Behavioural acceptability of Sitona lepidus (Coleoptera : Curculionidae) to the parasitoid Microctonus aethiopoides (Hymenoptera : Braconidae) using the pathogenic bacterium Serratia marcescens Bizio

Microctonous aethiopoides Loan has been introduced into New Zealand to cont rol the lucerne pest Sitona discoideus Gyllenhal (Coleoptera. Curculionidae ) Sitona lepidus Gyllenhal (Coleoptera: Curculionidae) a pest of clover (Tr ifolium spp.), has recently established in New Zealand. Laboratory experime nts to test the potential of M. aethiopoides to parasitize S. lepidus has r esulted in very low levels of parasitism. To investigate whether there were behavioural or physiological barriers to successful parasitism, two experi ments were conducted using the insect pathogenic bacterium. Serratia marces cens Bizio as a marker for parasitoid ovipositor penetration. Firstly, M. a ethiopoides 'treated' with S. marcescens were exposed to weevils and rapid weevil mortality, was used to indicate ovipositor penetration. Up to 50% mo rtality Of S. lepidus occurred, which was comparable with mortality observe d in the permissive host Listronotus bonariensis. Dissection of S. lepidus exposed to parasitoids treated with distilled water showed that ca. 21% con tained parasitoid eggs of which 98% were nonviable. In the second experimen t, exposure periods of 24, 48 and 72 h to S. marcescens-treated parasitoids produced an increase in S. lepidus mortality of 14, 28 and 38%, respective ly There was 3% successful parasitoid development in weevils exposed for 72 h to parasitoids treated with distilled water M. aethiopoides has been sho wn to develop successfully in a wide range of non-target weevil species bot h in the laboratory and field. Possible reasons for poor survival of M. aet hiopoides immature stages in S. lepidus are discussed.