Non-additive effects of multiple natural enemies on aphid populations

The question of whether multiple natural enemies often interact to produce lower host mortality than single enemies acting alone has not yet been resolved. We compared the effects of four different combinations of natural enemies-parasitoids, predators, parasitoids plus predators, and no enemies-on caged aphid populations on marsh elder, Iva frutescens, in west-central Florida. Using starting densities of natural enemies commonly found in the field, we showed that parasitoid wasps reduced aphid population densities more than predatory ladybird beetles. The addition of predators to cages containing parasites reduced the ability of parasitoids to decrease aphid population densities. Because the experiments ran only over the course of one generation, such a reduction in the effectiveness of parasites is likely caused by interference of predators with parasitoid behavior. Parasitism in the cages containing both parasitoids and predators was reduced when compared to percent parasitism in parasitoid-only cages, but this could also be due to predation. Our experiments showed that ladybird beetles prey on parasitized aphids. Thus over the long-term, the effectiveness of parasites is impaired by the interference of predators on ovipositing parasitoids and by the predation of parasitized aphids. The effects of natural enemies in this system are clearly non-additive.     

Host nutritional status mediates degree of parasitoid virulence

Parasitic organisms rely on the resources of their hosts to obtain nutrients essential for growth and reproduction. Insect parasitoids constitute an extreme condition since they develop in a single host from which they typically consume all available resources. As a result, the host is killed following parasitism. However, a few intriguing cases of host survival have been reported wherein hosts resume foraging and may even reproduce following parasitoid emergence. Yet, the ultimate and proximate mechanisms responsible for host recovery remain unresolved. We tested the impact of host nutrition on host fate and parasitoid fitness, using the association between Dinocampus coccinellae and the spotted lady beetle Coleomegilla maculata. Under laboratory conditions, we fed parasitized ladybirds on different aphid diets, with or without pollen. In the field, we followed the fate of parasitized ladybirds during seasonal variations in pollen and aphid abundance. We found that ladybirds fed on aphids or a combination of aphids and pollen recovered more frequently from parasitism (from 65 to 81%) than those eating only pollen (48%). Field data suggest that the fate of parasitized ladybirds is also related to food availability. On the other hand, when hosts fed on a combination of aphids and pollen, consequences for parasitoid fitness were often ‘all‐or‐nothing’: parasitoid emergence rate was the lowest of all host nutrition regimes (～50%), but parasitoids that did emerge were larger than individuals emerging from other host nutrition regimes. Laboratory and field results concur to show that host nutritional status during parasitoid development significantly influences both host fate and parasitoid fitness.     

Detection of secondary predation by PCR analyses of the gut contents of invertebrate generalist predators

Predation by generalist predators is difficult to study in the field because of the complex effects of positive and negative interactions within and between predator species and guilds. Predation can be monitored by molecular means, through identification of prey DNA within predators. However, polymerase chain reaction (PCR) amplification of prey DNA from predators cannot discriminate between primary and secondary predation (hyperpredation), in which one predator feeds on another that has recently eaten the target prey. Here we quantify, for the first time, the potential error caused by detection of prey DNA following secondary predation, using an aphid-spider-carabid model. First, the aphid Sitobion avenae was fed to the spider Tenuiphantes tenuis and the carabid Pterostichus melanarius, and the postconsumption detection periods, for prey DNA within predators, were calculated. Aphids were then fed to spiders and the spiders to carabids. Aphid DNA was detected in the predators using primers that amplified 245- and 110-bp fragments of the mitochondrial cytochrome oxidase I gene. Fragment size and predator sex had no significant effect on detection periods. Secondary predation could be detected for up to 8 h, when carabids fed on spiders immediately after the latter had consumed aphids. Beetles tested positive up to 4 h after eating spiders that had digested their aphid prey for 4 h. Clearly, the extreme sensitivity of PCR makes detection of secondary predation more likely, and the only reliable answer in future may be to use PCR to identify, in parallel, instances of intraguild predation.     

Generalist predators disrupt parasitoid aphid control by direct and coincidental intraguild predation

Generalist predators and parasitoids are considered to be important regulators of aphids. The former not only feed on these pests, but might also consume parasitoids at all stages of development. This direct or coincidental interference affects the natural control of aphids, the scale of which is largely unknown, and it has rarely been examined under natural conditions. Here, molecular diagnostics were used to track trophic interactions in an aphid-parasitoid-generalist predator community during the build-up of a cereal aphid population. We found that generalist predators, principally carabid and staphylinid beetles as well as linyphiid spiders, had strong trophic links to both parasitoids and aphids. Remarkably, more than 50% of the parasitoid DNA detected in predators stems from direct predation on adult parasitoids. The data also suggest that coincidental intraguild predation is common too. Generalist predators, hence, disrupt parasitoid aphid control, although the levels at which the predators feed on pests and parasitoids seem to vary significantly between predator taxa. Our results suggest that taxon-specific trophic interactions between natural enemies need to be considered to obtain a more complete understanding of the route to effective conservation biological control.     

Complementary effects of resident natural enemies on the suppression of the introduced moth Epiphyas postvittana

Generalist predators may disrupt or complement biological control by parasitoids. Past studies have examined how predators and parasitoids interact to affect aphid suppression, but more information is needed from other host taxa. Here, we explore the interactive effects of a spider (Cheiracanthium mildei) and a generalist parasitoid (Meteorus ictericus) on the light brown apple moth (Epiphyas postvittana), a recent introduction to North America. The spider negatively affected adult parasitoids in a field experiment, and reduced numbers of parasitized larvae in the laboratory. Nonetheless, the combined effects of parasitoids and spiders on larval mortality of the moth were additive. Percent parasitism was not affected by the presence of the spider in field or laboratory experiments, and results were similar when single or multiple larval instars of the moth were included. The spider’s lack of prey preference for unparasitized or parasitized larvae likely precluded any disruptive effects on parasitism. Results suggest that resident generalist parasitoids and predators can work in conjunction to hinder the invasion success of a novel herbivore prey species.     

Host Handling Time in a Polyembryonic Wasp is Affected both by Previous Experience and by Host State (Parasitized or Not)

Foraging behavior for hosts in parasitoids resembles that of predators with respect to finding, evaluating and manipulating of the prey. Host handling time may depend on the life history of the parasitoid and can be affected by oviposition experience. Additionally, handling time can be affected by host aggregation, species, size and state (parasitized or not). We studied handling times in the egg-larval parasitoid wasp Copidosoma koehleri. We allowed naïve female wasps to oviposit into three consecutive unparasitized hosts, and measured time until oviposition, and the duration of ovipositor insertion. We recorded the same data for naïve females ovipositing into already parasitized hosts. We found that both previous experience by females and previous parasitism of hosts reduced handling time. The results suggest that host handling durations reflect the interplay between host state and parasitoid internal state.     

The importance of intraguild interactions to the combined effect of a parasitoid and a predator on aphid population suppression

Intraguild predation (IGP) occurs when consumers competing for a resource also engage in predatory interactions. A common type of IGP involves aphid predators and parasitoids: since parasitoid offspring develop within aphid hosts, they are particularly vulnerable to predation by aphid predators such as coccinellid beetles. Other intraguild interactions that include non-lethal behavioral effects, such as interference with foraging and avoidance of IGP, may also hamper parasitoid activity and reduce their effectiveness as biological control agents. In this study, we quantified mortality in and behavioral effects on Aphidius colemani Viereck (Hymenoptera: Aphidiidae) by its IG-predator Coccinella undecimpunctata L. (Coleoptera: Coccinellidae), and compared the impact of two release ratios of these natural enemies on aphid populations. Parasitoids did not leave the plant onto which they were first introduced, regardless of the presence of predators, even when alternative prey was offered on predator-free plants nearby. In 2-hour experiments, predator larvae interfered with wasp activity, and the level of aphid parasitism was lower in the presence of predators than in their absence. In these experiments, the parasitoids contributed more to aphid mortality than the predators and aphid suppression was higher when a parasitoid acted alone than in combination with a predator larva. These results were confirmed in a 5-day experiment, but only at one parasitoid:predator release ratio (4:3) not another (2:3). The over-all impact on aphid population growth was non-the-less stronger when both enemies acted together than when only one of them was present. Results indicate that for given release ratios and time scale, the negative lethal and non-lethal effects of the predator on parasitoid performance did not fully cancelled the direct impact of the predator on the aphid population.     

Interspecific competition between Diadegma semiclausum Hellen (Hym., Ichneumonidae) and Cotesia plutellae (Kurdjumov) (Hym., Braconidae) in parasitizing Plutella xylostella (L.) (Lep., Plutellidea)

Abstract:  Interspecific competition between Diadegma semiclausum and Cotesia plutellae was investigated at 25°C in the laboratory, by exposing the third instar larvae of the diamondback moth, Plutella xylostella to both species together, either species alone or by exposing the host larvae already parasitized by one species, at different intervals, to the other. When host larvae were exposed simultaneously to two species in one arena, parasitism rates of the host by each species were not reduced by the presence of the other species; joint parasitism rate by two species was not significantly higher than that by either parasitoid alone. Both parasitoids could lay eggs into the host larvae which had previously been parasitized by the other species, leading to the occurrence of multiparasitized hosts. When host larvae were parasitized first by D. semiclausum and then being followed within 1–2 h by exposing to C. plutellae , or vice versa, ensuing parasitoid cocoons from the multiparasitized host larvae were nearly all C. plutellae . When host larvae were parasitized initially by C. plutellae and then being followed by D. semiclausum two or more days later, all parasitoids ensued from the multiparasitized hosts were C. plutellae . In contrast, when host larvae were parasitized initially by D. semiclausum and then being followed by C. plutellae two or more days later, most host larvae could not survive to prepupae and most of the ensuing parasitoid adults from the surviving hosts were D. semiclausum . Dissections of host larvae at various time intervals after parasitization by the two parasitoids showed that development of both parasitoids in multiparasitized hosts were somewhat arrested, and that the first instar larvae of C. plutellae could initiate a physical attack on the larvae of D. semiclausum and remove the latter.     

闭弯尾姬蜂与菜蛾盘绒茧蜂寄生菜蛾幼虫时的种间竞争

在室内25℃下,以菜蛾3龄初幼虫作寄主,研究了菜蛾盘绒茧蜂Cotesia plutellae和半闭弯尾姬蜂Diadegma semiclausum的种间竞争。当寄主供2种蜂同时产卵寄生时,2种蜂各自的寄生率与其单独寄生时无显著差异,合计寄生率比一种蜂单独存在时有所提高,但差异不显著。2种蜂均能产卵寄生已被另一种蜂寄生了的寄主幼虫。当寄主被2种蜂寄生的间隔时间很短（少于10 h）时,所育出的蜂绝大部分（80%以上）为绒茧蜂;当寄主先被绒茧蜂寄生,并饲养2天以上再供弯尾姬蜂寄生时,所育出的全为绒茧蜂;当寄主先被弯尾姬蜂寄生,并饲养2天以上再供绒茧蜂寄生时,寄主幼虫绝大部分不能存活,只有少部分能育出寄生蜂,且多为弯尾姬蜂。当2种蜂的幼虫存在于同一寄主体内时,2种蜂的发育均受到另一种蜂的抑制;绒茧蜂1龄幼虫具有物理攻击能力,能将弯尾姬蜂卵或幼虫致死。这些结果表明,菜蛾盘绒茧蜂与半闭弯尾姬蜂在同一寄主中发育时,前者具有明显的竞争优势。     

Food-deprived host-feeding parasitoids kill more pest insects

Some parasitoids not only parasitize their hosts but also destructively feed on them which could result in host mortality. We hypothesized that host-feeding parasitoids that are food-deprived before being released for biological control would destructively feed on more hosts, and an optimal duration of food deprivation of parasitoids would enhance host feeding without negatively affecting parasitism. We investigated the effects of food-deprivation durations (0, 3, 6, 10, 24 h and 20% honey solution offered), before being released, on the host mortality and parasitism by a destructive host-feeding parasitoid of whitefly nymphs, Encarsia sophia (Girault & Dodd) (Hymenoptera: Aphelinidae). In another experiment, we compared host feeding, parasitization and longevity throughout the lifespan between food-deprived and non-food-deprived parasitoids. Using fourth-instar Bemisia tabaci B biotype as the host, the responses of E. sophia differed significantly with the durations of food deprivation before they were exposed to their hosts. E. sophia adults food-deprived for 6 h killed more hosts through feeding than those that were food-deprived for 0, 3, 10 and 24 h or those that were fed with honey solution. Moreover, parasitoids that were food-deprived for 6 h parasitized more hosts than those held for other durations of food deprivation. Similar results on host-feeding were found when third-instar whitefly nymphs were used. We confirmed that E. sophia food-deprived for 6 h not only killed more whiteflies through host feeding, but also lived significantly longer and parasitized more hosts than the non-food-deprived ones throughout their lifespan. Our results could be extended to improve the effectiveness of augmentative biological control of pests through improved host feeding and parasitizing capacity by starving the parasitoids for a certain period of time before they will be used.     

Inter-larval Competition and its Subsequent Effect on Pediobius furvus (Hym.: Eulophidae) Broods for the Management of Graminaceous Stem Borers

The effects of superparasitism on the size, rate of development, progeny production, sex ratio and percentage parasitism of the parasitoid Pediobius furvus (Gah.) and the number of parasitized hosts producing mature parasitoids were investigated. High parasitoid egg densities in hosts were obtained using two host densities (1 and 2) and three arbitrary parasitoid densities (1, 2 and 5) under two exposure times (12 and 24 h). Superparasitism in P. furvus resulted in a reduction in the number of adults that matured and a reduction in the size and preponderance of male production. Dissections of the parasitized host pupae of the stem borer Chilo partellus (Swinhoe) showed no combat among developing wasps. Immature supernumeraries were eliminated by starvation and/or suffocation. The results showed that P. furvus larvae are not harmed by surplus food material in their hosts and that the female parasitoid preferred to lay more eggs into unparasitized than into parasitized host pupae.     

Interactions between an ectoparasitoid and a nucleopolyhedrovirus when simultaneously attacking Spodoptera exigua (Lepidoptera: Noctuidae)

Insect pathogenic viruses and parasitoids represent distinct biological entities that exploit a shared host resource and have similar effects in suppressing host populations. This study explores the interactions between the ectoparasitoid Euplectrus plathypenae (Hymenoptera: Eulophidae) and the Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) in larvae of S. exigua (Lepidoptera: Noctuidae). Parasitoid progeny failed to complete development in hosts that had been infected prior to parasitism. However, infection of S. exigua fourth instars at 48 h post‐parasitism had no significant effects on the survival of parasitoid progeny. Larval and pupal development times of E. plathypenae that survived on virus‐infected S. exigua did not differ significantly from that of parasitoids on healthy hosts. Virus‐induced mortality and the production of occlusion bodies were very similar in parasitized and non‐parasitized S. exigua. The virus was genetically stable over three passages in parasitized and unparasitized hosts. These results suggest that applications of SeMNPV‐based insecticides are unlikely to disrupt pest control exerted by the parasitoid E. plathypenae in biological pest control programs as long as virus applications are timed not to coincide with parasitoid releases.     

Host ontogeny determines parasitoid use of a forest caterpillar

For most organisms, patterns of natural enemy‐mediated mortality change over the course of development. Shifts in enemy pressure are particularly relevant for organisms that exhibit exponential growth during development, such as juvenile insects that increase their mass by several orders of magnitude. As one of the dominant groups of insect herbivores in most terrestrial plant communities, larval lepidopterans (caterpillars) are host to a diverse array of parasitoids. Previous research has described how the frequency of herbivore parasitism varies among host plants or habitats, but much less is known about how parasitism pressure changes during host development. To test whether the two major parasitoid taxa, wasps and flies, differentially attack shared hosts based on host developmental stage, we simultaneously exposed early‐ and late‐instar Euclea delphinii Boisduval (Lepidoptera: Limacodidae) caterpillars to parasitism in the field. We found strong evidence that parasitoids partition hosts by size; adult female wasps preferentially parasitized small caterpillars, whereas adult female flies preferred to attack large caterpillars. Our results demonstrate that host ontogeny is a major determinant of parasitoid host selection. Documenting how shifts in enemy pressure vary with development is important to understanding both the population biology and evolutionary ecology of prey species and their enemies.     

Evidence for adaptive self-superparasitism in the dryinid parasitoid Haplogonatopus atratus when conspecifics are present

Self-superparasitism – ovipositing in or on a host that was previously parasitized by the same individual – had generally been considered maladaptive for solitary parasitoids. Recently, however, some researchers have suggested that self-superparasitized hosts produce higher fitness returns per host than singly parasitized hosts when these hosts are parasitized by conspecifics. The present study is the first to verify this possibility in the parasitoid Haplogonatopus atratus . Fitness returns from the first and second ovipositions under triple parasitism were compared with those from the first oviposition under double parasitism. Fitness returns were assessed in terms of survival rate, adult size, and developmental time. No differences were found between the two groups in adult size and developmental time. When the interval between the second and third ovipositions was 1 h, the total survival rate of the first and second comers under triple parasitism was higher than the survival rate of the first comer under double parasitism, and as high as the total survival rate of two first comers under double parasitism. This suggests that the value of self-parasitized hosts is as high as that of unparasitized hosts. When the second-to-third interval of triple parasitism was 24 h, self-superparasitism produced no fitness returns. Moreover, fitness returns from ovipositing into self-superparasitized hosts were similar to those from ovipositing into singly parasitized hosts.     

The effects of temperature on detection of prey DNA in two species of carabid beetle

PCR-based techniques to investigate predator-prey trophic interactions are starting to be used more widely, but factors affecting DNA decay in predator guts are still poorly understood. Here, we investigated the effects of time since feeding, temperature and amplicon size on the detectability of prey DNA in the gut content of two closely related predator species. Cereal aphids, Sitobion avenae, were fed to the carabid beetles Pterostichus melanarius and Nebria brevicollis. Beetles were allowed to digest their meal at 12 degrees C, 16 degrees C and 20 degrees C, and batches of beetles were subsequently frozen at time periods from 0-72 h after feeding. Aphid DNA was detected within beetles' gut contents using primers amplifying fragments of 85, 231, 317 and 383 bp. Prey DNA detection rates were significantly higher in N. brevicollis than in P. melanarius, indicating fundamental dissimilarities in prey digestion capacities. High temperatures (20 degrees C) and large amplicons (383 bp) significantly decreased detection rates. The shortest amplicon gave the highest prey DNA detection success, whereas no differences were observed between the 231 bp and the 317 bp fragment. Our results indicate that factors such as ambient temperature, predator taxon and amplicon size should all be considered when interpreting data derived from PCR-based prey detection. Correction for such factors should make calculation of predation rates in the field more accurate and could help us to estimate when predation events occur in the field.     

Intraguild predation of the aphid parasitoid Aphelinus certus by Coccinella septempunctata and Harmonia axyridis

Coincidental intraguild predation is expected to be less disruptive to biological control than omnivorous intraguild predation, and strong intraguild predation is not expected to occur in natural systems. Coincidental intraguild predation in a foodweb involving introduced pest and natural enemy species was examined to determine whether intraguild predation would be disruptive of biological control services in soybean agroecosystems. Introduced natural enemies are important regulators of soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), populations in North America. Seven-spotted lady beetles, Coccinella septempunctata L., and multicolored Asian lady beetles, Harmonia axyridis Pallas (Coleoptera: Coccinellidae), are key predators of soybean aphid in North America while the chalcidoid wasp, Aphelinus certus Yasnosh (Hymenoptera: Aphelinidae), is the most common parasitoid of soybean aphid in Ontario, Canada. Predation of parasitized soybean aphids at two stages (newly parasitized aphids and mummified aphids) by adults and third instar larvae of both C. septempunctata and H. axyridis was examined in laboratory experiments. In choice experiments, all stages of lady beetles preferred non-parasitized aphids over mummified aphids. In cage experiments, third instar larvae and male and female adults of both lady beetles did not discriminate between newly parasitized and non-parasitized aphids. The influence of coincidental intraguild predation on the efficacy of parasitoids as biological control agents, and implications for soybean aphid management decisions based on natural enemies, are discussed.     

EVOLUTION OF AN APHID-PARASITOID INTERACTION: VARIATION IN RESISTANCE TO PARASITISM AMONG APHID POPULATIONS SPECIALIZED ON DIFFERENT PLANTS

The evolution of associations between herbivorous insects and their parasitoids is likely to be influenced by the relationship between the herbivore and its host plants. If populations of specialized herbivorous insects are structured by their host plants such that populations on different hosts are genetically differentiated, then the traits affecting insect-parasitoid interactions may exhibit an associated structure. The pea aphid (Acyrthosiphon pisum) is a herbivorous insect species comprised of genetically distinct groups that are specialized on different host plants (Via 1991a, 1994). Here, we examine how the genetic differentiation of pea aphid populations on different host plants affects their interaction with a parasitoid wasp, Aphidius ervi. We performed four experiments. (1) By exposing pea aphids from both alfalfa and clover to parasitoids from both crops, we demonstrate that pea aphid populations that are specialized on alfalfa are successfully parasitized less often than are populations specialized on clover. This difference in parasitism rate does not depend upon whether the wasps were collected from alfalfa or clover fields. (2) When we controlled for potential differences in aphid and parasitoid behavior between the two host plants and ensured that aphids were attacked, we found that pea aphids from alfalfa were still parasitized less often than pea aphids from clover. Thus, the difference in parasitism rates is not due to behavior of either aphids or wasps, but appears to be a physiologically based difference in resistance to parasitism. (3) Replicates of pea aphid clones reared on their own host plant and on a common host plant, fava bean, exhibited the same pattern of resistance as above. Thus, there do not appear to be nutritional or secondary chemical effects on the level of physiological resistance in the aphids due to feeding on clover or alfalfa, and therefore the difference in resistance on the two crops appears to be genetically based. (4) We assayed for genetic variation in resistance among individual pea aphid clones collected from clover fields and found no detectable genetic variation for resistance to parasitism within two populations sampled from clover. This is in contrast to Henter and Via's (1995) report of abundant genetic variation in resistance to this parasitoid within a pea aphid population on alfalfa. Low levels of genetic variation may be one factor that constrains the evolution of resistance to parasitism in the populations of pea aphids from clover, leading them to remain more susceptible than populations of the same species from alfalfa.     

Intraguild interference and biocontrol effects of generalist predators in a winter wheat field

Arable land typically harbours communities of polyphagous invertebrate natural enemies, among them numerous soil-surface dwelling predators such as ground beetles (Carabidae) and spiders (Lycosidae, Linyphiidae). Numbers of these predators were experimentally manipulated in a winter wheat field in order to study the predation impact of a generalist predator assemblage on herbivorous insects, the possible interferences among the predators concerned, and subsequent effects on wheat plant parameters. Removing ground beetles doubled numbers of Lycosidae indicative of intraguild interference between these two predator groups. Aphid densities were highest in carabid removal plots implying a substantial predation impact of ground beetles on the pest population. The predation impact of ground beetles was strongest earlier and disappeared later in the season. In mid-season, at intermediate aphid densities, the combined impact of carabid beetles and spiders appeared to be responsible for the reduction in aphid abundance. This result was probably due to a biomass effect rather than to a synergistic effect of the predator community. Thysanoptera decreased when spiders were removed (perhaps because spiders were preying on a predator of thrips), while Cicadellidae and Delphacidae showed no effect at all. The rise of aphid numbers in carabid removal plots corresponded to an increase in protein content of the wheat grains, while other plant parameters such as plant numbers and grain mass were not affected. In conclusion, this study provided field evidence for intraguild interference among generalist ground predators in arable land. Despite this interference the polyphagous predator community was able to depress numbers of aphids in winter wheat, a result cascading down to plant quality parameters.     

Varying degree of physiological integration among host instars and their endoparasitoid affects stress‐induced mortality

In natural populations of insect herbivores, genetic differentiation is likely to occur due to variation in host plant utilization and selection by the local community of organisms with which they interact. In parasitoids, engaging in intimate associations with their host during immature development, local variation may exist in host quality for parasitoid development. We compared the development of a gregarious endoparasitoid, Cotesia glomerata L. (Hymenoptera: Braconidae), collected in The Netherlands, in three strains and three caterpillar instars (L1–L3) of its main host, Pieris brassicae L. (Lepidoptera: Pieridae). Hosts had been collected in The Netherlands and France, and were reared in the laboratory for one generation. We also used an established Dutch laboratory strain that had not been exposed to parasitoids for at least 24 generations. Parasitoid survival to adulthood was inversely correlated with host instar at parasitism. Adult parasitoid body mass was largest when hosts were parasitized as L1 and smallest when hosts were parasitized as L3, whereas egg‐to‐adult development time was quickest on L3 hosts and slowest on L1 hosts. Higher survival and faster development of C. glomerata on French L2 hosts also showed that there is variation in host‐instar‐related suitability. Many L2 and most L3 caterpillars that were parasitized exhibited signs of pathogen infection and perished within a few days of parasitism, whereas this never happened when hosts were parasitized as L1 or in non‐parasitized control caterpillars. Our results reveal that, irrespective of the host strain, L1 hosts are optimally synchronized with C. glomerata development. By contrast, the high precocious mortality of L3 larvae may be due to stress‐induced regulation by the parasitoid in order to ‘force’ its developmental program into synchrony with the developing parasitoid larvae. Our results underscore a potentially important role played by pathogens in mediating herbivore–parasitoid interactions that are host‐instar‐dependent in their expression.