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

Trichilogaster acaciaelongifoliae (Froggatt) was introduced into South Africa in 1982 for the biological control of Acacia longifolia (Andr.) Willd. (Fabaceae). Since its introduction it has substantially reduced the reproductive potential of this weed and is widely regarded as a successful biological control agent. Nonetheless, with growing concern about the risks of biological control, follow-up monitoring of agents that have been released is strongly 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. acaciaelongifoliae.The level of infestation and success of T. acaciaelongifoliae on Acacia floribunda Sieber at the study site in Gauteng Province was found to compare favourably with its performance in the most successful areas of its release. Although climatic matching may be important for the initial establishment of biocontrol agents, in the long-term it is clear that these species 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 acquired parasitoid species was unexpectedly high. However, no relationship was found between the fate of T. caciaelongifoliae individuals in galls and gall size. Although this species is an effective biocontrol agent, its long-term behavior (range expansion and association with local natural enemies) has proved to be less predictable.