An accidental biological control agent? Host specificity of the willow sawfly Nematus oligospilus (Hymenoptera: Tenthredinidae) in Australia

Classical biological control (CBC), the reunification of a pest with its natural enemies from its area of origin, can be a sustainable, long-term and cost-effective way of controlling invasive plants. However, non-target impacts of biological control agents cause significant concerns. To prevent non-target effects, rigorous host-specificity testing is undertaken before biological control agents can be introduced. Willows (Salix spp.) have been widely introduced in Australia where they have become important weeds, due to their high invasiveness and detrimental impacts on waterways and riparian zones. A natural enemy of willows, the willow sawfly Nematus oligospilus, has been inadvertently introduced in Australia. Despite having many characteristics of an efficient biological control agent, host-specificity testing has never been performed for this species. In this study, host specificity was investigated with non-choice feeding tests on 35 native plants. Plants were chosen based on their habitat similarities, and economical and ecological importance. The non-choice test was followed by choice feeding and non-choice oviposition tests. Nematus oligospilus fed extensively and oviposited only on Salix leaves. Although minor feeding occurred on Betula pendula (silver birch) and Populus nigra (poplar), N. oligospilus completed its life cycle only on Salix. Results show that N. oligospilus in Australia has a very narrow host range and was not found to attack any native plant species tested. Despite this, there is a need for vigilance in the field to assess the realised host range and future potential non-target effects of N. oligospilus.