Three-trophic level food webs support the safety of a biocontrol agent 3 years after release

Abstract

Biological control (biocontrol) is a powerful tool for managing invasive alien species and assisting the restoration of native ecosystems. Rigorous post-release monitoring of biocontrol agents is critical to evaluate the success of biocontrol programs; however, this is still rarely implemented. Here, we combined the use of species interaction networks with a Before-After Control-Impact design to evaluate the target and non-target, direct and indirect effects of the Australian gall wasp Trichilogaster acaciaelongifoliae, released to control the invasive plant Acacia longifolia in Portugal. We compared the structure of plant-galling insect-parasitoid food webs before and 3 years after the release of the biocontrol agent. Exhaustive sampling did not detect any non-target effects, either direct (on non-target plants) or indirect (on other galling insects via shared plants). Additionally, no significant changes were detected in network structure that could be related to the establishment of the biocontrol agent. This study shows that monitoring biocontrol at the community level is possible and that, when carefully planned, biocontrol poses minimal risk of non-target effects.