To treat or not to treat: Diminishing effectiveness of emamectin benzoate tree injections in ash trees heavily infested by emerald ash borer

Abstract

Emerald ash borer (EAB), a non-native invasive tree-boring beetle, is the primary agent behind the widespread mortality of ash trees (Fraxinus spp.) in both natural forests and urban areas of North America. While a variety of insecticide options have been adopted for protection against EAB attacks, little has been reported on the success of insecticide treatments on EAB-infested trees. This is particularly important because EAB is difficult to detect in early stages of its infestation and protective treatment may be undertaken on trees already in decline. Here, we investigated the effectiveness over a four-year period of biennial emamectin benzoate injections in trees exhibiting different initial EAB infestation levels (estimated using visual ash canopy condition metrics) in an ash forest in central Ohio. Results indicate that emamectin benzoate treated trees exhibited less canopy decline relative to non-treated control trees over the course of the experiment. In fact, all untreated trees died over the course of the four-year experiment, while on average treated trees did not exhibit a significant decline. Furthermore, initially healthy and moderately EAB impacted trees treated with insecticide either maintained or improved their canopy condition, while initially heavily EAB impacted trees stabilized, declined slightly, or died. Our results suggest that by using trunk injections of emamectin benzoate, private and public landowners may effectively be able to preserve lightly or moderately EAB infested trees or delay the replacement of ash trees at varying levels of EAB infestation.