Tri-Trophic Effects of Seasonally Variable Induced Plant Defenses Vary across the Development of a Shelter Building Moth Larva and Its Parasitoid

Plant chemical defenses can negatively affect insect herbivore fitness, but they can also decrease herbivore palatability to predators or decrease parasitoid fitness, potentially changing selective pressures on both plant investment in production of chemical defenses and host feeding behavior. Larvae of the fern moth Herpetogramma theseusalis live in and feed upon leaf shelters of their own construction, and their most abundant parasitoid Alabagrus texanus oviposits in early instar larvae, where parasitoid larvae lay dormant for most of host development before rapidly developing and emerging just prior to host pupation. As such, both might be expected to live in a relatively constant chemical environment. Instead, we find that a correlated set of phenolic compounds shows strong seasonal variation both within shelters and in undamaged fern tissue, and the relative level of these compounds in these two different fern tissue types switches across the summer. Using experimental feeding treatments, in which we exposed fern moth larvae to different chemical trajectories across their development, we show that exposure to this set of phenolic compounds reduces the survival of larvae in early development. However, exposure to this set of compounds just before the beginning of explosive parasitoid growth increased parasitoid survival. Exposure during the period of rapid parasitoid growth and feeding decreased parasitoid survival. These results highlight the spatial and temporal complexity of leaf shelter chemistry, and demonstrate the developmental contingency of associated effects on both host and parasitoid, implying the existence of complex selective pressures on plant investment in chemical defenses, host feeding behavior, and parasitoid life history.     

To mate or not to mate,and subsequent host search by a haplodiploid female parasitoid wasp

Mated females of haplodiploid species can vary the sex ratio of their offspring, but virgin or sperm‐depleted females can produce only males. Depending on the costs and benefits, the theory of constrained sex allocation states that female haplodiploids may vary in their propensity to mate, with important implications for the populations’ sex ratio. Unmated female parasitoid wasps Alabagrus texanus (Braconidae) have been observed to reject matings with males, even under highly confined spatial conditions. We performed field trials to determine whether unmated female A. texanus would mate. We then compared the preferences of the resulting unmated (constrained) and mated (unconstrained) female wasps for shelters constructed and occupied by their hosts, fern moth caterpillars Herpetogramma theseusalis (Crambidae) on fern fronds. We presented the wasps with pairs of shelters collected from the field that differed in size, as well as freshness, fern species, and presence or absence of caterpillars. Unconstrained females searched small shelters more frequently than did constrained females and tended to favor senesced (old) shelters over fresher ones. They did not differ in choice between shelters on marsh and sensitive ferns or between shelters containing caterpillars and empty shelters. The preference for small shelters by the unconstrained wasps may enhance their rate of contacting favorable hosts, but the foraging regime of the constrained females should expose them to caterpillars at sites infrequently searched by the unconstrained females.     

Growth rate adjustment of two Drosophila parasitoids in response to the developmental stage of hosts

1. Generalist koinobiont parasitoids often exhibit high flexibility in their development; their larvae shorten or prolong the developmental period depending on the host quality at parasitisation. However, flexibility of the growth rate of parasitoid larvae has rarely been investigated so far. 2. This study investigated how the koinobiont parasitoid wasps Asobara japonica and Leptopilina ryukyuensis regulate their larval growth when they parasitise host Drosophila larvae with varying larval periods. 3. In both parasitoid species, the preimaginal period was longer when they parasitised 1‐day‐old larvae of Drosophila rufa than when they parasitised older larvae of D. rufa or when they parasitised larvae of Drosophila simulans, a species with a shorter larval period than D. rufa. After host pupariation, A. japonica accelerated its growth, thereby showing a biphasic growth curve. On the other hand, L. ryukyuensis did not accelerate its growth after host pupariation. 4. Growth retardation of parasitoid larvae in 1‐day‐old D. rufa larvae would contribute to avoiding excess growth before host pupariation, because the excess growth of parasitoid larvae would have negative effects on host growth. The growth rate acceleration of A. japonica after host pupariation suggests that they enhance resource utilisation in a host that has reached maximum body mass. It remains uncertain as to why L. ryukuensis does not show clear accelerated growth after host pupariation. Nonetheless, these results suggest that parasitoid larvae have the ability to detect the developmental stage of hosts in a species‐specific manner.     

Dynamic effects of parasitism by an endoparasitoid wasp on the development of two host species: implications for host quality and parasitoid fitness

1. The study reported here examined growth and developmental interactions between the gregarious larval koinobiont endoparasitoid Cotesia glomerata (Hymenoptera: Braconidae) and two of its hosts that vary considerably in growth potential: Pieris rapae and the larger P. brassicae (Lepidoptera: Pieridae). At pupation, healthy larvae of P. brassicae are over twice as large, in terms of fresh body mass, as those of P. rapae. 2. Clutch size of C. glomerata was manipulated artificially, and the relationship between parasitoid burden and the maximum weight of the parasitised host (= host–parasitoid complex) was measured. In both hosts, the maximum complex weight was correlated positively with parasitoid burden. Compared with unparasitised hosts, however, the growth of P. rapae was increased at significantly lower parasitoid burdens than in P. brassicae. Emerging wasp size was correlated negatively with parasitoid burden in both host species, whereas development time was less affected. 3. After larval parasitoid egress, the weight of the host carcass increased slightly, but not significantly, with parasitoid burden, although there was a strong correlation between the proportion of host mass consumed by C. glomerata larvae during development and parasitoid burden. 4. Clutch size was generally correlated positively with instar parasitised in both hosts, and greater in P. brassicae than in P. rapae. Sex ratios were much more female biased in L1 and L2 P. rapae than in all other host classes. Adult parasitoid size was correlated inversely with host instar at parasitism, and wasps emerging from P. brassicae were larger, and completed development faster, than conspecifics emerging from P. rapae. 5. The data reveal that parasitism by C. glomerata has profound species‐specific effects on the growth of both host species. Consequently, optimality models in which host quality is often based on host size at parasitism or unparasitised growth potential may have little utility in describing the development of gregarious koinobiont endoparasitoids. The results of this investigation are discussed in relation to the potential effectiveness of gregarious koinobionts in biological control programmes.     

Host discrimination of a larval parasitoid: the quick movement of Microplitis demolitor

Many parasitoids discriminate previously parasitised hosts from unparasitised ones to avoid mortality of offspring. Parasitoids that parasitise aggressive hosts such as lepidopteran larvae are known to attack hosts very quickly to avoid being attacked. However, little is known about host discrimination of such quick attacking parasitoids. We investigated host discrimination of Microplitis demolitor (Wilkinson) (Hymenoptera: Braconidae) a quick attacking parasitoid of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). Results showed that ratios of female wasps that rejected the hosts after antennal examination did not differ between parasitised and unparasitised hosts, indicating that M. demolitor did not discriminate hosts by antennal examination. However, 95% of females that inserted ovipositor into unparasitised hosts actually laid eggs, whereas it was only 31% for parasitised hosts, indicating that females discriminated hosts by oviposition insertion. Analyzing video recordings revealed that the ovipositor exploration of the host took 0.3 s. Female wasps that had experienced high-host density of unparasitised hosts readily rejected parasitised hosts, while wasps with experience of low host availability of parasitised hosts tended to accept parasitised hosts. This suggests that host discrimination behaviour of M. demolitor is affected by previous experience of different host availability and host quality.

     

Effect of host instar on host discrimination of heterospecific‐parasitised hosts by sympatric parasitoids

1. Interspecific competition among hymenopteran parasitoids may shape their behavioural strategies for host resource exploitation. In order to reduce or prevent competition, many parasitoid species have evolved the ability to discriminate between unparasitised hosts and hosts parasitised by another parasitoid species (i.e. heterospecific host discrimination). However, discriminatory ability might be affected by host instar. 2. This study reports the first results on whether host instar can influence the use of heterospecific‐parasitised hosts by sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae). 3. Aphytis melinus and Aphytis chrysomphali discriminated between unparasitised and heterospecific‐parasitised hosts when they found a third‐instar host (high quality), with a tendency to multi‐parasitise. However, this discrimination was not observed in the second instar (lower size). 4. The behavioural strategies adopted towards multi‐parasitise third‐instar hosts varied between both species. Aphytis chrysomphali reduced its clutch size in heterospecific‐parasitised hosts, whereas A. melinus tended to probe them for longer than healthy hosts. 5. Overall, our results highlight the importance of host instar in the study of intrinsic competition between parasitoids.     

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.     

The effect of host developmental stage at parasitism on sex‐related size differentiation in a larval endoparasitoid

• 1 For their larval development, parasitoids depend on the quality and quantity of resources provided by a single host. Therefore, a close relationship is predicted between the size of the host at parasitism and the size of the emerging adult wasp. This relationship is less clear for koinobiont than for idiobiont parasitoids.
• 2 As size differentiation in host species exhibiting sexual size dimorphism (SSD) is likely to occur already during larval development, in koinobiont larval endoparasitoids the size of the emerging adult may also be constrained based on the sex of the host caterpillar.
• 3 Sex‐specific growth trajectories were compared in unparasitised Plutella xylostella caterpillars and in second and fourth instar hosts that were parasitised by the solitary larval koinobiont endoparasitoid Diadegma semiclausum. Both species exhibit SSD, where females are significantly larger than males.
• 4 Healthy female P. xylostella caterpillars developed significantly faster than their male conspecifics. Host regulation induced by D. semiclausum parasitism depended on the instar attacked. Parasitism in second‐instar caterpillars reduced growth compared to healthy unparasitised caterpillars, whereas parasitism in fourth‐instar caterpillars arrested development. The reduction in growth was most pronounced in hosts producing male D. semiclausum.
• 5 Parasitism itself had the largest impact on host growth. SSD in the parasitoid is mainly the result of differences in growth rate of the parasitoid–host complex producing male and female wasps and differences in exploitation of the host resources. Female wasps converted host biomass more efficiently into adult biomass than males.

     

Four‐level interactions: herbivore use of ferns and subsequent parasitoid–hyperparasitoid performance

Abstract 1. Variables affecting species at the ends of trophic chains may modify the success of members with which they do not directly interact. The majority of such examples involve three trophic levels, but hyperparasitoids provide an excellent opportunity to examine four‐level relationships. 2. The gregarious hyperparasitoid Aprostocetus sp. (Hymenoptera: Eulophidae) commonly attacks the primary parasitoid Alabagrus texanus (Hymenoptera: Braconidae), by far the commonest parasitoid of the moth Herpetogramma theseusalis (Lepidoptera: Crambidae). 3. Larvae of this moth feed on ferns of two families, sensitive fern Onoclea sensibilis (Dryopteridaceae) and marsh fern Thelypteris palustris (Thelypteridaceae), in the study area, an old field in Maine, U.S.A. 4. I test the hypotheses that the ferns indirectly affect the reproductive success of the hyperparasitoids and that the ferns produce similar effects at intermediate links. 5. The moths experienced similar success on the two ferns, and the primary parasitoid performed similarly on moths reared from both ferns. The hyperparasitoid parasitized similar proportions of the primary parasitoid from moths that fed on sensitive fern and marsh fern. 6. However, hyperparasitoid broods on primary parasitoids from moths feeding on marsh fern contained approximately one‐third more offspring, whose individuals were significantly larger than those from sensitive fern, even though their hosts’ sizes did not differ significantly. 7. An indirect effect, related to the primary producers, thus strongly affected Trophic Level 4 in the absence of a significant effect at intermediate levels. To the best of my knowledge, this relationship has not been previously reported in a multi‐year or field‐based study of a natural system.     

Larval parasitism of the autumnal moth reduces feeding intensity on the mountain birch

Plants respond to grazing by herbivorous insects by emitting a range of volatile organic compounds, which attract parasitoids to their insect hosts. However, a positive outcome for the host plant is a necessary precondition for making the attraction beneficial or even adaptive. Parasitoids benefit plants by killing herbivorous insects, thus reducing future herbivore pressure, but also by curtailing the feeding intensity of the still living, parasitised host. In this study, the effect of parasitism on food consumption of the 5th instar larvae of the autumnal moth (Epirrita autumnata) was examined under laboratory conditions. Daily food consumption, as well as the duration of the 5th instar, was measured for both parasitised and non-parasitised larvae. The results showed that parasitism by the solitary endoparasitoid Zele deceptor not only reduced leaf consumption significantly but also hastened the onset of pupation in autumnal moth larvae. On the basis of the results, an empirical model was derived to assess the affects on the scale of the whole tree. The model suggests that parasitoids might protect the tree from total defoliation at least at intermediate larval densities. Consequently, a potential for plant–parasitoid chemical signalling appears to exist, which seems to benefit the mountain birch (Betula pubescens ssp. czerepanovii) by reducing the overall intensity of herbivore defoliation due to parasitism by this hymenopteran parasitoid.     

The roles of foraging environment,host species,and host diet for a generalist pupal parasitoid

Even for parasitoids with a wide host range, not all host species are equally suitable, and host quality often depends on the plant the host feeds on. We compared oviposition choice and offspring performance of a generalist pupal parasitoid, Pteromalus apum (Retzius) (Hymenoptera: Pteromalidae), on two congeneric hosts reared on two plant species under field and laboratory conditions. The plants contain defensive iridoid glycosides that are sequestered by the hosts. Sequestration at the pupal stage differed little between host species and, although the concentrations of iridoid glycosides in the two plant species differ, there was no effect of diet on the sequestration by host pupae. The rate of successful parasitism differed between host species, depending on the conditions they were presented in. In the field, where plant‐associated cues are present, the parasitoid used Melitaea cinxia (L.) over Melitaea athalia (Rottemburg) (Lepidoptera: Nymphalidae), whereas more M. athalia were parasitised in simplified laboratory conditions. In the field, brood size, which is partially determined by rate of superparasitism, depended on both host and plant species. There was little variation in other aspects of offspring performance related to host or plant species, indicating that the two host plants are of equal quality for the hosts, and the hosts are of equal quality for the parasitoids. Corresponding to this, we found no evidence for associative learning by the parasitoid based on their natal host, so with respect to these host species they are truly generalist in their foraging behaviour.     

Compound effects of induced plant responses on insect herbivores and parasitoids: implications for tritrophic interactions

1. Induced plant responses can affect herbivores either directly, by reducing herbivore development, or indirectly, by affecting the performance of natural enemies. Both the direct and indirect impacts of induction on herbivore and parasitoid success were evaluated in a common experimental system, using clonal poplar trees Populus nigra (Salicales: Salicaceae), the gypsy moth Lymantria dispar (L.) (Lepidoptera: Lymantriidae), and the gregarious parasitoid Glyptapanteles flavicoxis (Marsh) (Hymenoptera: Braconidae). 2. Female parasitoids were attracted to leaf odours from both damaged and undamaged trees, however herbivore‐damaged leaves were three times more attractive to wasps than undamaged leaves. Parasitoids were also attracted to herbivore larvae reared on foliage and to larval frass, but they were not attracted to larvae reared on artificial diet. 3. Prior gypsy moth feeding elicited a systemic plant response that retarded the growth rate, feeding, and survival of gypsy moth larvae, however induction also reduced the developmental success of the parasitoid. 4. The mean number of parasitoid progeny emerging from hosts fed foliage from induced trees was 40% less than from uninduced trees. In addition, the proportion of parasitised larvae that survived long enough to issue any parasitoids was lower on foliage from induced trees. 5. A conceptual and analytical model is provided to describe the net impacts of induced plant responses on parasitoids, and implications for tritrophic interactions and biological control of insect pests are discussed.     

Rearing temperature and parasitoid load determine host and parasitoid performance in Manduca sexta and Cotesia congregata

1. Temperature strongly influences the rates of physiological processes in insects, including the herbivore Manduca sexta and its larval endoparasitoid Cotesia congregata. Parasitisation by C. congregata decreases the growth and consumption of food by larval M. sexta. However, the effects of temperature on parasitised caterpillars and the developing wasp larvae are largely unknown. 2. In this study, parasitised and unparasitised caterpillars were reared at three constant temperatures (20, 25 and 30 °C) throughout larval development. Caterpillar mass gain and consumption were monitored daily until wandering (unparasitised control group) or wasp emergence (parasitised group) was observed. Development time and survival to emergence were measured as metrics of parasitoid performance. 3. Parasitised M. sexta developed more slowly than unparasitised controls, but had similar cumulative consumption until the terminal instar. Parasitised caterpillars with relatively large parasitoid loads had higher rates of consumption and growth than those with smaller loads. Both temperature and parasitoid load strongly affected wasp success. Mean development time to wasp emergence increased with low temperatures and with large loads. The combination of warm temperature and large parasitoid loads greatly reduced wasp survival. 4. These results demonstrate the interactive effects of rearing temperature and parasitisation on host consumption and growth rates throughout larval development. In addition, wasp performance was affected by the interaction of temperature and parasitoid load size. High temperatures alter the dynamics of the interaction between the parasitoid and its caterpillar host, which could have far-reaching impacts as the global temperatures continue to rise.     

Herbivore egg deposition induces tea leaves to arrest the egg‐larval parasitoid Ascogaster reticulata

Plants are able to activate direct and indirect defences against egg deposition by herbivorous insects. A known indirect defence is the production of synomones to help egg‐ and egg‐larval parasitoids to locate their hosts. The wasp Ascogaster reticulata Watanabe (Hymenoptera: Braconidae) is a solitary egg‐larval parasitoid of the moth Adoxophyes honmai Yasuda (Lepidoptera: Tortricidae), which lays eggs and feeds as caterpillars on the leaves of the tea plant Camellia sinensis (L.) Kuntze (Theaceae). Here, we studied whether or not oviposition by A. honmai induces tea plants to produce synomones that help the parasitoid to locate its host. An olfactometer bioassay suggested that synomones produced by the infested plants did not attract the parasitoid over a short range. However, a contact bioassay showed that tea leaves were induced to arrest the parasitoid 24 h after egg deposition and remained induced until the host‐egg masses were no more attractive to the parasitoids. Wing scales and deposits of adult moths and the contents of the egg masses did not induce the tea leaves to arrest the parasitoid, but the contents of the female moth's reproductive system did. Synomone induction was systemic: uninfested leaves in the vicinity of egg‐laden leaves also arrested the parasitoid.     

Host discrimination by the solitary endoparasitoid Dolichogenidea tasmanica (Hymenopotera: Braconidae)

Successful parasitism of a host partly depends on a female's assessment of its quality, including whether the host has already been parasitised or not. We conducted experiments to elucidate host discrimination by Dolichogenidea tasmanica (Hymenoptera: Braconidae). It is the most commonly collected parasitoid of light brown apple moth, Epiphyas postvittana (Lepidoptera: Tortricidae). To assess the rate of superparasitism avoidance by D. tasmanica, female wasps were given choices between (1) unparasitised hosts versus freshly self-parasitised hosts, (2) unparasitised hosts versus hosts at 24 h post-self-parasitisation and (3) freshly self-parasitised hosts versus hosts freshly parasitised by a conspecific female. Results confirm that host discrimination occurs in D. tasmanica. Females avoid laying eggs in hosts that have been parasitised by themselves or conspecifics, even though the frequency of first encounter with either an unparasitised or a parasitised host was the same for all choices. Thus, it appears that females are not able to discriminate the host parasitisation status prior to contacting a host, but host acceptance is not random. Host discrimination is time-dependent, with greater avoidance of superparasitism after 24 h. The ability of female D. tasmanica to distinguish healthy from parasitised hosts suggests that it could be an effective biological control agent in regulation of host populations. It should also ensure production efficiency in parasitoid mass-rearing.     

Histories of host shifts and cospeciation among free‐living parasitoids of Rhagoletis flies

Host shifts by specialist insects can lead to reproductive isolation between insect populations that use different hosts, promoting diversification. When both a phytophagous insect and its ancestrally associated parasitoid shift to the same novel host plant, they may cospeciate. However, because adult parasitoids are free living, they can also colonize novel host insects and diversify independent of their ancestral host insect. Although shifts of parasitoids to new insect hosts have been documented in ecological time, the long‐term importance of such shifts to parasitoid diversity has not been evaluated. We used a genus of flies with a history of speciation via host shifting (Rhagoletis [Diptera: Tephritidae]) and three associated hymenopteran parasitoid genera (Diachasma, Coptera and Utetes) to examine cophylogenetic relationships between parasitoids and their host insects. We inferred phylogenies of Rhagoletis, Diachasma, Coptera and Utetes and used distance‐based cophylogenetic methods (ParaFit and PACo) to assess congruence between fly and parasitoid trees. We used an event‐based method with a free‐living parasitoid cost model to reconstruct cophylogenetic histories of each parasitoid genus and Rhagoletis. We found that the current species diversity and host–parasitoid associations between the Rhagoletis flies and parasitoids are the primary result of ancient cospeciation events. Parasitoid shifts to ancestrally unrelated hosts primarily occur near the branch tips, suggesting that host shifts contribute to recent parasitoid species diversity but that these lineages may not persist over longer time periods. Our analyses also stress the importance of biologically informed cost models when investigating the coevolutionary histories of hosts and free‐living parasitoids.     

Herbivore species identity rather than diversity of the non‐host community determines foraging behaviour of the parasitoid wasp Cotesia glomerata

Extensive research has been conducted to reveal how species diversity affects ecosystem functions and services. Yet, consequences of diversity loss for ecosystems as a whole as well as for single community members are still difficult to predict. Arthropod communities typically are species‐rich, and their species interactions, such as those between herbivores and their predators or parasitoids, may be particularly sensitive to changes in community composition. Parasitoids forage for herbivorous hosts by using herbivore‐induced plant volatiles (indirect cues) and cues produced by their host (direct cues). However, in addition to hosts, non‐suitable herbivores are present in a parasitoid's environment which may complicate the foraging process for the parasitoid. Therefore, ecosystem changes in the diversity of herbivores may affect the foraging efficiency of parasitoids. The effect of herbivore diversity may be mediated by either species numbers per se, by specific species traits, or by both. To investigate how diversity and identity of non‐host herbivores influence the behaviour of parasitoids, we created environments with different levels of non‐host diversity. On individual plants in these environments, we complemented host herbivores with 1–4 non‐host herbivore species. We subsequently studied the behaviour of the gregarious endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) while foraging for its gregarious host Pieris brassicae L. (Lepidoptera: Pieridae). Neither non‐host species diversity nor non‐host identity influenced the preference of the parasitoid for herbivore‐infested plants. However, after landing on the plant, non‐host species identity did affect parasitoid behaviour, whereas non‐host diversity did not. One of the non‐host species, Trichoplusia ni Hübner (Lepidoptera: Noctuidae), reduced the time the parasitoid spent on the plant as well as the number of hosts it parasitized. We conclude that non‐host herbivore species identity has a larger influence on C. glomerata foraging behaviour than non‐host species diversity. Our study shows the importance of species identity over species diversity in a multitrophic interaction of plants, herbivores, and parasitoids.     

A chalcid wasp acts chiefly as a hyperparasitoid by mostly using small uncommon hosts

Although ovipositing insects may predominantly use resources that lead to high offspring quality, exceptions to this rule have considerably aided understanding of oviposition decisions. We report the frequency of host species use by a solitary facultative hyperparasitoid, Brachymeria subrugosa Blanchard (Hymenoptera: Chalcididae). In our samples, the wasp attacks the large pupae of the moth Gonioterma indecora Zeller (Lepidoptera: Elachistidae), as well as the considerably smaller, and rarer, pupae of two of its other parasitoids. Consistent with conditional sex allocation models, the wasp produced mainly female offspring on the largest (moth) host, an unbiased sex ratio on the middle‐sized (parasitoid) host, and only males on the smallest (parasitoid) host. Adult offspring size was correlated with the size of the host attacked. These features strongly suggest that the two smaller, primary parasitoid, hosts produce lower‐quality offspring. Despite being more common, the proportion of hosts from which parasitoids emerged was lowest (14%) on the largest host species, and highest on the rarer middle‐sized (34%) and smallest (30%) hosts. This suggests that costs or constraints on attacking high‐quality primary hosts may be a selective force favouring the evolution of hyperparasitism.     

Reproductive biology and related host-parasitoid interactions between the gypsy moth,Lymantria dispar and Glyptapanteles flavicoxis,a gregarious endoparasitoid*

The mean total progeny produced by female Glyptapanteles flavicoxis was significantly greater when the parasitoid attacked 4th instar gypsy moth rather than younger larvae. The development of parasitoids within older instars was faster than that in younger hosts. The proportion of female progeny was not altered by the host instar parasitised. The total reproductive output per female G. flavicoxis increased as the number of host larvae increased. However, since the reproductive output was spread among more available hosts, the reproductive output per host declined as the number of host larvae increased. There was no trend in the relationship between total progeny produced or the proportion of female progeny and the number of matings of parental females.     

Diamondback moth females oviposit more on plants infested by non‐parasitised than by parasitised conspecifics

Abstract 1. When offered a choice, female diamondback moths (Plutella xylostella) oviposited more eggs on plants with non‐parasitised conspecific larvae than on plants with parasitised larvae. 2. The leaf area consumed by parasitised larvae was significantly lower than that by non‐parasitised larvae. However, this quantitative difference in larval damage did not explain the female’s ability to discriminate between plants with parasitised and non‐parasitised larvae, as females showed an equal oviposition preference for plants infested by higher or lower densities of non‐parasitised larvae. 3. Pupal weight and duration of the larval stage of P. xylostella were independent of whether larvae were reared on plants that were previously infested by either non‐parasitised or parasitised larvae. 4. The larval parasitoid Cotesia vestalis did not distinguish between plants infested by non‐parasitised larvae and plants infested by larvae that had already been parasitised by conspecific wasps. 5. Based on these data, it can be concluded that the moth oviposition preference for plants infested by non‐parasitised conspecifics relative to plants infested by parasitised conspecifics was not explained by plant quality or by the attractiveness of plants towards wasps. It is hypothesised that one of the reasons for this preference is avoidance of plants where a relatively high risk of parasitism is expected due to the emergence of parasitoids from the parasitised host larvae.