FACTORS AFFECTING HOST SELECTION IN AN INSECT HOST-PARASITOID INTERACTION

1. Many parasitoids can develop successfully in different stages of th e same host but the costs of parasitism may vary between the stages. T he stage of host attacked has generally been determined when there is no choice, giving a misleading impression of host selection or prefere nce. 2. The rate of parasitism by a solitary endoparasitoid, Venturia canescens, of each larval stage of the Indian meal moth, Plodia interp unctella, was examined with and without a host refuge from parasitism. In addition, when given a choice of host stages, with and without a r efuge, the influence of parasitoid age on host selection was examined. 3. Wasps were able to parasitize all except the first instar, but sec ond instars experienced significantly reduced parasitism, in both refu ge treatments, compared with third to fifth instars. Whilst parasitoid emergence was always significantly less when all host stages had a re fuge, the reduction was only marginally significant when second instar s were attacked. 4. When given a choice of second- and fifth-instar la rvae, 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 i ncreasing age of the parasitoids. 5. Host selection by V. canescens is discussed in terms of host-parasitoid population dynamics and structu re.