Factors influencing efficacy of an area-wide pest management program in three urban forests

Abstract

Managing exotic invasive pests like emerald ash borer can strain budgets and the capacity of cities to protect their urban forest. Area-wide management approaches, like SLAM (SL.ow A.sh M.ortality), can potentially protect trees at a greatly reduced cost. We tested this strategy in three urban forests in Indiana by treating 40 % of the ash trees with insecticides. While the urban SLAM approach reduced overall mortality of untreated ash trees, survivorship varied considerably between sites. SLAM was most successful (54 % survival) where initially < 10 % of the ash trees were moribund (canopy thinning > 30 %) and 40 % of all trees were treated with emamectin benzoate every two years. The approach was less successful (38 % survival) in a site with similar initial ash morbidity but where 15 % of trees were treated with emamectin benzoate and 25 % with annual applications of imidacloprid. In the third site, where 51 % of ash forest were initially moribund and 40 % of the ash trees were treated, only 23 % survived. Overall survival of treated ash trees declined by 18–22 %, and trees that were not moribund were most likely to survive. Although many treated trees that were initially moribund regained their health by the end of the project, this was not the case for untreated ash trees. Where SLAM was most successful, both untreated and treated white ash were more likely to survive than green ash trees. Untreated ash trees at all three sites were more likely to survive when closer to trees treated with emamectin benzoate, but not to those treated with imidacloprid. Our findings suggest that the SLAM approach can protect urban ash trees, but its success is strongly influenced by initial tree condition, species composition and proximity to treated ash trees.