Functional response and switching behaviour of Campoletis sonorensis (Cameron) (Hymenoptera: Ichneumonidae) to the native host Trichoplusia ni and the non-native host Chrysodeixis chalcites mediating host range expansion

Host range expansion by parasitoids into a non-native host is a common ecological process in nature mediated by multiple factors. Here, we addressed whether foraging behaviour responses of parasitoids to the abundance of the different host species to which they are exposed is one mediating factor. We investigate the parasitism levels, the functional response and the switching behaviour of two strains of the larval parasitoid Campoletis sonorensis (Cameron) (Hymenoptera: Ichneumonidae) exposed to different patch densities and host ratios of the native host Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) and the non-native Chrysodeixis chalcites (Esper) (Lepidoptera: Noctuidae). With a few exceptions, the parasitism levels were similar on both hosts. Both strains of the parasitoid demonstrated a Type III functional response with parasitism level positively related to increasing host abundance. The switching response was positive for both strains of C. sonorensis when both species of hosts are present in the same host patch. Yet a switching response was demonstrated whereby the parasitised proportion of each host at each patch changed depending on host ratio, mediated by their preference for C. chalcites. The results of this study indicate that the foraging behaviour responses of C. sonorensis to the abundance of the host T. ni are factors mediating its host range expansion from the native T. ni to the non-native C. chalcites.