Expansion of host-plant range of a biocontrol agent Trichilogaster acaciaelongifoliae (Pteromalidae) released against the weed Acacia longifolia in South Africa

Abstract Since 1982 an eastern Australian gall wasp, Trichilogaster acaciaelongifoliae Froggatt, has been successfully used to reduce the reproductive potential of the weed Acacia longifolia (Andr.) Willd. in South Africa. During the past 4 years, however, the gall wasp has expanded its host range in South Africa to include Acacia melanoxylon R. Br. (from eastern Australia) and Paraserianthes lophantha (Willd.) Nielsen (from western Australia). Both of these plant species are invasive in South Africa, but A. melanoxylon is commercially cultivated for fine furniture wood in the southern Cape Province. Comparison of a range of parameters including galling intensities, gall mass and survival, effects on plant reproduction and survival and mass of the female wasps on the three species indicates that the gall wasp should have negligible detrimental effect on either of its new hosts. Paraserianthes lophantha appears to be a more suitable host than A. melanoxylon ; the association of T. acaciaelongifoliae with P. lophantha is new as the insect has not previously been exposed to this western Australian plant, while the wasp occurs sympatrically with A. melanoxylon in eastern Australia. Examination of female mass and survival in multiple galls (galls containing more than one insect but in separate chambers) indicates that there is no competition between developing wasps within the galls. Competition between ovipositing females has probably induced the host range expansion. This host range expansion is noteworthy because only about 20 host shifts of introduced biocontrol agents have been recorded to date, and plant gallers are allegedly highly host specific.