Factors affecting host selection in an insect host-parasitoid interaction

1. Many parasitoids can develop successfully in different stages of the same host but the costs of parasitism may vary between the stages. The stage of host attacked has generally been determined when there is no choice, giving a misleading impression of host selection or preference. 2. The rate of parasitism by a solitary endoparasitoid, Venturia canescens, of each larval stage of the Indian meal moth, Plodia interpunctella, was examined with and without a host refuge from parasitism. In addition, when given a choice of host stages, with and without a refuge, the influence of parasitoid age on host selection was examined. 3. Wasps were able to parasitize all except the first instar, but second instars experienced significantly reduced parasitism, in both refuge treatments, compared with third to fifth instars. Whilst parasitoid emergence was always significantly less when all host stages had a refuge, the reduction was only marginally significant when second instars were attacked. 4. When given a choice of second- and fifth-instar larvae, wasps consistently parasitized more fifth instars, both with and without a refuge. Moreover, significantly fewer second-instar larvae were parasitized in the presence of fifth instars than when presented alone to the wasps. This pattern of parasitism was unaffected by the increasing age of the parasitoids. 5. Host selection by V. canescens is discussed in terms of host-parasitoid population dynamics and structure.