Patterns of host use in solitary parasitoids (Hymenoptera, Ichneumonidae):field evidence from a homogeneous habitat

We detected a significant inter- and intraspecific host preference on the l evel of individual host use in a system, in which three moth species (Lepid optera: Noctuidae), feeding on a cattail Typha latifolia, are parasitized b y three solitary parasitoid species (Hymenoptera: Ichneumonidae). The biolo gy of the host species is similar but they exhibit remarkable inter- and in traspecific variance in body size. Ail the parasitoid species preferred the largest host species in this system whereas other host species were used o nly occasionally. We found that parasitoids which emerged from females of t he preferred host species were larger than those which developed in males o f the same species. Accordingly, two of the parasitoid species had a signif icant within-host-species preference: females of the largest moth species w ere used more often than males. No dependence of the preference pattern on host density was found. This pattern of host use is discussed in the light of the switching theory and the optimal host selection theory. Our results indicate that non-random host use by parasitoids may have significant effec ts on host populations and communities, and forms a potential selective fac tor against large body size in herbivorous insects. Unlike the majority of ichneumonid wasps, these three parasitoid species have no remarkable female -biased sexual size dimorphism. In accordance with the predictions of Charn ov's sex allocation theory for this case, we did not observe any significan t host quality dependent biases in sex allocation: there was no association between host sex and parasitoid sex; neither did parasitoid sex ratio diff er between years with different host quality.