The incidence of parasitism in Trichilogaster acaciaelongifoliae (Froggatt) (Hymenoptera: Pteromalidae), a gall-forming biological control agent of Acacia longifolia (Andr.) Willd. (Fabaceae) in South Africa.
Native parasitoid species became associated with the galls of Trichilogaster acaciaelongifoliae within 2 years of its introduction into South Africa during 1982 for biological control of Acacia longifolia, a perennial, leguminous shrubby weed from Australia. The main parasitoid species were Antistrophoplex [Microdontomerus] sp. (89% of total) and Eupelmus sp. (11%). Emergence of adult parasitoids from galls collected during the 1994 austral summer indicated that levels of parasitism were low (0.6%) and had not increased from previous years. Inspections of immature T. acaciaelongifoliae within galls showed that the incidence of parasitism was much higher (8.6%) than was deduced from adult emergence. Levels of parasitism varied from 0 to 21.3% at 4 different localities. Most parasitoids were found in chambers close to the surface of the gall, irrespective of gall size or the numbers of chambers per gall, and only a small proportion of the occupants of deep-seated chambers were parasitized. There were no parasitoids in the galls until late November, by which time adult emergence of T. acaciaelongifoliae was well advanced. Consequently, T. acaciaelongifoliae is protected from generalist parasitoids, both by its physical position and by its asynchronized phenology, and persists as an important biological control agent of A. longifolia in South Africa.