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. none