Abstract: We tested whether a plant's life time seed production is increased by parasitization of herbivores in a tritrophic system, Arabidopsis thaliana (Brassicaceae) plants, Pieris rapae (Lepidoptera: Pieridae) caterpillars and the solitary endoparasitoid Cotesia rubecula (Hymenoptera: Braconidae). We established seed production for intact A. thaliana plants, plants that were mechanically damaged, plants fed upon by parasitized caterpillars and plants fed upon by unparasitized caterpillars. In the first experiment, with ecotype Landsberg (erecta mutant), herbivory by unparasitized P. rapae caterpillars resulted in a strongly reduced seed production compared to undamaged plants. In contrast, damage by P. rapae caterpillars that had been parasitized by C. rubecula did not result in a significant reduction in seed production. For the second experiment with the ecotype Columbia, the results were identical. Plants damaged by unparasitized caterpillars only produced seeds on regrown shoots. Seed production of plants that had been mechanically damaged was statistically similar to that of undamaged plants. Production of the first ripe siliques by plants fed upon by unparasitized caterpillars was delayed by 18–22 days for Landsberg and 9–10 days for Columbia. We conclude that parasitization of P. rapae by C. rubecula potentially confers a considerable fitness benefit for A. thaliana plants when compared to plants exposed to feeding damage by unparasitized P. rapae larvae. Plants that attract parasitoids and parasitoids that respond to herbivore-induced plant volatiles will both experience selective advantage, justifying the use of the term mutualism for this parasitoid-plant interaction. This type of mutualism is undoubtedly very common in nature.

Abstract: Summary 1. Quantitative food webs were constructed describing the community of leafmining moths in the genus Phyllonorycter (Gracillariidae) and their parasitoids at a site in southern England. 2. Twelve species of Phyllonorycter were reared from four species of tree. The moths were attacked by 27 species of hymenopterous parasitoids. 3. Four webs were constructed for successive moth generations over 2 years. A summary web was built and a comparison made between webs expressed in terms of number of mines and number of females recruiting to the next generation. 4. All hosts and most parasitoids were recorded in the first year, but new host–parasitoid associations were discovered throughout the sampling period. The diversity of mines and parasitoids was similar across generations. 5. The structure of the parasitoid community was strongly influenced by the host plant. 6. The extent of potential indirect interactions between hosts was assessed by constructing quantitative parasitoid overlap diagrams. The measure of influence used was the importance of one species as a source of parasitoids attacking the other. The greatest potential for apparent competition was between species of host attacking the same species of plant.

Abstract: Several studies have documented associative learning in insects, but the adaptive value of such learning is not yet well understood. To evaluate this issue, we quantified long-term fitness consequences of associative learning in the parasitoid wasp, Biosteres arisanus. We compared individual wasps that were allowed to choose host substrate based on experience (‘‘learning’’ wasps) to wasps that could only make random substrate choice (‘‘random’’ wasps) in an environment where only one out of two substrates contained host eggs. In two experiments, the average number of host eggs parasitized and offspring produced were significantly larger for learning than for random wasps. Our results allow detailed examination of the conditions under which learning would have positive fitness effects in ecological systems similar to ours. These conditions include relatively long search duration for hosts; the ability to remember a learned preference over extended periods of interfering activities; and large mean differences between alternatives, and small variances, which together allow rapid evaluation of alternatives and long duration of exploiting the superior one. Key words: Biosteres arisanus, decision making, fitness, learning, parasitoid wasps. [Behav Ecol 11: 536–543 (2000)]

Abstract: Ficus and their species-specific pollinator wasps (Agaonidae) form a remarkable plant–insect obligate mutualism Each monoecious fig species also shelters a community of nonpollinating chalcids, composed of both gallmakers and parasitoids The few previous studies that took these species into account aimed at determining their potential effect on the mutualism To go further, we argue that the fig wasps represent an interesting model for studies of community ecology Figs and fig wasps of different groups were studied in Cote d'Ivoire to quantify local ecological interactions Regardless of their fig host or taxonomical position, we identified three ecological groups of nonpollinating fig wasps on the basis of their timing of oviposition in the fig and on their oviposition behavior Pollinating and nonpollinating fig wasps were shown to interact intimately, and to have a significant effect on the host plant in at least some cases Each species of the community seems to preferentially oviposit in a part of the female flowers, depending on their depth inside the fig Convergence in fig exploitation thus exists in various chalcid lineages, and the community structure and organization are very similar between fig species Concepts developed in parasitoid community ecology could now be tested on this promising model

Abstract: Naive female Diachasmimorpha longicaudata (Ashmead), a solitary endoparasitoid of tephritid fruit flies, exhibited positive responses toward volatiles of host fruits in olfactometer and wind tunnel bioassays. Although no significant preference for one of the test fruits, mango, Mangifera indica L., or grapefruit, Citrus paradisi Macfaden, was observed, the number of flights in the wind tunnel was higher in the presence of mangos. In the olfactometer trials, parasitoids preferred fly infested over noninfested grapefruits, and infested over noninfested mangos. Reproductive performance bioassays were conducted using Anastrepha ludens (Loew) larvae (Diptera: Tephritidae) reared in grapefruit, orange, mango, or artificial diet, and Anastrepha obliqua (Macquart) larvae reared in mango as parasitoid hosts. Parasitoid performance was compared for 2 Anastrepha species and 3 fruit species. Significant effects of fruit fly species and of the diet of fruit fly larvae on longevity, size, and progeny production of D. longicaudata were observed. Anastrepha ludens reared in grapefruit was the best host in terms of offspring longevity, size, and number of female progeny, but parasitoids that developed in A. ludens reared in mango had higher overall fecundity. Anastrepha ludens reared in mango was a better host than A. obliqua in the same fruit. No correlation between parasitoid size and demographic parameters was found. The results of this laboratory study showed that host preference and offspring performance are partially related.

Abstract: Summary 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.

Abstract: The functional response and the effect of superparasitism of Diachasmimorpha longicaudata (Ashmead) in larvae of Anastrepha ludens (Loew) was determined under laboratory conditions. Adult parasitoids were tested individually and in groups of 5. Third-instar A. ludens larvae were exposed for 3 h to experienced, 5-d-old females at the ratios of 1, 5, 20, 30, 40, 50, and 60 host larvae per wasp. For individual females, functional response was type III, whereas for females in groups, a type II curve was observed. In the presence of conspecifics, females increased their parasitization activity. Females showed a strong tendency for self-superparasitism (the same female laying >1 egg in the same host) with a range of 38.9-57.9% of larvae superparasitized, even when there was no competition and a high availability of host larvae. In the superparasitism experiment, when the parasitoid/host ratio was 1 or greater, larval mortality was consistently high but never exceeded 90%. Successful parasitoid emergence decreased as the parasitoid/host ratio increased. We discuss and question the detrimental effects that have been attributed to superparasitism in solitary endoparasitoids.

Abstract: Parasitoid wasps have long been favored organisms for fundamental studies on reproductive strategies and life-history evolution. Progeny allocation models designed with parasitoids in mind assume that offspring develop by consuming most or all of the resources available from a single host, and that size is the most important factor affecting offspring fitness. Many parasitoids exhibit host usage patterns consistent with these assumptions, but our recent observations suggested that endoparasitic wasps in the family Braconidae often do not. To investigate how differences in host usage patterns might affect developmental strategies, we compared two related braconids with contrasting host usage patterns. Apanteles carpatus consumed virtually all host tissues during immature development, whereas Microplitis demolitor fed exclusively on host hemolymph and consumed a relatively small proportion of available host resources. Development time of M. demolitor was unaffected by host size, whereas development time of A. carpatus was much longer in small hosts than in large hosts. On the other hand, offspring size in M. demolitor correlated strongly with host size, but it correlated only weakly with host size in A. carpatus. Our results collectively suggest that selection has favored rapid development at the potential cost of reduced size in M. demolitor, and increased size at the potential cost of increased development time in A. carpatus. Tissue feeding appears to be more prevalent among parasitoids overall, but hemolymph feeding is the predominant pattern of host usage in several subfamilies of endoparasitic braconids. We argue that the relative importance of offspring size and development time will be influenced by host ecology and the effects of selected traits on parasitoid survival.

Abstract: Cocoons of the specialist parasitoid Cotesia melitaearum, which attacks the Glanville fritillary butterfly in the Aland islands of SW Finland, are parasitized by the generalist hyperparasitoid Gelis agilis. We added experimentally to the system a second host species for G. agilis, C. glomerata, with which C. melitaearum does not compete for resources. After the one-time addition of the second parasitoid the natural populations of C. melitaearum declined in the treatment, as predicted by the apparent competition theory.

Abstract: Summary 1. In this study we examined the clutch size of Hyssopus pallidus (Askew) (Hymenoptera, Eulophidae), a gregarious ectoparasitoid of codling moth (Lepidoptera, Tortricidae) larvae that is characterized by a host handling time of 2 days approximately. 2. Clutch size increased asymptotically with host larval size (fresh weight) to a maximum of 32 eggs. A single male developed from each clutch, brood survival was 83% and pupal size of female offspring was 0·45 mg, all independent of clutch size. 3. A comparison of the response of both experienced and naive adult parasitoids to varying host larval size, indicated that experience was not necessary for H. pallidus to be able to accurately estimate host size. 4. Clutch size increased by 50% for medium size host larvae (30–40 mg) and 83% for large host larvae (60–76 mg) when the encounter rate was reduced from one host every second day to one every seventh day. A significantly lower clutch size was produced on the last day at the highest encounter rate for large host larvae, suggesting exhaustion of the egg supply for H. pallidus. 5. Manipulation of parasitoid clutch size on constant size host larvae did not influence sex ratio (single male) or brood survival (93%), but produced a linear trade-off between clutch size and the size (fresh weight) of female pupae. Longevity, lifetime fecundity, total number of hosts attacked, and clutch size all increased linearly with the adult size (female pupal weight). 6. The observed clutch size in newly emerged H. pallidus is consistent with a strategy of maximizing lifetime fitness gain rather than the gain from individual hosts (Lack clutch size) despite a significant investment in the time required to attack each host.

Abstract: The parasitoid assemblages associated with grass-feeding chalcid wasps in Great Britain were used to examine the relationships between diversity (species richness), community function (total parasitism rate) and stability (variability in parasitism rate over time). Species-rich communities did not generate higher parasitism rates than species-poor communities, nor was temporal variation of parasitism rates related to parasitoid community richness. The mechanisms underlying hypotheses linking species richness and community function and stability are discussed in the light of these results. Because all parasitoid species represent a single functional group, a lack of complementarity in the ways they use their resources may explain why diversity is not linked to function or community stability. A second likely reason is that these parasitoid communities are under bottom-up control, thus exerting little or no influence on total system function and variability. This is likely to be common in parasitoid communities.

Abstract: The effects of sweet alyssum (Lobularia maritima) (Brassicaceae) flowers on the longevity of two augmentatively-released parasitoids, Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae) and Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae), were studied in a greenhouse experiment. C. marginiventris and D. insulare survived approximately 4.8 and 12.7 times longer, respectively, when provisioned with honey or with sweet alyssum than with water alone. Sweet alyssum planted in northern Florida cabbage fields may be one way to improve biological control by augmentatively-released natural enemies of lepidopteran pests by increasing adult parasitoid longevity during times when few wild plants are in bloom.

Abstract: Tropical rain forests are characterized by their rich plant diversity and highly diverse insect faunas containing mainly rare species. Phytophagous and parasitoid insects utilizing such fragmented resources often must travel considerable distances to find suitable hosts. For small, weak-flying insects, entry into the fast-flowing air above the canopy can provide one way by which long-distance dispersal is achieved. Using sticky traps placed at different heights in a lowland rain forest of Borneo, we compared the diurnal and nocturnal flight heights of chalcids, a group of mainly very small parasitoids and phytophages, to determine if the air above the canopy was used for dispersal. Most families were represented throughout the range of trap heights, including those above the general canopy. A higher proportion of individuals were trapped above the canopy at night than during the day. Fig wasps were exceptional in that they were trapped almost entirely above the canopy. They included species associated with host trees that do not fruit in the canopy, suggesting that these short-lived, slow-flying insects actively fly up above the canopy and then use the wind to passively carry them the long distances needed to reach their highly localized and ephemeral hosts. Once the fig wasps detect the species-specific volatiles released by their host figs, they then may fly down into the canopy, where the lower wind speeds would allow them to fly actively upwind to their hosts.

Abstract: Field experiments with foraging parasitoids are essential to validate the conclusions from laboratory studies and to interpret differences in searching and host selection behaviour of parasitoid species. Furthermore, field experiments can indicate whether the parameters measured in the laboratory are relevant to elucidation of the ecological processes under study, such as adaptation or species interactions. In previous extensive laboratory studies we studied plant- and host-searching behaviour, host acceptance, host suitability; host plant preference, and learning of two congeneric parasitoids of Pieris caterpillars: the generalist Cotesia glomerata, which has been reported to attack several Pieridae species, and C. rubecula, a specialist of the small cabbage white Pieris rapae. In the present field study our aim was to verify the importance of these previous laboratory findings for explaining the performance of these two species in the field. We investigated experimentally whether parasitism on three Pieris species varied with parasitoid species and with food plant of the caterpillars. We exposed different types of host plants, infested with different Pieris species, to parasitism by natural populations of Cotesia species, by setting the experimental plants out in Brussels sprouts cabbage fields. Furthermore we made direct observations of parasitoid foraging in the field. In general, the field results confirmed our predictions on the range of host plant and host species used in the field. The two Cotesia species appear to coexist through niche segregation, since C. glomerata was mainly recovered from P. brassicae and C. rubecula from P. rapae. Although C. glomerata is a generalist at the species level, it can be a specialist at the population level under certain ecological circumstances. Our study shows the importance of variation in host plant attraction and host species acceptance in restricting host plant and host diet in the field. Furthermore the results suggest that, at least in the Netherlands, specialisation of C. glomerata on P. brassicae may occur as a result of C. rubecula outcompeting C. glomerata in P. rapae larvae.

Abstract: We tabulated flowering plant species visited by 151 species of parasitic Hymenoptera. These data were extracted from records of C. Robertson who collected >15,000 insect visitors of flowering plants in central Illinois during a 33-yr period. Species diversity of parasitoids was highest on plant species of the Apiaceae and Asteraceae. The most abundant parasitoids were 2 tiphiid and a scoliid species. Parasitoid species in 6 dominant families (Ichneumonidae, Braconidae, Chrysididae, Tiphiidae, Pteromalidae, and Eucoilidae) showed similar preferences for 10 plant species. Most parasitoid species visited a limited range of host plants, which may have implications for conservation biological control and conservation biology.

Abstract: ABSTRACT Tropical rain forests are characterized by their rich plant diversity and highly diverse insect faunas containing mainly rare species. Phytophagous and parasitoid insects utilizing such fragmented resources often must travel considerable distances to find suitable hosts. For small, weak‐flying insects, entry into the fast‐flowing air above the canopy can provide one way by which long‐distance dispersal is achieved. Using sticky traps placed at different heights in a lowland rain forest of Borneo, we compared the diurnal and nocturnal flight heights of chalcids, a group of mainly very small parasitoids and phytophages, to determine if the air above the canopy was used for dispersal. Most families were represented throughout the range of trap heights, including those above the general canopy. A higher proportion of individuals were trapped above the canopy at night than during the day. Fig wasps were exceptional in that they were trapped almost entirely above the canopy. They included species associated with host trees that do not fruit in the canopy, suggesting that these short‐lived, slow‐flying insects actively fly up above the canopy and then use the wind to passively carry them the long distances needed to reach their highly localized and ephemeral hosts. Once the fig wasps detect the species‐specific volatiles released by their host figs, they then may fly down into the canopy, where the lower wind speeds would allow them to fly actively upwind to their hosts.

Abstract: Parasitoids of dried-fruit insects were surveyed at a culled fig warehouse in Fresno, CA. Three parasitoids of pyralid larvae were found: Habrobracon hebetor (Say), Venturia canescens (Gravenhorst), and at least one species in the genus Goniozus Forster. Two parasitoids of pyralid pupae also were noted: Mesostenus gracilis (Cresson) and a new species of Psilochalcis Keifer. The latter is a new host association. Several beetle parasitoids were present, including Anisopteromalus calandrae (Howard), three species of Cephalonomia Westwood, Laelius centratus (Say), and Cerchysiella utilis Noyes. C. utilis, a parasitoid of driedfruit beetle, Carpophilus hemipterus (L.), is a new record for California. Most activity by parasitoids (detected by yellow flight traps) occurred directly above the fig mass. Pyralid parasitoids exhibited two peaks of seasonal activity; one in late summer through early fall, shortly after new figs were brought into the warehouse, and one in the spring. H. hebetor generally attacked older host larvae, whereas V. canescens equally attacked older and younger larvae, indicating that these two parasitoids may coexist by exploiting different portions of the host population. H. hebetor was active throughout the winter, suggesting that winter release of H. hebetor could be used to control diapausing pyralid populations in dried fruit and nut storage areas.

Abstract: The larvae of mutillid wasps are parasitoids of insect host stages which are not actively feeding and are enclosed in some sort of package (cell, cocoon, puparium). The literature dealing with mutillids living in association with eusocial insects is surveyed and evaluated, and some new host records are provided. Relatively few mutillid species are involved despite the potential for prolific reproduction on social hosts. The problems faced by such parasitoids are greater than those encountered when using solitary hosts. Several species of mutillids parasitise social halictine bees, a few parasitise bumble bees and a few occasionally attack honey bees; a few are parasitoids of commensals or symbionts of ants. Their possible occurrence in the nests of other eusocial insects is briefly discussed.

Abstract: The performance, as measured by daily rate of parasitism and total parasitism, of five aphelinid species found in Australia parasitising Bemisia tabaci were compared on cotton, hibiscus, rockmelon, soybean and tomato. Two Eretmocerus spp., both indigenous to Australia, gave the highest levels of parasitism on each of the plant host species tested. The tritrophic interactions between B. tabaci, host plant species and Eret. mundus (Australian parthenogenetic form) (APF) were also examined. In general, more whiteflies were parasitised when cotton was the source host or rockmelon the test host. Parasitism was always low when tomato was either the source or test host. When parasitoids were transferred from rockmelon to cotton, parasitism declined. In contrast, parasitism increased when parasitoids were transferred from cotton to rockmelon. Parasitism also increased when parasitoids were transferred from soybean to rockmelon, yet failed to do so when shifted from soybean to cotton despite cotton normally being a better host. However, when parasitoids were transferred from cotton to soybean there was a marked increase in parasitism. Possible causes are discussed. The field cage trial demonstrated that parasitism by both Eretmocerus spp. increased with increasing whitefly density. Further, the increase in parasitism was not due to the presence of more parasitoids as neither the parasitoid-whitefly ratio nor the total number of parasitoids present had a significant effect on parasitism. The combination of the two species, gave similar levels of parasitism to that achieved by Eret. mundus (APF) alone. Subsequent identification of the emerged individuals indicated that over 50% of the parasitism was due to this species suggesting that it out-competed Eret. queenslandensis. Despite this competition, there was no evidence that overall control was compromised.

Abstract: The impact of host age on the number of hosts killed, survival of progeny, progeny allocation, and sex allocation was examined for several Trichogrammatidae (Hymenoptera) species in laboratory choice tests. Individual female parasitoids were provided with young, medium-aged and old eggs of one of three lepidopterous host species: Trichoplusia ni (Hubner) (Noctuidae), Pieris rapae (L.) (Pieridae), or Plutella xylostella (L.) (Plutellidae). Trichogrammatid species behaved as gregarious parasitoids with the first two host species, and as solitary parasitoids with eggs of the smaller latter one. They mostly preferred young eggs of T. ni, but did not discriminate among P. rapae eggs of different ages, and often preferred young or medium-aged P. xylostella eggs over old eggs. Survival of progeny did not vary constantly with host age, although it was often very low in P. rapae eggs of any age. Clutch size frequently decreased with host age in both T. ni and P. rapae. Offspring sex ratio did not change with age of T. ni and P. rapae eggs, and rarely did so in P. xylostella eggs. In regard to host age, the results with T. ni are the ones which are the most in agreement with optimal foraging theoretical predictions, as clutch size was the highest in preferred younger eggs.

Abstract: Thiacloprid, a new chloronicotinyl insecticide, is targeted chiefly to control aphid pest species in orchards and vegetables. In a standard IOBC glass plate assay, residues of thiacloprid (SC 480) severely affected adult stages of the cereal aphid parasitoid Aphidius rhopalosiphi whereas pre-imaginal stages (which develop within the host) remained unaffected. As for A. rhopalosiphi, the pre-imaginal stages of the crop-relevant egg parasitoid T. cacoeciae were not adversely affected by a spray treatment with thiacloprid (SC 480). However, when treated host eggs were stored under greenhouse conditions, the emergence success of ecdysed parasitoids was significantly reduced. This is assumed to be caused by thiacloprid residues that had been deposited on the host egg cuticle during spray treatment and which were ingested by the parasitoids during emergence, when they were biting its way through the host egg cuticle. In contrast, if treated host eggs were subjected to field exposure conditions before emergence of the parasitoids, no statistically significant reduction in emergence success was recorded. Moreover, the fitness of parasitoids emerging from treated host eggs was not reduced compared to untreated parasitoids. Since pre-imaginal stages comprise a major part of a parasitoid population and were not affected in a lethal or sublethal way by thiacloprid (SC 480), we conclude that a spray treatment with thiacloprid (SC 480) in the field will not significantly interfere with the pest control function of a parasitoid fauna in the target crops.

Abstract: A survey of fruit flies (Diptera: Tephritidae) from wild and cultivated host plants was conducted in Thailand and Malaysia between 1986 and 1994. In addition to fruit flies, host samples also yielded parasites of those flies, predominantly opiine wasps (Hymenoptera: Braconidae: Opiinae). Although used extensively in classical biological control programmes, very little is known about the host relationships of these parasites in their native environment. From the survey work, host records are given for 13 described species (viz. Diachasmimorpha albobalteata [Cameron], D. dacusii [Cameron], D. longicaudata [Ashmead], Fopius arisanus [Sonan], F. deeralensis [Fullaway], F. persulcatus [Silvestri], F. skinneri [Fullaway], F. vandenboschi [Fullaway], Opius bellus Gahan, Psvtallia flelcheri [Silvestri], P. incisi [Silvestri], P. makii [Sonan] and Utetes bianchii [Fullaway]) and three undescribed opiines. The parasitoid species are listed in relation to the fruit fly species within fruit samples, and the plant species from which the flies and wasps were reared.

Abstract: Despite extensive research on parasitoid-prey interactions and especially the effects of heterogeneity in parasitism on stability, sources of heterogeneity other than prey density have been little investigated. This research examines parasitism rates by three parasitoid species in relationship to prey density and habitat spatial pattern. The herbivore Itame andersoni (Geometridae) inhabits a subdivided habitat created by patches of its host plant, Dryas drummondii, in the Wrangell Mountains of Alaska. Dryas colonizes glacial moraines and spreads clonally to form distinct patches. Habitat subdivision occurs both on the patch scale and on the larger spatial scale of sites due to patchy successional patterns. Itame is attacked by three parasitoids: an ichneumonid wasp (Campoletis sp.), a braconid wasp (Aleiodes n. sp.), and the tachinid fly (Phyrxe pecosensis). I performed a large survey study at five distinct sites and censused Itame density and parasitism rates in 206 plant patches for 1-3 years. Parasitism rates varied with both plant patch size and isolation and also between sites, and the highest rates of overall parasitism were in the smallest patches. However, the effects of both small- and large-scale heterogeneity on parasitism differed for the three parasitoid species. There was weak evidence that Itame density was positively correlated with parasitism for the braconid and tachinid at the patch scale, but density effects differed for different patch sizes, patch isolations, and sites. At the site scale, there was no evidence of positive, but some indication of negative density-dependent parasitism. These patterns do not appear to be driven by negative interactions between the three parasitoid species, but reflect, rather, individual differences in habitat use and response to prey density. Finally, there was no evidence that parasitism strongly impacted the population dynamics of Itame. These results demonstrate the importance of considering habitat pattern when examining spatial heterogeneity of parasitism and the impacts of parasitoids.

Abstract: Models explaining the appearance of gregariousness in insect parasitoids assume the presence of a tolerance gene that enables nonsiblicidal behaviour in gregarious larvae. Under this assumption, nontolerant individuals should attack and kill tolerant individuals when sharing a host, making this behaviour susceptible to invasion by nontolerant individuals. We propose an alternative hypothesis where gregarious larvae retain their aggressiveness but are less mobile. We tested this hypothesis with two sympatric and congeneric species of Mymaridae, Anaphes victus and Anaphes listronoti, respectively, solitary and gregarious egg parasitoids of a Curculionidae, Listronotus oregonensis. Results obtained in competition experiments and from direct observation of movement in mymariform larvae of both species support the reduced mobility hypothesis. By being immobile while retaining their fighting capacity, A. listronoti mymariform larvae appear to optimize host utilization through gregarious development, but can still defend themselves against sympatric aggressive species.

Abstract: Summary 1. Oviposition behaviour and host size − fitness relationships of a gregarious, idiobiont ectoparasitoid, Elachertus cacoeciae (Howard) (Hymenoptera: Eulophidae), were studied by implanting one fourth-, fifth-, and sixth-instar spruce budworm Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae) larva per tree in a stand where the density of the wild C. fumiferana population was low. 2. Development time of E. cacoeciae larvae was quickest on fifth-instar C. fumiferana larvae, which were the preferred hosts for oviposition. 3. Brood sex ratio (proportion of females) was related positively to increasing C. fumiferana instar, indicating that more females were laid on larger hosts. 4. Parasitoid offspring size increased with increasing C. fumiferana instar and decreased with increasing brood size on smaller hosts. Female but not male size was related positively to increasing brood sex ratio (proportion of females). 5. Under laboratory conditions, parasitoid longevity was related positively to parasitoid size and realised lifetime fecundity, and clutch size was related positively to host size. 6. These results suggest that selection of intermediate-sized C. fumiferana larvae may be adaptive for E. cacoeciae.

Abstract: Locating potential hosts for egg laying is a critical challenge in the life history of many insects. Female insects in several orders have evolved mechanisms to find hosts by using olfactory and visual signals derived from their hosts. We describe visual and chemical cues used by the dipteran parasitoid Apocephalus paraponerae (Diptera: Phoridae) in the location and acceptance of its host ant Paraponera clavata (Hymenoptera: Formicidae: Ponerinae). Our results show that A. paraponerae uses the visual cue of ant body size when locating hosts at short range and that these flies lay more eggs in ants that retain their surface chemicals than in ants with these chemicals removed. We compare the cues used by A. paraponerae with cues used by tephritid fruit flies in location and acceptance of their hosts, and we suggest further avenues for the study of host location, acceptance, and host discrimination of A. paraponerae and other parasitoids of ants.

Abstract: 1. The effect of spatial scale on the interactions between three hymenopteran parasitoids and their weevil hosts was investigated. The parasitoid Mesopolobus incultus (Walker) parasitised Gymnetron pascuorum Gyll.; the parasitoids Entodon sparetus (Walker) and Bracon sp. parasitised Mecinus pyraster Herbst. Both of these weevils develop inside the seedhead of Plantago lanceolata L. but occupy different niches. Seedheads were sampled annually from 162 plants at each of two experimental sites consisting of a series of habitat patches of two distinct sizes. Data were analysed from three site-years. 2. Parasitoid densities at each site-year were closely related to the abundance of their respective weevil hosts. The overall proportion of hosts parasitised was more variable for M. incultus than for E. sparetus and Bracon sp. 3. Changes in spatial scale affected the variability of parasitoid densities. For M. incultus, there was generally a greater degree of additional heterogeneity for all increases of scale; for E. sparetus, this was true only at the largest scales; for Bracon sp., all components of variance were negative. 4. The rate of parasitism was related to host density in different ways at different spatial scales. Mesopolobus incultus exhibited inverse density dependence at the finest (seedhead) scale, direct density dependence at the intermediate (plant) scale, and density independence at the large (habitat area 729 m2) scale. Entodon sparetus showed no response to variation in host density at any spatial scale. Bracon sp. showed direct density dependence only at the intermediate and largest scales. 5. Parasitoids E. sparetus and Bracon sp. seemed able to detect more than one M. pyraster individual in seedheads with multiple host occupancy; a greater incidence of conspecific parasitoids than expected emerged from such seedheads.