Range expansion and success of the weed biocontrol agent Trichilogaster acaciaelongifoliae (Froggatt) (Hymenoptera : Pteromalidae) in South Africa

Trichilogaster acaciaelongifoliae (Froggatt) was introduced into South Afri ca in 1982 for the biological control of Acacia longifolia (Andr.) Willd. ( Fabaceae). Since its introduction it has substantially reduced the reproduc tive potential of this weed and is widely regarded as a successful biologic al control agent. Nonetheless, with growing concern about the risks of biol ogical control, follow-up monitoring of agents that have been released is s trongly advocated. The present contribution examines the geographical range expansion and success of T. acaciaelongifoliae 18 years after its release. This study was carried out at a locality with climatic conditions similar to those that were originally predicted to be unsuitable for T. acaciaelong ifoliae The level of infestation and success of T. acaciaelongifoliae on Ac acia floribunda Sieber at the study site in Gauteng Province was found to c ompare favourably with its performance in the most successful areas of its release. Although climatic matching may be important for the initial establ ishment of biocontrol agents, in the long-term it is clear that these speci es are potentially able to expand their geographical ranges. The sex ratio of T acaciaelongifoliae was strongly female-biased, unlike that previously recorded on A. floribunda. The level of parasitism by a single, newly acqui red parasitoid species was unexpectedly high. However, no relationship was found between the fate of T. acaciaelongifoliae individuals in galls and ga ll size. Although this species is an effective biocontrol agent, its long-t erm behaviour (range expansion and association with local natural enemies) has proved to be less predictable.