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New Zealand Plant Protection 55 (2002): 280-286

Parasitism by Microctonus aethiopoides on a novel host, Listronotus bonariensis, in Canterbury pastures

M.R. McNeill, J.M. Kean and S.L. Goldson


The parasitoids Microctonus aethiopoides and M. hyperodae have been introduced as biological control agents against Sitona discoideus and Argentine stem weevil Listronotus bonariensis respectively. Collections from ryegrass/white clover pastures in mid-Canterbury found that M. aethiopoides was widespread in L. bonariensis, despite the weevil being a novel host. Microctonus aethiopoides was recovered from 83% of the 65 sites sampled in Canterbury, with parasitism rates of 0.4-19%. Studies in lucerne crops showed that M. aethiopoides preferentially parasitised its natural host S. discoideus, although in some cases up to 10% of L. bonariensis found in the lucerne were parasitised. A possible reason for the prevalence of M. aethiopoides in Canterbury pasture was that the widely-dispersive flights of S. discoideus from aestivation sites provided a source for re-infection of the parasitoid into ryegrass pasture. Analysis of field parasitism suggested that parasitism of L. bonariensis by M. aethiopoides was negatively correlated with parasitism by M. hyperodae.

Keywords: Argentine stem weevil, Sitona discoideus, novel hostassociation, biological control, parasitoid competition.

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