An assessment scheme for identifying sentinel trees to detect invasive borers in urban forests

Early detection of aggressive exotic woodboring pests, such as emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), can trigger timely management responses and greatly reduce the extent of damage to city trees and prevent catastrophic losses. However the relationship between an assessment of tree vigor provided by city foresters during tree inventories and the likelihood of woodborer infestation remains unclear. To detect exotic borers, foresters need a tool to focus their efforts on trees whose degraded condition make them most susceptible to these pests. In this study, we explore the relationship between rankings of tree vigor and the probability that a borer will attack a tree. We first developed an assessment scheme of tree vigor that is correlated with the incidence of borer infestation. Secondly, we used this borer-specific assessment scheme to measure the health of 150 ash trees in urban areas either with or without EAB (Indianapolis and Lafayette, Indiana, respectively). Specifically, we determined whether our assessment scheme could be used as a putative predictor of borer attack. On an annual basis over the course of three summers, we marked insect emergence holes and determined the annual emergence of borers from each tree. The influence of tree vigor as measured by the scheme on the emergence of borers was analyzed in a repeated measures randomized complete block ANOVA. Probabilities of transitioning to a lower vigor class were calculated in each city to identify the vigor class of trees most likely to decline in the presence of EAB.