none Clover root weevil, Sitona lepidus, remains a threat to New Zealand's white-clover based pastoral production system. Unfortunately, the New Zealand-established strain of M. aethiopoides (originally from Morocco) active against Sitona discoideus, has been found to have no potential as a control agent against S. lepidus. It was therefore unexpected that a European strain of M. aethiopoides was found to be highly active against the weevil. This contribution examined the effect of Moroccan x European strain crosses on the value of M. aethiopoides as a control agent of S. discoideus and S. lepidus. It was found that such strain hybridisation led to significant reductions in the impact of this parasitoid species as a control agent against either S. lepidus or S. discoideus. The widespread implications of this finding for biological control are discussed. Animals and man; Control; Biological control; Genetics; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Sitona discoideus; Sitona lepidus (Curculionidae ); Control by; Microctonus aethiopoides (Hymenoptera); Hymenopteran parasites; Microctonus aethiopoides; Parasite geographical strains hybridization impact on efficacy as biocontrol agent Microctonus aethiopoides (Braconidae); Control of; Sitona (Coleoptera); Hybridization; Coleopteran hosts; Sitona discoideus & Sitona lepidus; Parasite geographical strains hybridization impact on efficacy as biocontrol agent