Dynamics of emerald ash borer invasion in an insecticide-treated urban environment

Invasive species threaten forest ecosystem functioning, composition, and structure. Emerald ash borer (EAB), Agrilus planipennis Fairmaire, is an invasive insect that has devastated urban ash forests across eastern North America. While insecticide treatments are commonly used to protect high-value urban ash trees, there is limited knowledge on EAB invasion dynamics in these treated urban forests. Our study aims to understand EAB invasion in Toronto, ON, Canada. The City of Toronto tracked the health of 13,125 ash trees that had been injected every two years since 2012 with a neem-based insecticide. We assessed EAB populations using green prism traps from 2016 to 2021 and evaluated the condition of surviving treated ash trees from 2019 to 2021. In keeping with invasion dynamics theory, we predicted that the EAB population in Toronto peaked around 2012 with densities shifting to a core phase around 2019, and ash decline lagging behind by approximately 3-4 years. We also predicted that there was likely more than one EAB population established in Toronto at the start of the invasion. During the core phase of invasion, parameters including ash density, EAB density, and potential spill-over of EAB from untreated ash to treated trees would influence the condition of treated ash. Our
study will help understand the dynamics of EAB invasion in a treated urban landscape and inform strategies for managing invasive forest insect pests.