous eggs carried by them.

The difference between years in the shape of the relationshipbetween body size and probability of capture is also interesting.The relationship between body size and capture is convex in2010, but concave in 2009 and 2012. If there is an optimumprey size that maximises caloric benefit for wasps whileminimising effort and time invested in capture and transportof prey, one would expect that the relationship would beconvex every year. As the size and shape of the wasp arerelated to the mass of prey that can be handled efficiently(Marden, 1987), wasps from whose nest prey were taken mayhave been smaller in 2010 and only able to carry smallerprey back to their nests. However, this relationship betweenpredator size and prey size is not always present in natural



conditions. In a similar study to this one, Grant (2006) foundno significant relationship between body mass of cicada killerwasps (Sphecius speciosus Drury, Hymenoptera: Sphecidae)and body mass of their cicada prey. Alternatively, the convexshape of the function I observed in 2010 may have beencaused by a mid-season shift in prey species: wasps may haveignored small Oecanthus prey early in the season to hunt largerorthoperans. I did observe juvenile Conocephalus katydidsmore often in wasp nests during this year.

The convex shape of relationships between body size andcapture in 2009 and 2012 may be explained by the fact thatwasps require a larger number of smaller prey to feed onelarva. The shape of the concave functions may be partiallydetermined by the fact that the functions integrate informationacross multiple life-history stages and sizes of prey. Thus, theymay represent a seasonal shift in foraging strategies by wasps,where wasps indiscriminately hunt many abundant small preyearly in the year, and as larger prey become available but aremore scarce, wasps switch to prey with a higher caloric benefitfor the amount energy expended in search.

The incidence of size-biased predation and female-biasedpredation in this system is relevant for several reasons. First,since female egg production is a primary determinant of pop-ulation growth, female-biased mortality is more likely thanmale-biased mortality to produce population collapse (Boukalet al., 2008). Secondly, predation by I. mexicana on largeradult members of the population is also expected to imposeviability selection favouring smaller adult tree crickets, and inpopulations under intense I. mexicana predation, such selectionpressures may lead to a significant phenotypic change in preybody size (although the evolutionary response would dependon the heritability of body size). However, as the relationshipbetween body size and predation risk is non-linear, and thenature of this relationship changes from year to year, it wouldbe difficult to predict the direction of this change. Other fac-tors, such as sexual selection, may also mitigate the effects ofsize-biased predation, since, in O. nigricornis , the largest indi-viduals of both sexes tend to mate most often (Brown, 2008).

Acknowledgements

I would like to thank Darryl Gwynne for his insight intowasp and cricket biology, help with experimental design, andconstructive writing criticism. I am grateful to Peter Hallettfor sharing his expertise with trap-nesting Hymenoptera andfor generously providing access to his sites and trap nests.Thank you to Chris Darling, Rob Baker, Kevin Judge, JerryBrunner, and Aaron Allen for their helpful discussions onexperimental methods and statistics. Edyta Piascik, KendraLahut, and Andrew Martinez-Novoa provided invaluable fieldand laboratory assistance. Kevin O’Neill and two anonymousreviewers provided insightful criticism and commentary onthe manuscript. Thank you also to Arthur Weis and the staffof Koffler Scientific Reserve at Joker’s Hill, and to the rareCharitable Research Reserve for access to study sites. Fundingfor this research came from an NSERC Discovery Grantto D.G.

Supporting Information

Additional Supporting Information may be found in the onlineversion of this article under the DOI reference:10.1111/een.12083

File S1. Testing possible bias in collection method.

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Accepted 12 October 2013First published online 18 November 2013


size and sex of cricket prey predict capture by a sphecid wasp

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