Questioning the effectiveness of seed-reducing agents on invasive Acacia: Pod production relative to gall abundance of classical biological control agents

Abstract

It has been widely accepted that the biological control agents released on invasive Australian Acacia in South Africa, have impacted on the population dynamics of their host plants. However, there are no quantitative data to support these claims and these invasive species remain at similar abundance levels prior to the release of their associated biological control agents and also show evidence of range expansion. The effect of biological control agents, two bud galling wasps ( Trichilogaster acaciaelongifoliae and T. signiventris ) and one gall-forming rust fungus ( Uromycladium morrisii ), on the pod production of their respective hosts ( Acacia longifolia , A. pycnantha and A. saligna ) was assessed by quantifying the reproductive and galled trees per hectare as well as the pods and galls produced per tree and per square meter over a one to two year period. It was shown that invasive Australian Acacia satiate their gall-forming biological control agents, with many pods surviving in their presence. Pod production and loss was placed into perspective using recent findings of seed rain and seed bank studies. It was concluded, based on pod production, seed rain and seed bank data that despite T. acaciaelongifoliae , T. signiventris and U. morrisii reducing pod production, they have had no impact on the population dynamics of their invasive Australian Acacia hosts in South Africa. • Biological control impact on invasive Australian Acacia reproductive output tested for first time. • Invasive Australian Acacia satiate their gall-forming biocontrol agents, allowing pods to survive the pre-dispersal stage. • Gall abundance of biological control agents positively correlated with invasive host pod production. • Invasive Australian Acacia satiate their seed-reducing biological control agents. • Importance of testing hypotheses with field collected data to evaluate the accuracy of expert opinion illustrated.