Host choice and offspring sex allocation in a solitary parasitic wasp

Summary Females of the parasitic wasp Antrocephalus pandens can detect differences in the quality of their hosts (pupae of Corcyra cephalonica, a stored-product moth) and allocate offspring of either sex accordingly. Larger and younger hosts are accepted more often in both dead and live hosts; more female offspring emerge from the perceived better hosts, while more males emerge from the smaller, older ones. These patterns are consistent with a sex allocation strategy by the mother, since females from a given size host tend to be larger than males and larger females produce more eggs. However, when wasps lay their eggs in groups of hosts of different size and age rather than encountering them one at a time, no difference in number or sex ratio of offspring is detected between groups. This result and evidence from the change in offspring sex ratio with female age and with numbers of females foraging on a group of hosts are interpreted and discussed in the context of sex allocation (Charnov 1979) and local mate competition (LMC, Hamilton 1967) theories.     

Adaptive responses to local mate competition by the parasitoid,Telenomus remus

Summary The parasitic wasp, Telenomus remus, lays her eggs in diserete patches of moth eggs, where her offspring develop and mate before dispersal, satisfying conditions for local mate competition (LMC). In the presence of other ovipositing females, wasps lay a higher sex ratio (proportion males), as predicted by LMC theory, and achieve this by a combination of two mechanisms, (1) avoidance of superparasitism and a sequence of sex allocation initially biased towards males and (2) a direct increase in sex ratio in the presence of other wasps, sex ratio increases with the proportion of previously parasitized hosts, as predicted by LMC theory. In both cases, chemical traces left by foraging wasps are indicated as the stimuli causing wasps to increase the proportion of males allocated to hosts.     

Learning and natal host influence host preference,handling time and sex allocation behaviour in a pupal parasitoid

The host choice and sex allocation decisions of a foraging female parasitoid will have an enormous influence on the life-history characteristics of her offspring. The pteromalid Pachycrepoideus vindemiae is a generalist idiobiont pupal parasitoid of many species of cyclorrhaphous Diptera. Wasps reared in Musca domestica were larger, had higher attack rates and greater male mating success than those reared in Drosophila melanogaster. In no-choice situations, na?ve female P. vindemiae took significantly less time to accept hosts conspecific with their natal host. Parasitoids that emerged from M. domestica pupae spent similar amounts of time ovipositing in both D. melanogaster and M. domestica. Those parasitoids that had emerged from D. melanogaster spent significantly longer attacking M. domestica pupae. The host choice behaviour of female P. vindemiae was influenced by an interaction between natal host and experience. Female P. vindemiae reared in M. domestica only showed a preference among hosts when allowed to gain experience attacking M. domestica, preferentially attacking that species. Similarly, female parasitoids reared on D. melanogaster only showed a preference among hosts when allowed to gain experience attacking D. melanogaster, again preferentially attacking that species. Wasp natal host also influenced sex allocation behaviour. While wasps from both hosts oviposited more females in the larger host, M. domestica, wasps that emerged from M. domestica had significantly more male-biased offspring sex ratios. These results indicate the importance of learning and natal host size in determining P. vindemiae attack rates, mating success, host preference and sex allocation behaviour, all critical components of parasitoid fitness. Electronic Publication     

Local mate competition,sex ratio and clutch size in bethylid wasps

Summary Parasitoid wasps of the family Bethylidae lay groups of eggs on the larvae of beetles and moths. The larvae develop together and pupate in the vicinity of the host. On hatching, the majority of copulations are between siblings leading to a high degree of local mate competition. Sex ratio theory predicts that as the number of individuals developing on a host increases, the population sex ratio will become progressively more female biassed. A comparative study of sex ratios across the family supports this prediction.     

Test of the adaptiveness of sex ratio manipulation in a parasitoid wasp

In behavioral ecology it is generally assumed that behavior is adaptive. This assumption is tested here for sex ratio manipulation in response to host size in the parasitoid wasp Spalangia cameroni. Females produce a greater proportion of daughters on larger hosts. If this behavior is adaptive, it is not through a positive effect of host size on the fitness of daughters, as theory suggests and as found for other species. Females that developed on larger hosts were not more successful at drilling into hosts, were not more successful at interspecific competition for hosts, and did not have greater dispersal ability as measured by wing loading (weight/area of wing and thorax). The possibility that S. cameroni's sex ratio manipulation may be adaptive through a negative effect of host size on the fitness of sons cannot be ruled out. Relative to males from larger hosts, males from smaller hosts had lower wing loading and thus potentially greater dispersal ability. The actual effect of wing loading on fitness remains to be tested.     

Fitness consequences of body-size-dependent host species selection in a generalist ectoparasitoid

In insect parasitoids, offspring fitness is strongly influenced by the adult females choice of host, particularly in ectoparasitoids that attack non-growing host stages. We quantified the fitness consequences of size-dependent host species selection in Dirhinus giffardii, a solitary ectoparasitoid of tephritid fruit fly pupae. We first showed a positive correlation between the size of emerged D. giffardii wasps and the size of their host fruit fly species (in order of decreasing size): Bactrocera latifrons, B. cucurbitae, B. dorsalis or Ceratitis capitata. We then manipulated individual wasps to show that the parasitoid preferred to attack the largest (B. latifrons) to the smallest (C. capitata) host species when provided with a choice, and laid a greater proportion of female eggs in B. latifrons than in C. capitata. There were no differences in developmental time or offspring survival between individuals reared from these two host species. Finally, we compared the foraging efficiency of large versus small wasps (reared from B. latifrons vs C. capitata) under two different laboratory conditions: high versus low host habitat quality, given that realized fecundity in parasitoids may be influenced by either egg-limited or time-limited factors. Under both conditions, large wasps parasitized more hosts than did small ones as a consequence of high searching efficiency in the host-poor habitat, and high capacity for adjusting egg maturation in response to host availability in the host-rich habitat. Considering the flexibility of body growth, the apparent lack of cost of achieving large body size in either development or survival, and the strong dependence of realized reproductive success on a females size, we argue that body size may be a key to understanding evolution of host species selection in ectoparasitoids. We also discuss constraints upon the evolution of size-dependent host species selection in parasitoids.Communicated by D. Gwynne     

Local mate competition in the solitary parasitoid wasp <Emphasis Type="Italic">Ooencyrtus kuvanae</Emphasis>

Local mate competition (LMC) occurs when brothers compete with each other for mating opportunities, resulting in selection for a female-biased sex ratio within local groups. If multiple females oviposit in the same patch, their sons compete for mating opportunities with non-brothers. Females, in the presence of other females, should thus produce relatively more sons. Sex ratio theory also predicts a more female-biased sex ratio when ovipositing females are genetically related, and sex-ratio responses to foundress size if it differentially affects fitness gains from sons versus daughters. The mating system of the parasitoid wasp Ooencyrtus kuvanae meets assumptions of LMC. Females insert a single egg into each accessible egg of gypsy moth, Lymantria dispar, host egg masses. Wasps complete development inside host eggs and emerge en masse, as sexually mature adults, resulting in intense competition among brothers. We tested the hypothesis that O. kuvanae exhibits LMC by manipulating the number of wasp foundresses on egg masses with identical numbers of eggs. As predicted by LMC theory, with increasing numbers of wasp foundresses on an egg mass, the proportions of emerging sons increased. In contrast, the presence of a sibling compared to a non-sibling female during oviposition, or the size of a female, did not affect the number or sex ratio of offspring produced. The O. kuvanae system differs from others in that larvae do not compete for local resources and thus do not distort the sex ratio in favor of sons. With no resource competition among O. kuvanae larvae, the sex ratio of emergent son and daughter wasps is due entirely to the sex allocation by ovipositing wasp foundresses on host egg masses.     

Intraspecific brood parasitism in the moorhen: parentage and parasite-host relationships determined by DNA fingerprinting

Parasitic female moorhens (Gallinula chloropus) lay from one to six eggs in the nests of conspecific neighbours. DNA fingerprinting was used to show that parasitic eggs could be correctly identified when they appeared in addition to or outside the host’s laying sequence. Moorhen hosts accept all parasitic eggs laid after the 2nd day of their laying period. To understand why moorhen hosts tolerate parasitic eggs, we tested two hypotheses. (1) The quasi-parasitism hypothesis: females lay their eggs in the evening when the host males are normally in attendance at the nest, so host males may allow parasitic females to lay in their nests in exchange for fertilizing their eggs. However, DNA fingerprinting showed that all the parasitic eggs were sired by the parasites’ mates. Parasitic moorhens frequently continue laying a clutch in their own nest, without a break in the laying sequence after a parasitic laying bout. The eggs laid by brood parasites in their own nests were also sired by their own mates. Therefore this hypothesis was rejected. (2) The kin selection hypothesis: if one or both members of the host pair are close relatives of the parasite, the costs of rearing parasitic chicks will be to some degree offset by inclusive fitness benefits. We examined the genetic relationships between parasites and their hosts using DNA fingerprinting and genealogical data. Natal philopatry by both sexes was relatively common in this population, and the probability that a neighbour of either sex was a first-order relative (parent-offspring) was calculated as 0.18. Although first-order relatives were not preferentially chosen as hosts over individuals that were not first-order relatives, even through random host selection there is almost a one-in-five chance that brood parasites in this population are closely related to their hosts. This may facilitate host tolerance of parasitic eggs. Other hypotheses are also discussed. Received: 3 February 1995/Accepted after revision: 27 August 1995     

Offspring sex ratio response to host size in the parasitoid wasp Spalangia endius

The host size model, an adaptive model for maternal manipulation of offspring sex ratio, was examined for the parasitoid wasp Spalangia endius. In a Florida strain, as the model predicts, daughters emerged from larger hosts than sons, but only when mothers received both small and large hosts simultaneously. The pattern appeared to result from the mother's ovipositional choice and not from differential mortality of the sexes during development. If sex ratio manipulation is adaptive in the Florida strain, it appears to be through a benefit to daughters of developing on large hosts rather than through a benefit to sons of developing on small hosts. Both female and male parasitoids were larger when they developed on larger hosts. For females, developing on a larger host (1) increased offspring production, except for the largest hosts, (2) increased longevity, (3) lengthened development, and (4) had no effect on wing loading. For males, development on a larger host had no effect on any measure of male fitness – mating success, longevity, development duration, or wing loading. In contrast, a strain from India showed no difference in the size of hosts from which daughters versus sons emerged, although both female and male parasitoids were larger when they developed on larger hosts. These results together with previous studies of Spalangia reveal no consistent connection between host-size-dependent sex ratio and host-size-dependent parasitoid size among strains of S. endius or among species of Spalangia. Received: 28 October 1998 / Received in revised form: 20 May 1999 / Accepted: 30 May 1999     

Precise sex ratios manifested by several encyrtid parasitoids (Hymenoptera: Encyrtidae) of brown soft scale, <Emphasis Type="Italic">Coccus hesperidum</Emphasis> L. (Hemiptera: Coccidae)

We examined whether several facultatively gregarious encyrtid (Hymenoptera: Encyrtidae) endoparasitoids of brown soft scale, Coccus hesperidum L., manifest precise sex allocation under field conditions. Metaphycus luteolus (Timberlake), Metaphycus angustifrons (Compere), Metaphycus stanleyi (Compere), and Microterys nietneri (Motshulsky) evince brood sex ratios that are female-biased and extremely precise (low variance in the number of sons per host). Typically, this sex allocation pattern is attributed to extreme local mate competition (LMC) in which only one foundress exploits a patch of hosts and mating occurs mostly between her offspring. However, such a pattern of sex allocation was not detected for Metaphycus helvolus (Compere). Also, a large proportion of the broods in all five species contained only daughters; thus, an excess of male-only broods was expected if unmated females (i.e., females that can produce only sons) contribute offspring before mating. All-male broods were rare in our samples. This finding coupled with the life history characteristics of these wasps, such as the exploitation of aggregated hosts and the long life span and mobility of males, suggest that nonlocal mating is frequent. Our empirical work suggests that it is advantageous to allocate precise sex ratios in cases in which mating opportunities for males are not restricted to their natal host and/or when multiple foundresses exploit large patches of hosts. Limited theoretical work also supports this prediction but more detailed studies of this taxon’s mating structure and other life history characteristics are necessary to understand their sex allocation decisions.     

Local mate competition and precise sex ratios in Telenomus fariai (Hymenoptera: Scelionidae), a parasitoid of triatomine eggs

Telenomus fariai is a gregarious endoparasitoid of the eggs of several species of Triatominae (Hemiptera) with a high degree of sibmating: males fertilize their sisters inside the host egg before emergence or emerge first and copulate with their sisters as these emerge. Our results show that, when laying alone, T. fariai behaves adaptively, minimizing offspring mortality and conforming to the prediction of local mate competition (LMC) theory by laying a single male, which is sufficient to fertilize all the sisters. When more than one wasp was placed with one host, sex ratios still conformed to LMC predictions but, despite the decreasing number of eggs laid per wasp, clutch size could not be completely adjusted to avoid mortality. This is not surprising, as superparasitism is rare in the field. Offspring production was independent of the contacts between conspecifics but was affected by the number of mothers laying on a single host egg. The sex of the progeny was precisely determined: a female produced one male per clutch when laying on both unparasitized or previously parasitized hosts. On the other hand, a mother produced less daughters when superparasitizing. Under crowded conditions, the number of eggs laid per female wasp and per host decreased as the number of mothers increased. Developmental mortality also increased with the number of T. fariai eggs per host, determining a maximum of approximately 14 emerged adults. Host resources per individual affected male and female adult size with similar intensity, and male adult mortality was slightly higher than that for females. These results, and previous findings, suggest that T. fariai attains Hamiltonian sex ratios by laying one male and a variable number of females, and that the detection of chemical marks left by conspecifics provides information on the number of foundresses sharing a patch. Received: 4 February 2000 / Received in revised form: 19 April 2000 / Accepted: 20 May 2000     

How temperature and habitat quality affect parasitoid lifetime reproductive success—A simulation study

Adult parasitoid females lay their eggs in or on host insects. Most species are incapable of de novo lipogenesis as adults, and lipids accumulated during the larval stage are allocated either to egg production or to adult survival. Lipid consumption increases with distance covered by the parasitoids and thus with the distance between available hosts within a habitat. Temperature should affect parasitoid fitness because it changes the constraint imposed by a limited reserve of lipids and because it influences behaviour. Climate change involves both an increase in average temperature and an increased frequency of extreme weather such as heat waves. We investigated how the predicted increase of temperature will affect parasitoid fitness and how this depends on habitat parameters (spatial distribution of hosts and lipid cost of habitat exploitation). We studied optimal behaviour and calculated fitness at different temperatures and in different habitats using a stochastic dynamic programming model for pro-ovigenic parasitoids (which mature all their eggs before becoming adult). We show that an increase in temperature decreases fitness of parasitoids adapted to lower temperatures. This decrease in fitness depends on habitat quality. In field conditions (assuming small costs of intra-patch foraging), the loss of fitness should be larger in habitats with high inter-patch distance and in habitats with a more aggregated distribution of hosts. The foraging behaviour of parasitoids is also affected; at higher temperature we show that intra-patch foraging becomes less efficient, and patch residence times are longer.     

Parental care and sexual size dimorphism in wasps and bees

Sexual size dimorphism, in which one sex is larger than the other, occurs when body size has differential effects on the fitness of males and females. Mammals and birds usually have male-biased size dimorphism, probably because of strong sexual competition among males. Invertebrates usually have female-biased size dimorphism, perhaps because their inflexible exoskeletons limit ovary size, leading to a strong correlation between female body size and fecundity. In this paper, we test whether an additional factor, the type of parental care provided, affects the degree of sexual size dimorphism. Among wasps and bees, there is a contrast between provisioning taxa, in which females must gather and transport heavy loads of provisions to nests they have constructed, and non-provisioning taxa, in which females lay eggs but do not construct nests or transport provisions. Males have no role in parental care in either case. An analysis of British wasps and bees shows that provisioning taxa have significantly more female-biased size dimorphism than non-provisioning taxa. This is true for simple cross‑species comparisons and after controlling for phylogeny. Our data imply that the demands of carrying provision loads are at least part of the explanation for this pattern. Thus, sexual size dimorphism is greatest in pompilid wasps, which carry the heaviest prey items. Bees, which transport minute pollen grains, exhibit the least dimorphism. We also find that cavity‑nesting species, in which nest construction costs may be minimized, exhibit reduced dimorphism, but this was not significant after controlling for phylogeny.     

Precise sex allocation, local mate competition, and sex ratio shifts in the parasitoid wasp Trichogramma pretiosum

We determined the sex, order, and clutch size of eggs laid by the parasitoid wasp, Trichogramma pretiosum Riley, in the eggs of one of its natural hosts, Trichoplusia ni (Hübner). The parasitoid allocated sex non-randomly to hosts in the laboratory with a variance significantly less than that of a binomial (random) distribution, our null model. More clutches of two or more eggs contained a single male egg as the second or third egg laid than would be expected by chance and none contained two or more male eggs. T. pretiosum also increased the sex ratio (% male) of its offspring with increasing foundress numbers by increasing the frequency of male offspring as the second egg in a two-egg clutch allocated to unparasitized hosts and as the single egg allocated to previously parasitized hosts. These results indicate that T. pretiosum allocates the sex of its offspring precisely. Precise sex allocation is favored under local mate competition because it reduces variation in the number of sons per patch thus maximizing the number of inseminated daughters emigrating from the patch. Similar combinations of female and male offspring emerged from T. ni eggs parasitized by T. pretiosum in the field, again with a sex ratio variance less than that expected for a binomial distribution. These results strongly suggest that this parasitoid species manifests local mate competition.     

Clutch size as an optimal foraging problem for insects

Summary Many insects oviposit on food patches (e. g., hosts) that represent a finite resource. Competition for food may occur among the developing progeny with the effect of reducing their survivorship, size, and/or some other aspect of fitness. Here a simple, but general, equation is given that captures tures some of the possible relationships between the number of progeny on a host and their fitness. This in turn specifies the clutch size that maximizes the fitness of a female ovipositing on a single host (the Lack clutch size). The equation is then used in an optimal foraging type model to specify optimal clutch sizes when a female searches for and oviposits on a series of hosts. The model makes three general predictions: (1) for a constant host size (or value), optimal clutch size varies directly with the inter-host search time, (2) for a constant search time, optimal clutch size varies directly with host value, and, (3) females should never lay more than the Lack clutch size. The model is then extended to include optimal clutch size in superparasitism.     

Sex ratio responses to other parasitoid wasps: multiple adaptive explanations

In an effort to distinguish among adaptive models and to improve our understanding of behavioral mechanisms of sex ratio manipulation, this study examines sex ratio responses to other wasps in the solitary parasitoid wasp Spalangia cameroni. Relative to when alone, females produced a greater proportion of sons in the presence of conspecifics, regardless of whether the conspecifics were female or male. In addition, females produced a greater proportion of sons after a day with a conspecific male, and after a day with a conspecific female, but only if the females had been ovipositing. Relative to when alone, females did not produce a greater proportion of sons in the presence of females of the confamilial Muscidifurax raptor or in response to hosts that had already been parasitized by a conspecific. A combination of evolutionary models may explain S. cameroni’s sex ratios. An increased proportion of sons in response to conspecific females is common among parasitoid wasps and is usually explained by local mate competition (LMC) theory. However, such a response is also consistent with the perturbation model, although not with the constrained females model. The response to conspecific males is not consistent with LMC theory or the perturbation model but is consistent with the constrained females model.     

Tactics of parasitic American coots: host choice and the pattern of egg dispersion among host nests

Summary I examined the tactics adopted by a conspecific brood parasite, the American coot (Fulica americana), and the degree to which these tactics reflect sources of mortality for parasitic eggs. Only 8% of parasitic eggs produced independent offspring, compared to a 35% success rate for non-parasitic eggs, and most mortality was due to egg-rejection by hosts or the consequences of laying eggs too late in the host's nesting cycle. Parasites usually laid parasitically before initiating their own nests and usually parasitized immediate neighbours. Parasites did not remove host eggs before laying their own egg, and egg disappearance in general was not more common at parasitized nests. I found no evidence for non-random host choice, either on the basis of stage of the host's nesting cycle or the host's brood size. The absence of adaptive host choice is likely a consequence of the fact that, due to host limitation, only a small proportion of parasites had meaningful variation among potential hosts to choose from. The pattern of egg dispersion among host nests by individual parasites appears to be a compromise between constraints imposed by host limitation and the increased success obtained from spreading eggs among nests. Most females laying fewer than five parasitic eggs laid them in a single host nest while females laying five or more eggs normally parasitized two or more hosts. An examination of egg rejection and survival rates showed that parasites would maximize success by laying a single egg per host nest, and the pattern of laying several eggs per host nest is likely a consequence of host limitation. However, no egg that was the fifth laid, or later, parasitic egg in a host nest was ever successful and this probably explains why most females laying five or more eggs parasitized more than one host.     

Recognition of individual-specific marked parasitized hosts by the solitary parasitoid Epidinocarsis lopezi

Summary In parasitoid wasps, self-superparasitism (oviposition into a host already parasitized by the female herself) often contributes less to the reproductive success of the parasitoid than oviposition into a host previously parasitized by a conspecific (conspecific superparasitism). It could therefore often be profitable for parasitoids to avoid self-superparasitism. This requires a mechanism for either (1) the avoidance of previously searched areas and/or (2) the rejection of hosts containing eggs laid by the searching female. We investigated whether the solitary parasitoid Epidinocarsis lopezi is able to avoid self-superparasitism. We show that visits to previously searched patches were shorter than visits to unsearched patches and conclude that E. lopezi females leave a trail odour on patches they have searched. No differences were found between the time on patches previously searched by the wasp itself and on patches visited by conspecifics. However, E. lopezi superparasitizes fewer hosts previously parasitized by itself than hosts parasitized by a conspecific. Thus, they recognize an individual-specific mark in or on the host. We discuss how patch marking and host marking enable E. lopezi to avoid self-superparasitism.     

Asymmetric larval competition in the parasitoid wasp <Emphasis Type="Italic">Nasonia vitripennis</Emphasis>: a role in sex allocation?

Sex allocation theory offers excellent opportunities for testing how animals adjust their behaviour in response to environmental conditions. A major focus has been on instances of local mate competition (LMC), where female-biased broods are produced to maximise mating opportunities for sons. However, the predictions of LMC theory can be altered if there is both local competition for resources during development and an asymmetry between the competitive abilities of the sexes, as has been seen in animals ranging from wasps to birds. In this paper, we test the extent to which asymmetric larval competition alters the predictions of LMC theory in the parasitoid wasp Nasonia vitripennis. We found that the body size of both sexes was negatively correlated with the number of offspring developing within the host. Further, we found that when faced with high levels of competition, the body size of females, but not males, was influenced by the sex ratio of the competing offspring; females were smaller when a higher proportion of the brood was female. This asymmetric competition should favour less biased sex ratios than are predicted by standard LMC theory. We then develop a theoretical model that can be parameterised with our data, allowing us to determine the quantitative consequences of the observed level of asymmetric larval competition for sex allocation. We found that although asymmetric competition selects for less biased sex ratios, this effect is negligible compared to LMC. Furthermore, a similar conclusion is reached when we re-analyse existing data from another parasitoid species where asymmetric larval competition has been observed; Bracon hebetor. Consequently, we suspect that asymmetric larval competition will have its greatest influence on sex ratio evolution in species that have smaller clutches and where local mate competition is not an issue, such as birds and mammals.     

Ecological costs on local adaptation of an insect herbivore imposed by host plants and enemies

Herbivore populations may become adapted to the defenses of their local hosts, but the traits that maximize host exploitation may also carry ecological costs. We investigated the patterns and costs of local adaptation in the pine processionary moth, Thaumetopoea pityocampa, to its host plants, Pinus nigra and P. sylvestris. The two hosts differ in needle toughness, a major feeding impediment for leaf-eating insects. We observed a west-to-east gradient of increasing progeny size in the Italian Alps, matching the pattern in toughness of their respective local host plant. Eastern populations that feed on the native P. nigra with tough needles had larger eggs, and neonate larvae with larger head capsules, than western populations that feed on the native P. sylvestris and the introduced P. nigra with softer foliage. In a reciprocal transfer experiment that involved the eastern-most and the western-most populations of T. pityocampa from this region, and excluded natural enemies, we found evidence for local adaptation to the host plant. Specifically, larvae from the western population only performed well when raised on their local hosts with soft needles, and they suffered near-complete mortality on the tough foliage at the eastern site. In contrast, larvae from the eastern population survived equally well at both sites. Local adaptation involved a trade-off between progeny size and the number of offspring. We hypothesized that an additional cost, imposed by natural enemies, may be associated with increased egg size: we also observed a west-to-east gradient of increased egg parasitism. We tested this hypothesis in a common garden by exposing eggs of both populations to parasitism by two native egg parasitoids, Ooencyrtus pityocampae and Baryscapus servadeii. The eastern population suffered a higher level of parasitoid attack by O. pityocampae than the western population, and performance of hatched adults of both parasitoids was enhanced in large eggs. Thus, increased neonate quality (larger eggs yielding larger larvae) confers an advantage on tough foliage but incurs the ecological cost of increased parasitism, which may constrain further adaptation by this herbivore.