Effect of the ratios of Listronotus bonariensis and Sitona discoideus (Coleoptera: Curculionidae) to their respective parasitoids Microctonus hyperodae and M-aethiopoides (Hymenoptera: Braconidae), on parasitism, host oviposition and feeding in the laboratory

The parasitoids Microctonus hyperodae Loan and M. aethiopoides Loan have been introduced into New Zealand to control the forage pests Listronotus bonariensis (Kuschel) and Sitona discoideus Gyllenhal, respectively. Laboratory experiments were carried out to investigate the effect of host-parasitoid ratio, and exposure time on host survival, parasitism, oviposition and feeding. Cages of 20 field collected weevils were exposed to 0, 1, 3 or 6 parasitoids for 0, 1, 12 or 48 hours, maintained until parasitoid pupal emergence, and surviving weevils dissected. Over the ranges studied, increasing parasite number, and to a greater extent, period of exposure of parasitoids to their hosts increased parasitism levels. Microctonus aethiopoides achieved higher levels of parasitism in S. discoideus at the lower parasitoid contact treatments than did M. hyperodae in L. bonariensis. Fecundity in unparasitized weevils was progressively reduced in both L. bonariensis and S. discoideus in relation to increasing exposure to parasitoids. Feeding was reduced by about 40% in L. bonariensis for at least four days after parasitoids were removed, and in S. discoideus for the duration of the experiment, averaging 15%. Results were discussed in relation to possible mechanisms of indirect effects of parasitoids on unparasitized hosts.