Spatial patterns of host exploitation in a larval parasitoid of the predatory dusky large blue Maculinea nausithous

Abstract

Summary The foraging behaviour of the parasitoid wasp Neotypus melanocephalus and factors affecting parasitism at the population level were studied. This specialised parasitoid attacks caterpillars of the butterfly Maculinea nausithous , which sequentially feed on the plant Sanguisorba officinalis and specific red Myrmica ants. Among M. nausithous populations, there is considerable variation in caterpillar densities. At low M. nausithous densities, foraging might be time consuming for N. melanocephalus. High host densities may not always be advantageous to foraging parasitoids due to the caterpillars’ frequent overexploitation of ant resources and subsequent density-dependent mortality. In order to disperse progeny, we hypothesised that N. melanocephalus should search in a non-random way at the level of the micro-habitat, i.e., single flower heads of S. officinalis . Our analysis of 32 natural populations in the Upper Rhine valley in Germany did not show a density-dependent relationship between M. nausithous caterpillars and parasitism. Furthermore, habitat parameters like patch size and density of the host's food plant did not affect the parasitism rate. Foraging N. melanocephalus females preferred to search on large flower heads. They probed host-occupied flower heads only, visiting non-host-exploited flower heads only briefly. Time spent on a flower head was independent of the number of caterpillars per flower head. This study indicates that N. melanocephalus increases its foraging efficiency by preferring large flower heads that were previously shown to contain more host caterpillars than small flower heads. Furthermore, oviposition increases the likelihood of continuing to search on a flower head, which is an adaptive strategy for parasitoids foraging for aggregated hosts. However, many host-occupied flower heads were not probed by N. melanocephalus . We discuss the possibility that temporal host refuges of M. nausithous caterpillars might contribute to heterogeneity of parasitism, and why spreading offspring might constitute a suitable strategy for a parasitoid of an ant-parasitic butterfly.