The host specificity of insect parasitoids and herbivores is thought to be shaped by a suite of traits that mediate host acceptance and host suitability. We conducted laboratory experiments to identify mechanisms shaping the host specificity of the aphid parasitoid Binodoxys communis. Twenty species of aphids were exposed to B. communis females in microcosms, and detailed observations and rearing studies of 15 of these species were done to determine whether patterns of host use resulted from variation in factors such as host acceptance or variation in host suitability. Six species of aphids exposed to B. communis showed no signs of parasitism. Four of these species were not recognized as hosts and two effectively defended themselves from attack by B. communis. Other aphid species into which parasitoids laid eggs had low suitability as hosts. Parasitoid mortality occurred in the egg or early larval stages for some of these hosts but for others it occurred in late larval stages. Two hypotheses explaining low suitability were investigated in separate experiments: the presence of endosymbiotic bacteria conferring resistance to parasitoids, and aphids feeding on toxic plants. An association between resistance and endosymbiont infection was found in one species (Aphis craccivora), and evidence for the toxic plant hypothesis was found for the milkweed aphids Aphis asclepiadis and Aphis nerii. This research highlights the multifaceted nature of factors determining host specificity in parasitoids.

Multifaceted determinants of host specificity in an aphid parasitoid

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.

https://www.jstor.org/stable/info/2647440

The structure of a leafminer–parasitoid community

Natural enemies have been proposed as important agents of natural selection on herbivorous insects that may facilitate host plant shifts and increases in diet breadth. However, there is little experimental field work to support claims of host-shifting via escape from natural enemies, i.e., to enemy-free space. In this study, we took the unique approach of experimentally creating a host shift for a specialized leafmining fly, Liriomyza helianthi (Diptera: Agromyzidae). We manually transferred leafminer larvae from their normal host plant, Helianthus annuus (Asteraceae), to a variety of novel plants: Helianthus maximilianii, Ambrosia artemisiifolia, Taraxacum officinale, and Centaurea solstitialis. By exposing trans- ferred larvae on normal and novel plants to natural enemy attack under field conditions, we were able to examine whether host-shifting can provide an herbivore with enemy-free space. Our data show that enemy-free space does exist for L. helianthi immediately following a host shift, as mortality in novel plants averaged 17% less than in the normal host. Nevertheless, there was significant within- and between-year heterogeneity in results over the 3-yr period of the study. We found that escape from natural enemies was related to annual variation in the diversity and abundance of parasitoid species. In years when par- asitoid assemblages were dominated by endoparasitoids, mortality of larvae averaged 22% lower in novel hosts. However, when generalist ectoparasitoids, Diglyphus spp. (Hyme- noptera: Eulophidae), were present, there was no advantage of developing in novel plants, a result that could be explained by the less discriminating nature of the ectoparasitoids. When overall levels of mortality from natural enemies were high, the benefit of novel plants was also reduced. This pattern suggests that, as available larval hosts become scarce, parasitoids may be more likely to forage on novel host plants in search for prey, thus diminishing the opportunity for enemy-free space. Nevertheless, our study showed that enemy-free space can exist for an herbivorous insect utilizing a novel host plant, and that natural enemies may, in some cases, offset physiological fitness costs often associated with developing in novel plants. If all else is equal, the balance of these factors may facilitate the inclusion of novel host plants into the feeding repertoire of an herbivore.

Does "enemy-free space" exist? experimental host shifts of an herbivorous fly

Classical biological control could have a major environmental cost if introduced natural enemies colonize and disrupt native systems. Although quantifying these impacts is difficult for systems already colonized by natural enemies, the a priori condition for such impacts can be evaluated based on the extent to which exotics have acquired native hosts. We use native host records for exotic parasitoids introduced into North America for biological control to document the number of exotic species that have been recorded from at least one native insect species. We also evaluate the ability of six biological and ecological variables to predict whether or not a parasitoid will move onto natives. Sixteen percent of 313 parasitoid species introduced against holometabolous pests are known from natives. Further, the likelihood that a parasitoid had colonized native hosts was largely unpredictable with respect to the independent variables. We conclude that given the quality of the data available either now or in the foreseeable future, coupled with inherent stochasticity in host shifts by parasitoids, there are no rules of thumb to assist biological control workers in evaluating if an introduced parasitoid will colonize native insect communities.

The colonization of native phytophagous insects in North America by exotic parasitoids

Despite the vast diversity of parasitic insects and their importance in natural and agricultural communities, our knowledge of what determines their patterns of association with hosts remains sparse. Unlike most parasites that tend to be specialized, parasitoid flies in the family Tachinidae exhibit a broad spectrum of host-specificity, with many species attacking a wide range of hosts. This variability in host-specificity makes them a useful model for examining the ecological and historical factors that determine host associations. We analyzed data collected from a 5-year rearing program of Lepidoptera in southern Arizona to investigate the factors that influence tachinid-host associations. After controlling for a strong effect of sample size, a significant portion of the remaining variance in host range was explained by differences among phylogenetic groups of tachinids and/or their correlated reproductive strategies. Relatively specialized tachinids tended to be associated with monophagous or narrowly oligophagous hosts and attacked them at relatively high frequencies, a pattern we suggest is related to host location efficiency. Cluster analysis indicated that host abundance, gregariousness, food-plant type, and morphology are all important determinants of tachinid host use. Little concordance was found between how tachinid species cluster according to characteristics of their hosts and their estimated phylogenetic relationships. Together, the results of this study suggest that ecological factors are important determinants of host use in these parasitoids and although phylogenetic history may influence the range of hosts used, its power to explain the ecological or taxonomic character of hosts used appears limited.

R. R. Askew

Papers

Parasitoids of leaf-mining Lepidoptera:what determines their host ranges?