Probability of emerald ash borer impact for Canadian cities and North America: a mechanistic model.
Emerald ash borer (EAB; Agrilus planipennis, Coleoptera: Buprestidae) causes large economic costs by killing ash trees (Fraxinus spp.): a process that takes several years of infestation. We suggest that the most important prediction regarding this species is not whether EAB can persist indefinitely in a new region, but whether it can persist long enough to kill trees. We use a mechanistic model of overwintering mortality of EAB prepupae to identify Canadian cities and more generally, those areas of North America at risk of impact. Although we have previously used a Newtonian cooling model to predict underbark temperatures of ash from meteorological data, we show that a linear regression model has smaller errors for the low winter temperatures relevant to EAB mortality. Using this regression model we generate distributions of predicted underbark temperatures which we then use to predict the return time of weather events cold enough to cause either complete (99%) or partial (75%) mortality of overwintering EAB prepupae. We find that most of North America does not experience extreme cold events frequently enough to prevent ash mortality from EAB (i.e., more frequently than every 6 years), and therefore conclude that large economic impacts are likely throughout the continent. However, if relatively frequent partial mortality events are sufficient to reduce ash mortality, there is a possibility of northern refugia for ash species, and some northern Canadian sites may escape the costs of this non-native pest.