Introduction

Species of the genus Sitona Germar (Col., Curculionidae) have been recorded on all continents and all develop at the expense of a number of Leguminosae. In their larval stages, they attack the root system and may be especially injurious to the nodules (Aeschlimann, 1986), whereas the longlived adults consume the stems and foliage of their host plants. The four palaearctic representatives which comprise the S. humeralis group of species all depend upon Medicago spp. as host plants (Aeschlimann, 1984). One of these, S. discoideus Gyllenhal, was accidentally introduced into the southern hemisphere, where it has become a pest of economic importance to both perennial cultivated lucerne and volunteer annual Medicago species used for pasture throughout southeastern Australia and subsequently in the south island of New Zealand.

As a consequence of the importance of S. discoideus, surveys were carried out from 1973 to 1985 over most of the Mediterranean basin (Aeschlimann, 1980) with the aim of identifying efficient natural enemies of the various stages of S. discoideus that would be suitable for deliberate introduction into Australia. These investigations demonstrated that the parasitoid Microctonus aethiopoides Loan (Hym., Euphorinae) was the most promising biological control agent for use against adult Sitona weevils. Based on these findings, several biotypes of this natural enemy were imported between 1975 and 1979 from various parts of the western Mediterranean region to Australia for mass-rearing in quarantine and subsequent field release against S. discoideus (Aeschlimann, 1983a). Initial establishment followed by spreading was recorded in Australia 2 years after the start of the release programme (Aeschlimann, 1983a).