Remarkable similarity in body mass of a secondary hyperparasitoid Lysibia nana and its primary parasitoid host Cotesia glomerata emerging from cocoons of comparable size

Lysibia nana is a solitary, secondary idiobiont hyperparasitoid that attacks newly cocooned pre-pupae and pupae of several closely related gregarious endoparasitoids in the genus Cotesia, including C. glomerata. Prior to oviposition, the female wasp injects paralysing venom into the host, thus preventing further development. Here, host fate, emerging hyperparasitoid mass, and egg-to-adult development time was compared in hosts parasitized at different ages over 24-h intervals. Cocoons of C. glomerata were parasitized by L. nana at 12, 36, 60, 84, and 108 h post-egression from the secondary host, Pieris brassicae. Hyperparasitoid survival exceeded 80% in hosts parasitized within the first 60 h after pupation, but dropped thereafter, with no hyperparasitoids emerging in hosts aged 108 h. The mass of hyperparasitoids was positively correlated with the mass of the host cocoon, and this relationship remained consistent in hosts up to 60 h old. Within each host age cohort, the mass of male and female wasps was not significantly different. Development time in L. nana was uniform in hosts up to 60 h old, but increased significantly in 84-h-old hosts, and male wasps completed their development earlier than female wasps. Regulation of host growth varied with the age of the host at parasitism, with the early growth of older hosts reduced much more dramatically than young hosts. Unlike most parasitoids, pupal hyperparasitoids do not make cocoons but instead pupate within the already prepared cocoon of the host parasitoid. Consequently, for a given mass of cocoon, newly emerged L. nana adults were remarkably similar in size with male and female adults of C. glomerata. This reveals that L. nana is extremely efficient at exploiting its primary parasitoid host.     

Does Manipulation by the Parasitoid Wasp Cotesia glomerata (L.) Cause Attachment Behaviour of Host Caterpillars on Cocoon Clusters?

Some parasitoid wasps appear to control the behaviour of their hosts. However, altered behaviours of parasitised hosts are not necessarily caused by parasitoids but are sometimes the result of traumatic side effects of parasitism. However, it was difficult for us to discriminate the cause of host's behaviours between manipulation by parasitoids and traumatic side effects. Larvae of the parasitoid wasp Cotesia glomerata form cocoon clusters after egression from the parasitised host caterpillar Pieris brassicae . Following parasitoid egression, host caterpillars survive for several days and remain near the cocoon clusters. These caterpillars may repel solitary pteromalid hyperparasitoid wasps, Trichomalopsis apanteloctena , that attempt to parasitise fresh C. glomerata pupae. We allowed hyperparasitoids to attack cocoon clusters in the field and laboratory and then assessed the costs and benefits to C. glomerata of attachment by the parasitised caterpillars. The eclosion success of C. glomerata in cocoon clusters with attached caterpillars was higher than that in clusters without attached caterpillars in both field and laboratory experiments. This difference was attributed to shorter hyperparasitoid visits to cocoon clusters with attached host caterpillars. However, large cluster size was potentially costly for host attachment, because the duration of host caterpillar attachment decreased with increasing numbers of C. glomerata per caterpillar. This trade-off may be related to shortages of fat body resources, which are shared between the development of wasp larvae and the survival of host caterpillars. Therefore, we concluded that caterpillar attachment satisfied some requirements of host manipulation by C. glomerata .     

Effects of relative humidity on cocoon formation and survival in the braconid wasp Cotesia glomerata

Abstract The effects of relative humidity (RH) on cocoon formation and survival in the braconid parasitoid wasp Cotesia glomerata (L.) (Hymenoptera: Braconidae) are investigated under various humidity conditions (50, 75, 90, 95 and 100% RH) at 20 °C and under an LD 16 : 8 h photoperiod. The mortality rate at the time of egression from hosts under 100% RH is significantly higher than for other RHs. Cocoon clusters formed at 100% RH spread significantly more than those formed at 50, 75, or 90% RH. Developmental periods differ significantly among RHs under which wasps developed. The mean period from the egression from hosts to adult emergence is 8.7 days when developed at 50–95% RHs, and 8.0 days at 100% RH. The emergence rates of C. glomerata that are maintained under the same humidity conditions after egression from hosts are not significantly different among RHs. However, emergence rates from cocoons that are transferred from 100% RH to 50 and 75% RH are < 70%, although the rates are > 90% in most cases. Some wasps do not emerge from cocoons: more than 60% die after adult eclosion at all RHs; the relative frequency of adult deaths is approximately 90% at 50% RH. Relative humidity influences the cluster and cocoon status strongly: both good clusters and cocoons are formed at low RHs. Emergence rates from cocoons of different ranks are significantly different: the rates of low‐rank cocoons are low at low RHs. The survival of C. glomerata is affected strongly by RH through cocoon formation.     

Comparing and contrasting development and reproductive strategies in the pupal hyperparasitoids Lysibia nana and Gelis agilis (Hymenoptera: Ichneumonidae)

In most animals, the optimal phenotype is determined by trade-offs in life-history traits. Here, I compare development and reproductive strategies in two species of solitary secondary hyperparasitoids, Lysibia nana and Gelis agilis, attacking pre-pupae of their primary parasitoid host, Cotesia glomerata. Parasitoid larvae of both species exploit a given amount of host resources with similar efficiency. However, adults exhibit quite different reproductive strategies. Both species are synovigenic, and female wasps emerge with no mature eggs. However, G. agilis must first host-feed to produce eggs, while L. nana does not host-feed but mobilizes internal resources carried over from larval feeding to initiate oogenesis. Further, G. agilis is wingless, produces large eggs, has a long life-span, and generates only small numbers of progeny per day, whereas these traits are reversed in L. nana. Given unlimited hosts, the fecundity curve in L. nana was “front-loaded,” whereas in G. agilis it was depressed and extended over much of adult life. In L. nana (but not G. agilis), wasps provided with honey but no hosts lived significantly longer than wasps provided with both honey and hosts. Differences in the fecundity curves of the two hyperparasitoids are probably based on differing costs of reproduction between them, with the wingless G. agilis much more constrained in finding hosts than the winged L. nana. Importantly, L. nana is known to be a specialist hyperparasitoid of gregarious Cotesia species that pupate in exposed locations on the food plant, whereas Gelis sp. attack and develop in divergent hosts such as parasitoid cocoons, moth pupae and spider egg sacs. Consequently, there is a strong match between brood size in C. glomerata and egg production in L. nana, but a mismatch between these parameters in G. agilis.     

Lifetime Reproductive Success in the Solitary Endoparasitoid, Venturia canescens

Parasitoid wasps have long been considered excellent organisms in studies examining the evolution of reproductive and life-history strategies. In examining the lifetime reproductive success of parasitoids in the laboratory, most investigations have provided the insects with excess hosts and food, where they exist in a relatively constraint-free environment. Importantly, these conditions may not accurately reflect the true heterogeneity of natural systems, where suitable hosts and food sources are likely to be limiting. This study examines the influence of differences in host and food availability on reproductive and life-history parameters in an asexual strain of the solitary endoparasitoid, Venturia canescens (Hymenoptera: Ichneumonidae). Lifetime reproductive success in V. canescens was measured in response to temporal variations in host and food (honey solution) access. Cohorts of parasitoids were provided with 200 fifth-instar larvae of the Indian meal moth, Plodia interpunctella (Lepidoptera: Pyralidae), and food for variable periods daily after eclosion. V. canescens is synovigenic, and host-deprived wasps continued to mature eggs over the first few days after eclosion until the egg storage capacity was reached in the oviducts. When these wasps were subsequently provided with hosts, oogenesis resumed and continued until later in adult life. Constantly fed wasps lived longer and produced more progeny than wasps from cohorts which were alternately fed and starved or were starved from eclosion. Moreover, wasps with constant host and food access produced most progeny early in life and usually experienced prolonged periods of postreproductive survival. In contrast, the reproductive period of wasps with limited host access was more evenly distributed throughout the adult life. Consequently, the cumulative progeny production by V. canescens with constant food access was fairly uniform irrespective of host availability. Longevity and fecundity in V. canescens were positively correlated with adult size. However, variable host access had little effect on the longevity of wasps which were constantly supplied with honey. Over the first 2 days of adult life, variation in food access also had no effect on progeny production by V. canescens. We argue that manipulating temporal host and food access to parasitoids in the laboratory more closely approximates natural conditions, where these resources are likely to be spatially separated. Moreover, our findings suggest that many highly synovigenic parasitoids like V. canescens, which produce microtype (=hydropic) eggs, have a considerably higher reproductive potential than ovary dissections have revealed. Our findings are discussed in relation to life-history evolution in the parasitic Hymenoptera.     

Opposing effects of mortality factors on progeny operational sex ratio may thwart adaptive manipulation of primary sex ratio

Despite extensive research on mechanisms generating biases in sex ratios, the capacity of natural enemies to shift or further skew operational sex ratios following sex allocation and parental care remains largely unstudied in natural populations. Male cocoons of the sawfly Neodiprion abietis (Hymenoptera: Diprionidae) are consistently smaller than those of females, with very little overlap, and thus, we were able to use cocoon size to sex cocoons. We studied three consecutive cohorts of N. abietis in six forest stands to detect cocoon volume‐associated biases in the attack of predators, pathogens, and parasitoids and examine how the combined effect of natural enemies shapes the realized operational sex ratio. Neodiprion abietis mortality during the cocoon stage was sex‐biased, being 1.6 times greater for males than females. Greater net mortality in males occurred because male‐biased mortality caused by a pteromalid parasitic wasp and a baculovirus was greater and more skewed than female‐biased mortality caused by ichneumonid parasitic wasps. Variation in the susceptibility of each sex to each family of parasitoids was associated with differences in size and life histories of male and female hosts. A simulation based on the data indicated that shifts in the nature of differential mortality have different effects on the sex ratio and fitness of survivors. Because previous work has indicated that reduced host plant foliage quality induces female‐biased mortality in this species, bottom‐up and top‐down factors acting on populations can affect operational sex ratios in similar or opposite ways. Shifts in ecological conditions therefore have the potential to alter progeny fitness and produce extreme sex ratio skews, even in the absence of unbalanced sex allocation. This would limit the capacity of females to anticipate the operational sex ratio and reliably predict the reproductive success of each gender at sex allocation.     

Egg resorption behaviour by the solitary egg parasitoid Anaphes nitens under natural conditions

We investigated under outdoor conditions and inside a climate chamber: (i) whether Anaphes nitens Girault (Hymenoptera: Mymaridae), an egg parasitoid of the Eucalyptus snout beetle, Gonipterus scutellatus Gyllenhal (Coleoptera: Curculionidae), experienced egg resorption, and (ii) how various treatments (location, food, and/or host supply) and body size influenced egg load, egg resorption, fecundity, and longevity. One‐day‐old females were mated and randomly assigned to five groups: (A) honey + hosts, (B) water + hosts, (C) only honey, (D) only water, and (E) control females killed at emergence. We compared the egg load of the newly emerged females, which represent the control group (egg load = ovarian eggs present at emergence), with the lifetime egg load (i.e., ovarian eggs + emerged offspring + not emerged offspring) of the females with various host and diet treatments, by dissection of the ovaries to find evidence of egg resorption. All groups reared outdoors had fewer eggs than the control, while indoors there was no significant difference. Outdoors, starved but host‐provided wasps (B) experienced the highest reduction of the lifetime egg load (51%). Groups without access to food (B + D) resorbed more eggs than groups provided with honey (A + C). Females with honey and hosts (A) had the highest lifetime fecundity, but those with water and hosts (B) showed a higher daily realized fecundity. Host‐deprived females with access to food (C) attained the longest lifespan. Our results suggest that under stress conditions, such as low temperature and food shortage, A. nitens females practice egg resorption, probably to save energy.     

Behavioral responses of the parasitoid Melittobia digitata to volatiles emitted by its natural and laboratory hosts

Responses of macropterous females of the ectoparasitoid Melittobia digitata Dahms (Hymenoptera: Eulophidae) to direct and indirect cues emitted by its natural hosts as well as laboratory hosts were investigated using a Y‐tube olfactometer. To locate the nest of mud dauber wasps, Trypoxylon politum Say (Hymenoptera: Crabronidae), and one of their inquilines, Anthrax spec., parasitoids exploit volatiles from the freshly built nest mud and the empty cocoon constructed by the wasps, as well as their meconium. However, the parasitoids did not respond to odors emitted by older nest mud or by the host stages that are attacked (T. politum prepupae and Anthrax spec. larvae). Melittobia digitata was not attracted to direct volatiles released by the dipteran hosts Anastrepha ludens Loew (Diptera: Tephritidae) (a natural host) and Sarcophaga bullata (Parker) (Diptera: Sarcophagidae) (a laboratory host). Based on our results, we suggest that M. digitata adopts a ‘sit and wait’ strategy to locate mud dauber wasps, relying mainly on indirect host‐related cues: females search for nests that are under construction and once found, they wait inside the cell until the host completes its cocoon and releases meconium, an indicator that is associated with host suitability. No attraction was found to dipteran hosts, suggesting that parasitization of these hosts may be incidental, due to the broad host plasticity of Melittobia wasps.     

Superparasitism in the solitary ectoparasitoid Aptesis nigrocincta: the influence of egg load and host encounter rate

Theory predicts that the acceptance of hosts already parasitized by a conspecific will depend both on egg load and the availability of hosts. In the present laboratory study, we tested the effect of egg load and host encounter rate on the propensity of superparasitism in the solitary parasitoid Aptesis nigrocincta Gravenhorst (Hymenoptera, Ichneumonidae), a synovigeneous ectoparasitoid of prepupae of the European Apple Sawfly. Parasitoid females carry few voluminous eggs at a time and the egg maturation rate is less than one egg per day. Egg load was manipulated by giving females access to hosts one week prior to the start of treatments and host availability by giving females access to either one host cocoon every day or every other day. In the first treatment where females had a high egg load of 5.3 egg in their ovaries and encountered host cocoons at low rates, we found that parasitized hosts were accepted to the same degree as healthy hosts. In females with significantly decreased egg load (3.8 eggs) encountering hosts at the same rate we found a slight but non-significant decrease in the acceptance of parasitized hosts compared with healthy hosts. In contrast, A. nigrocincta females accepted significantly fewer parasitized hosts at a high host encounter rate that would lead them to the point of egg limitation in the near future. Within the range of egg loads tested, the host encounter rate appears to be the most important determinant for a females decision to oviposit onto hosts already parasitized by a conspecific.     

The effect of different dietary sugars and honey on longevity and fecundity in two hyperparasitoid wasps

In nature adult insects, such as parasitic wasps or 'parasitoids' often depend on supplemental nutritional sources, such as sugars and other carbohydrates, to maximize their life-expectancy and reproductive potential. These food resources are commonly obtained from animal secretions or plant exudates, including honeydew, fruit juices and both floral and extra-floral nectar. In addition to exogenous sources of nutrition, adult parasitoids obtain endogenous sources from their hosts through 'host-feeding' behavior, whereby blood is imbibed from the host. Resources obtained from the host contain lipids, proteins and sugars that are assumed to enhance longevity and/or fecundity. Here we conducted an experiment exploring the effects of naturally occurring sugars on longevity and fecundity in the solitary hyperparasitoids, Lysibia nana and Gelis agilis. Although both species are closely related, L. nana does not host-feed whereas G. agilis does. In a separate experiment, we compared reproduction and longevity in G. agilis reared on either honey, a honey-sugar 'mimic', and glucose. Reproductive success and longevity in both hyperparasitoids varied significantly when fed on different sugars. However, only mannose- and water-fed wasps performed significantly more poorly than wasps fed on four other sugar types. G. agilis females fed honey produced twice as many progeny as those reared on the honey-sugar mimic or on glucose, whereas female longevity was only reduced on the mimic mixture. This result shows not only that host feeding influences reproductive success in G. agilis, but also that non-sugar constituents in honey do. The importance of non-sugar nutrients in honey on parasitoid reproduction is discussed.     

Role of the cocoon ofAcrolepiopsis assectella (Lep., Hyponomeutoidae) in host recognition by the parasitoidDiadromus pulchellus (Hym., Ichneumonidae)

The study of various behavioural criteria of femaleDiadromus pulchellus parasitoids in the presence of theirAcrolepiopsis assectella hosts has shown the essentially chemical nature of the stimulant determining host recognition. The physical stimuli of the cocoon seem not to be implicated. Thus cocoons whose original texture has been completely altered, either mechanically or chemically, as well as the silk excreted by the host caterpillars significantly stimulates the female parasitoids. The cocoon contact kairomones are detected in testing the aqueous extracts ofA. assectella which provoke a positive behavioural response from the females in a threshold concentration of 1 cocoon-equivalent. The comparison of aqueous extracts ofA. assectella host cocoons, and of non-host species:Bombyx mori, Ephestia kuehniella andCacoecimorpha pronubana demonstrate the kairomone specificity of the silk, the extracts of non-host cocoons being ignored by the parasitoid, as were the silk threads left byE. kuehniella caterpillars. Finally the contact kairomones linked to the silk seem to be independent of the host plant and of the nutrient diet of the host caterpillars. The cocoons spun by the caterpillars reared on leeks or on artificial diets with or without powdered leek provoke similar responses in the parasitoids.     

Relationships between parasitoid host range and host defence: a comparative study of egg encapsulation in two related parasitoid species

Abstract. Parasitoid host range may proceed from traits affecting host suitability, traits affecting parasitoid foraging behaviour, or both. We tested the hypothesis that encapsulation can be used as a reliable indicator of parasitoid host range in two closely related larval endoparasitoids of Lepidoptera. Cotesia glomerata (L.) (Hymenoptera: Braconidae) is gregarious and a generalist on several species of Pieridae, whereas C. rubecula (Marshall) is solitary and specific to Pieris rapae (L.). We determined the effects of host species ( Pieris brassicae (L.), P. napi (L.) and P. rapae ) (Lepidoptera: Pieridae) and host developmental stage (early first, second and third instar) on encapsulation of parasitoid eggs. Host species and parasitoid species, as well as the resulting interaction between these two factors had significant effects on encapsulation of Cotesia eggs. Encapsulation in Pieris hosts was much lower for C. glomerata (<34%, except for second and third instar of P. rapae ) than for C. rubecula (>32%), even when the latter was parasitizing P. rapae. Encapsulation increased with the age of the larvae, although the only significant difference was for C. glomerata. Overall, P. rapae showed a stronger encapsulation reaction than P. brassicae and P. napi. Encapsulation levels of C. glomerata corresponded well to patterns of female host species and host age preference for oviposition and parasitoid larval performance. In contrast, percentages of encapsulation of C. rubecula were not consistent with host preference and host suitability. We argue that encapsulation alone is unlikely to provide a sufficient explanation for C. glomerata and C. rubecula host range.     

Parasitoid complex of the pistachio twig borer moth,Kermania pistaciella,in Iran

The parasitoid complex of the pistachio twig borer moth, Kermania pistaciella Amsel (Lepidoptera: Tineidae), a native pest of pistachio trees, was investigated at 27 pistachio plantation sites in Kerman province, the major pistachio growing area of Iran. The present study was conducted to document the naturally established parasitoid complex and to assess the need for improving the biological control of this species. In total, 22,390 moth cocoons were collected from 186 samples collected from commercial orchards during 2006–2008 and kept singly in controlled conditions to rear immature insects. An average of 2.8% of moth cocoons had been attacked by predators at time of sampling. Of the collected cocoons, on average 46.7% completed development and emerged as adult moths, no insects emerged from 8%, suggesting that the moth or wasp died before maturing, and parasitoids emerged from the remaining 42.5%. The overall percentage of host cocoons from which wasps emerged ranged from 25.6 to 59%. Fifteen hymenopterous parasitoid species were recovered from cocoons, of which three species were primary parasitoids, two were obligatory hyperparasitoids and the remaining 10 species were facultative hyperparasitoids. The primary parasitoid, Chelonus kermakiae (Tobias) (Hymenoptera: Braconidae), was the most abundant comprising almost 85% of the total emerging parasitoids. In addition, a further four species of larval parasitoid developed within the PTBM's larval tunnels in pistachio fruit cluster-stem tissue. Conservation of these parasitoids in the pistachio growing areas is recommended since a high level of parasitized moths’ cocoons was found at the majority of experimental sites.     

Contrasting patterns of ovarian development and oogenesis in two sympatric host-feeding parasitoids,Diglyphus isaea and Neochrysocharis formosa (Hymenoptera: Eulophidae)

Diglyphus isaea (Walker) and Neochrysocharis formosa (Westwood) (Hymenoptera: Eulophidae) are common idiobiont parasitoids of leafminers attacking vegetable crops. They exhibit differing levels of synovigeny, and host feeding enhances their fecundity and longevity. The reproductive systems of these two parasitoids are typical of hymenopteran eulophids, consisting of two ovaries, each usually comprising three polytrophic meroistic ovarioles. Diglyphus isaea possesses two obvious oviduct accessory glands, which are absent in N. formosa. Both parasitoids underwent oosorption when starved, while feeding on host larvae promoted oogenesis and egg maturation. In both, oogenesis and vitellogenesis commenced on the first day of the pupal stage rather than after eclosion. Formation of ovarioles in D. isaea commenced 1 day earlier than in N. formosa. Mature eggs were rarely observed in ovaries of newly emerged D. isaea, but usually a few were present in N. formosa. When hosts (second–third instar Liriomyza sativae larvae) were provided, the number of mature eggs in D. isaea ovaries initially increased and then stabilized, while in N. formosa, the number first increased and then decreased. Diglyphus isaea had fewer but larger eggs than N. formosa did. Thus, synovigenic divergence begins at the pupal stage and may result in different life-history traits of adults.     

Genetic relationship between olfactory response and fitness in Cotesia glomerata (L.)

The flight response of the parasitic wasp Cotesia glomerata (L.) to semiochemicals from a plant-host complex is subject to genetic variation. The significance of additive genetic variance for the odour-guided behaviour has been demonstrated by bidirectional selection. In order to understand the potential and constraints for phenotypic evolution in olfactory response under the pressure of natural selection, this study was to investigate genetic covariation between the odour-guided behaviour and life-history traits and its genetic correlation with the efficiency of parasitism. A paternal half-sib analysis revealed that there was no significant genetic correlation between this behavioural character and any of three life-history traits examined (the development time of immature stages, the body size of female wasps, the number of female wasps per brood). Comparisons between the selected high and low olfactory-response strains showed the lack of correlated responses in these life-history traits to bidirectional selection on the odour-guided behaviour. On the other hand, genotypic differences in the ability of olfactory response significantly affected the efficiency of parasitism. In comparison with the low olfactory-response strain, female wasps from the high olfactory-response strain were able to parasitize more host larvae in a wider area of habitats. This study provides the first evidence of links between olfactory response and population success in parasitoids from a genetic perspective.     

Plant Species Loss Affects Life-History Traits of Aphids and Their Parasitoids

The consequences of plant species loss are rarely assessed in a multi-trophic context and especially effects on life-history traits of organisms at higher trophic levels have remained largely unstudied. We used a grassland biodiversity experiment and measured the effects of two components of plant diversity, plant species richness and the presence of nitrogen-fixing legumes, on several life-history traits of naturally colonizing aphids and their primary and secondary parasitoids in the field. We found that, irrespective of aphid species identity, the proportion of winged aphid morphs decreased with increasing plant species richness, which was correlated with decreasing host plant biomass. Similarly, emergence proportions of parasitoids decreased with increasing plant species richness. Both, emergence proportions and proportions of female parasitoids were lower in plots with legumes, where host plants had increased nitrogen concentrations. This effect of legume presence could indicate that aphids were better defended against parasitoids in high-nitrogen environments. Body mass of emerged individuals of the two most abundant primary parasitoid species was, however, higher in plots with legumes, suggesting that once parasitoids could overcome aphid defenses, they could profit from larger or more nutritious hosts. Our study demonstrates that cascading effects of plant species loss on higher trophic levels such as aphids, parasitoids and secondary parasitoids begin with changed life-history traits of these insects. Thus, life-history traits of organisms at higher trophic levels may be useful indicators of bottom-up effects of plant diversity on the biodiversity of consumers.     

How two different host species influence the performance of a gregarious parasitoid: host size is not equal to host quality

1. Hyssopus pallidus Askew (Hymenoptera, Eulophidae) is a gregarious ectoparasitoid of the two tortricid moths species Cydia molesta Busck and C. pomonella L. (Lepidoptera, Tortricidae). It paralyses and parasitizes different larval instars of both species inside the apple fruit, which leads to the death of the caterpillar. 2. We assessed the influence of host species characteristics and host food on the performance of the parasitoid female in terms of clutch size decisions and fitness of the F(1) generation. 3. A comparison of clutch size revealed that female parasitoids deposited similar numbers of eggs on the comparatively smaller C. molesta hosts as on the larger C. pomonella hosts. The number of parasitoid offspring produced per weight unit of host larva was significantly higher in C. molesta than in C. pomonella, which is contrary to the general prediction that smaller hosts yield less parasitoid offspring. However, the sex ratio was not influenced by host species that differed considerably in size. 4. Despite the fact that less host resources were available per parasitoid larva feeding on C. molesta caterpillars, the mean weight of emerging female wasps was higher in the parasitoids reared on C. molesta. Furthermore, longevity of these female wasps was neither influenced by host species nor by the food their host had consumed. In addition we did not find a positive relationship between adult female weight and longevity. 5. Parasitoid females proved to be able to assess accurately the nutritional quality of an encountered host and adjust clutch size accordingly. These findings indicate that host size is not equal to host quality. Thus host size is not the only parameter to explain the nutritional quality of a given host and to predict fitness gain in the subsequent generation.     

Effect of host availability on reproduction and survival of the parasitoid wasp Trichogramma minutum

Abstract.

• 1 We tested the hypothesis that females of the egg parasitoid, Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae), could adjust their fecundity schedule according to host availability and that there was a negative correlation between reproduction and survival in these wasps.
• 2 Newly-emerged females were provided with an unlimited or limited number of hosts in the first trial and with either unlimited, limited or zero hosts in the second trial.
• 3 When hosts were unlimited, wasps had the highest rate of reproduction in the first day, which decreased dramatically thereafter. When hosts were limited, wasps from the two trials differed in their response. In Trial I, females with limited hosts had lower first-day fecundity than, and the same subsequent-day fecundity as, those with unlimited hosts. However, in Trial II, females with limited host had a lower first-day but a higher subsequent-day fecundity than those with unlimited hosts. This indicates variation in Trichogramma's ability to shift its fecundity schedule in response to host availability.
• 4 There was a positive (rather than a negative) correlation between reproduction and survival. Wasps that oviposited (in host-unlimited treatment) had greater longevity than those that could not (in host-unavailable treatment).
• 5 The sex ratio of the progeny produced by wasps in both host-unlimited and limited treatments shifted gradually from a female to a male bias as the wasps aged.
• 6 We consider the ability of parasitoids to adjust their fecundity schedule as an adaptation to changing host resources and discuss our findings with regard to theories of life history evolution.

     

State dependent superparasitism in a solitary parasitoid: egg load and survival

In solitary parasitoids, superparasitism (the allocation ofan egg to an already parasitized host) has a payoff, measuredin offspring produced and costs, measured in eggs and time invested.Solitary parasitoids that are capable of host discriminationmust adopt the strategy that ensures the best use of both theiregg load and available lifetime. In this paper, we develop astate-dependent model defining the optimal strategy of superparasitismfor a solitary parasitoid species with overlapping generations.The fitness measure we use is based on the growth rate of thenumber of genotype copies. The model predicts that the tendencyto superparasitize should increase as the egg load of the parasitoidincreases, or as its life expectancy decreases. The model alsopredicts that under particular conditions wasps should showpartial preferences for parasitized hosts. These predictionswere tested with the parasitoid Venturia canescens (Hymenoptera:Ichneumonidae). The tendency of the wasps to superparasitizein the presence of both healthy and parasitized hosts was correlatedto egg load and access to food before the experiment A complementaryexperiment, where parasitized hosts were given sequentiallyto parasitoids, showed that V. canescens exhibits partial preferencestoward superparasitism. These experimental results and a previouswork support the predictions of the model.     

Pre‐Patch Experience Affects the Egg Distribution Pattern in a Polyembryonic Parasitoid of Moth Egg Batches

According to foraging theory, female parasitoids should alter their host choice in response to cues that indicate a limitation of resources. We tested whether females of the polyembryonic parasitoid Ageniaspis fuscicollis (Hymenoptera: Encyrtidae), which attack egg batches of small ermine moths (Lepidoptera: Yponomeutidae), would alter their host acceptance pattern in response to different pre‐patch experience. We kept females of the parasitoid prior to a patch visit under different conditions, which should indicate different levels of competition for hosts. With increased competition as pre‐patch experience, females laid more eggs per host egg and self‐superparasitized more often, and the resultant egg distributions showed a trend from more regular distributions to increasingly Poisson and aggregated distributions. Consequently, females with a pre‐patch experience that would indicate low competition for hosts had the most even egg distributions. We conclude that pre‐patch experience of competitors may lead to a significant change of mutual interference patterns in egg‐laying A. fuscicollis wasps.