Gall wasp biocontrol of invasive Acacia longifolia: implications of strong bottom‐up effects

Abstract

AbstractThe population dynamics of insect herbivore biocontrol agents is central to the successful control of invasive weeds. Although the importance of agent population dynamics is recognized, it is rarely considered in assessments of the biocontrol potential of herbivore agents. Herbivore insect population dynamics are influenced by a combination of top‐down effects from natural enemies, bottom‐up effects from plant resource availability (resource quality and quantity), and potential interactions between these effects. To better understand the tri‐trophic interactions that are likely to determine biocontrol success in a host plant–gall wasp–parasitoid system, the relative importance of top‐down and bottom‐up effects for the survival of a herbivore biocontrol agent (Trichilogaster acaciaelongifoliae), on two Acacia host plants in their native range, was estimated using path analysis. On both host plants, there was a strong positive relationship between gall mass per chamber and gall wasp survival and a strong negative relationship between gall mass per chamber and gall parasitism, with parasitoids being less common in large than small galls. There was, however, no significant correlation between parasitism and gall wasp survival and, therefore, no evidence for top‐down effects in this system. Strong bottom‐up effects of host plant resources on both gall wasp survival and gall parasitism have implications for the spatio‐temporal variability of biocontrol success. Such variation should be considered in pre‐release assessments and post‐release monitoring of gall wasps used as herbivore biocontrol agents.