Biological control of invasive golden wattle trees (Acacia pycnantha) by a gall wasp, Trichilogaster sp. (Hymenoptera: Pteromalidae), in South Africa

An undescribed species of gall wasp, Trichilogaster sp. (Hymenoptera: Pteromalidae), has been introduced to South Africa for biological control of the invasive Australian tree, golden wattle (Acacia pycnantha). After a slow start, Trichilogaster sp. is now common and widespread on the weed. The wasps usually lay their eggs in immature inflorescences, although about 10% are laid in meristematic vegetative tissues. The developing larvae induce substantial galls (up to 3 cm in diameter) to develop on the host plant. The gall tissues have calorific values that are comparable with buds and seedpods, but the wasps drain resources from the plants by: (i) developing much earlier in the season than normal flowers and buds; (ii) reaching masses that are almost three times greater than seedpods; and (iii) causing proportionately more inflorescences to become galls than would normally develop into seedpods. This ‘forced commitmentʹ affects the viability of the host plants and even moderate levels of galling substantially curtail seed production. No impairment of vegetative growth was detected, probably because this study was conducted during the early stages of colonization by the gall wasps. In areas where the wasps are already abundant, branches and trees frequently collapse under the weight of galls and dramatic effects on vegetative growth are still anticipated. All indications are that A. pycnantha is being brought under biological control as successfully as Acacia longifolia was by a congeneric gall wasp, Trichilogaster acaciaelongifoliae.