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 Trichilo gaster acacinelongifoliae (Froggatt) within two years of its deliberat e introduction into South Africa during 1982 for biological control of Acacia longifolia (Andr.) Willd., a perennial, leguminous shrubby wee d from Australia. Emergence of adult parasitoids from galls collected during the 1994 austral summer indicated that levels of parasitism wer e low (0.6 %) and had not increased from previous years. Inspections o f immature T. acaciaelongifoliae within galls showed that the incidenc e of parasitism was much higher (8.6 %) than was deduced from adult em ergence. Levels of parasitism varied from 0 % to 21.3 % at four differ ent localities. Most parasitoids were found in chambers located close to the surface of the gall, irrespective of gall size or the numbers o f chambers per gall, and only a small proportion of the occupants of d eep-seated chambers were parasitized. There were no parasitoids in the galls until late-November by which time adult emergence of T. acaciae longifoliae was well advanced. Consequently, T. acaciaelongifoliae is protected, both by its physical position and by its asynchronized phen ology, from the generalist parasitoids it has acquired, and the wasp p ersists as an important biological control agent of A. longifolia in S outh Africa.