Why does Dasineura dielsi-induced galling of Acacia cyclops not impede vegetative growth?

1. Conventionally, agent host specificity has been the major concern in the selection of prospective agents for biological control of weeds. However, subtle interactions between agent and host plant that could influence the outcome of the biological control programme also need to be identified and assessed when selecting potential agents. 2. A gall-forming cecidomyiid Dasineura dielsi was introduced into South Africa for biological control of an invasive tree species, Acacia cyclops. Galls of D. dielsi form on flowers and prevent seed production, the intended aim of the biological control programme. Possible indirect effects of gall induction on vegetative growth of A. cyclops were investigated. 3. The carbon costs of producing and maintaining galls and pods were compared by measuring their gas exchange characteristics, and those of subtending phyllodes. Relocation of ¹⁴C from phyllodes to galls and pods, and nutrient composition of galls and pods, were compared. Insecticide was used to exclude D. dielsi from trees, enabling comparison of growth rates of predominantly gall-bearing and predominantly podded branches. 4. Removal of galls, but not of pods, elevated photosynthetic rates of subtending phyllodes, indicating greater photosynthate demand by galls than pods. In light, the photosynthetic rates of expanded pods exceeded respiration rates. In contrast, there was a net release of CO₂ from all age classes of galls in both light and dark. ¹⁴C allocation to mature galls was approximately three times as high as it was to young galls or immature pods. 5. Pod production required more mineral nutrient than gall production, with N, P and Ca contents of mature pods (including seeds) being nine-fold those of mature galls. 6. Increases in stem diameters (i.e. vegetative growth rates) were similar for predominantly gall and predominantly pod bearing trees. 7. Synthesis and applications. Although nutrient costs were lower for galls than pods, this advantage was offset by higher carbon costs of gall production and the influence on plant growth was neutral. These findings illustrate that subtle physiological effects of herbivorous insects may influence the ecology of their hosts and should be considered during screening of potential agents for biological control.