Comparisons and contrasts in host-foraging strategies of two larval parasitoids with different degrees of host specificity

In theory, the degree of specificity of the signals a parasitoid species needs to successfully locate its host correlates with its level of specialization. We examined this question by comparing the foraging strategies of two parasitoids that differ in their host ranges. In wind-tunnel experiments, we investigated how systemically released herbivore-induced volatiles were used by the generalist parasitoid, Cotesia marginiventris (Cresson) and the specialist, Microplitis croceipes (Cresson). We determined the relative influence of these volatiles as compared to other signals emitted in the host orientation of the two parasitoids. Both the generalist and the specialist parasitoid strongly preferred Spodoptera exigua (Hubner) leaf-induced systemic plants over undamaged plants when no other information was available. When wasps were given a choice between leaf-induced and undamaged plants carrying other plant- or host-related materials, the responses differed for the two species. C. marginiventris appeared to cue primarily on recent damage volatiles, whereas M. croceipes appeared to cue primarily on host frass volatiles. However, recent damage on previously leaf-induced plants, was strongly preferred to recent damage on plants previously damaged by both species. When plants were induced at the squares by Helicoverpa tea (Boddie), only M. croceipes exhibited a preference for these plants over undamaged plants. The adaptive significance of the behaviors as related to dietary specializations of the parasitoids is discussed.