Host concealment: A determinant for host acceptance and feeding in an ectoparasitoid wasp

In general, ectoparasitoids attack concealed hosts in protected situations whereas endoparasitoids use both concealed and exposed hosts. The difference is assumed to be the consequence of ecological constraints; ectoparasitic larvae are vulnerable both to predation and to climatic factors such as rainfall, and, hence, require some structures to protect themselves. I hypothesized that such ecological constraints should act as a within-species selection pressure to female ectoparasitoids, and hence that females should recognize the degree of concealment of the host and prefer concealed over exposed hosts for oviposition. To test this hypothesis, I examined 1) whether host concealment could influence host acceptance by the ectoparasitoid wasp Agrothereutes lanceolatus and 2) whether host concealment could influence the fitness of the offspring. Female wasps recognized and attacked (probed) both cocooned and exposed host prepupae in equal proportions, but discriminated between them after ovipositor insertion, and preferred the former for oviposition and the latter for host-feeding. They also selected to oviposit on hosts concealed in paper tubes. Thus host concealment was important for host selection in A. lanceolatus. Offspring fitness (measured as survival and size) was much lower on exposed hosts than on cocooned and paper-concealed hosts, even under laboratory conditions. Thus, host concealment influenced the fitness of wasp offspring, and, hence, is a good indicator of host quality for female wasps. Adaptiveness of host selection and host-feeding in A. lanceolatus in relation to host concealment is discussed.