Managing emerald ash borer in urban forests: Integrating biocontrol and insecticide treatments

Ash trees (Fraxinus spp.) were commonly planted street trees in urban environments prior to the arrival of emerald ash borer (EAB), Agrilus planipennis Fairmaire, in the 1990 s and its subsequent impacts. Insecticide treatments can provide consistent control of EAB and are used to protect high-value trees; however, control only lasts a few years before reapplication is needed. An EAB biological control program seeks to provide long-term control and several parasitoids have been released to that end, including the larval parasitoids Tetrastichus planipennisi Yang (Eulophidae) and Spathius galinae Belokobylskij (Braconidae). In 2015, we released parasitoids in three cities (Syracuse NY, Naperville IL, Boulder CO) while city arborists were simultaneously treating high-value street trees with insecticides. We hypothesized parasitoids would be able to establish on EAB in untreated trees and spread throughout the cities while treated trees remained healthy. We also hypothesized EAB densities would fall as non-treated ash died and would not subsequently rebound as insecticide treatments were removed from street trees due to the parasitism of biocontrol agents. Three-hundred trees were selected for monitoring in each city: 100 untreated, 100 treated, and 100 temporarily treated. Tetrastichus planipennisi successfully established in all three cities, while S. galinae established in Syracuse and Boulder. Untreated trees rapidly declined as expected while treated trees remained healthy. During the study subsets of trees were removed from the treatment cycle as EAB densities fell (≤ 10 larvae/m²). Minor canopy decline was present in 2021–2023 in trees from which treatment was removed, with median crown classes declining from 1 to 2. Despite this shift, most of these trees remained un-infested (∼ 75 %) and retained healthy canopies. Trees that did become infested had high apparent parasitism and low EAB densities. Due to the high value of urban trees, we recommend managers continue monitoring trees and consider selectively reinstating insecticide treatments on trees that start to decline and/or are subject to additional urban stressors.