Insecticide treatment of invasive ant colonies leads to secondary ant invasions and promotes the spread of invasive ants

Invasive ants are among the world’s most damaging invaders and are considered a significant threat to urban, natural, and agricultural habitats worldwide. Populations of invasive ants are notoriously difficult to eradicate once established and are typically managed using chemical tools, predominantly toxic baits and residual sprays. Most studies evaluating control efforts do not quantify post-treatment community-level response to control efforts, so the overall outcome of management efforts remains unknown and the efficacy of management efforts in mitigating invader impacts remains unclear. The potential of insecticide treatments to cause secondary ant invasions has not been previously examined. Secondary ant invasions, the proliferation of non-target invasive ants following efforts to suppress the dominant target invader is a potentially ubiquitous, yet rarely studied problem. Additionally, limited understanding of the interactions between co-occurring invaders can be problematic for predicting how the removal of only one invasive, a common management scenario, will affect the other invaders and native communities. The current study reports on the potential threat of secondary ant invasions following insecticide treatments and highlights future research needs to address this problem. Residual spray insecticide treatments were applied in an urban setting to control the invasive Argentine ant, Linepithema humile. While the study was limited to a single geographic area, results demonstrate that insecticide treatments can affect target and non-target species in unpredicted ways. Specifically, insecticides applied to control invasive L. humile provide effective short-term control but degrade relatively quickly and lead to secondary invasions by other invasive ants. Therefore, insecticide treatments are capable of causing secondary invasions by multiple invaders. Results demonstrate that invasive ant control is not simply precision removal of the target invader but a form of ecological disturbance with multiple positive and negative impacts on the ecosystem.