Assessing geographic variation in the risk of invasion by emerald ash borer (Agrilus planipennis)

Simple models of the spread of invasive insect species often assume that the rate of spread is consistent in all directions from the point of invasion. This assumption may not hold if the environment through which the insect is spreading is heterogeneous or if data are truncated by international borders. Such biological or methodological phenomena may result in spread rates that vary spatially and temporally, termed anisotropy. We evaluated the hypothesis that the spread of emerald ash borer is affected by geographic variation by analyzing historical records of emerald ash borer (Agrilus planipennisi) county level detection in the United States and Canada. Emerald ash borer is an invasive wood boring beetle that arrived near Detroit, Michigan, USA in the 1990s from eastern Asia. These beetles attack ash trees (genus Fraxinus), which has resulted in high mortality of ash trees in the beetle’s introduced range in North America. Here, we used generalized additive models to asses variability in the spread rate of emerald ash borer. Additionally, we found that both minimum winter temperatures and the percent of ash in the total above ground biomass were associated with different rates of spread across the invaded range of emerald ash borer.