Associational protection and potential non-target effects of systemic insecticide treatments against emerald ash borer

Abstract

Emerald ash borer (EAB), Agrilus planipennis, is an invasive beetle that was accidentally introduced to North America from Asia. In North America, EAB attacks and kills ash trees (Fraxinus spp.). Since the 1990s the insect has spread to 35 American states and 5 Canadian provinces. A primary management strategy against EAB in urban centers is the use of systemic insecticide treatments. However, insecticide application can be very costly and induces concern about potential non-target effects on other invertebrate species and the environment. The objectives of this dissertation were to investigate the use of a treatment regime that could reduce costs, amounts of insecticides, and potential indirect negative effects on ash trees and other invertebrates. We hypothesized that treating a subset of ash trees in an urban setting could confer protective benefits to untreated trees, a strategy known as associational protection. We set up a study from summers 2017 to 2021 where 50% of boulevard ash trees at twelve sites distributed across eight cities in central and southeastern Minnesota were treated in a spatial gradient with one of two insecticides: emamectin benzoate (TREE-äge® G4) or azadirachtin (AzaSol®). Each site covered an area of approximately 1 km2 and contained approximately 100 ash trees. In general, treating a subset of trees increased the overall crown health of the study trees. We also conducted field and laboratory studies to analyze the indirect effect of insecticides on ash tree phenology and seed resources as well as non-target effects on other insect species. No adverse effects of chemical treatments were observed on ash tree phenology or seed germination or viability or time to germination. Non-target insect studies showed that there will likely be minimal effects on other invertebrates. Results from this study will assist in amending current treatment regimens using systemic insecticides and elucidate potential risks these insecticides may have on non-target invertebrates and ash trees.