Effects of systemic insecticides against emerald ash borer on ash seed resources

Abstract

Emerald ash borer (EAB), Agrilus planipennsis, is an invasive insect that was accidentally introduced to North America from Asia. It continues to spread rapidly across North America and is responsible for the death of tens of millions of ash trees (Fraxinus spp.). All North American species of ash are susceptible to EAB infestation threatening the ash resource and diversity. Measures such as systemic insecticide treatments in urban forests and collection of ash seeds provide a means of conserving genetic resources. Understanding the effect these insecticides could have on ash seed viability is therefore important to informing conservation efforts. Another potential concern for effective conservation of ash seeds is the ash seed weevil (Lignyodes spp.) whose larvae develop in and feed on ash seeds. However, the effect of EAB insecticides on weevil infestation levels in ash seeds has not been investigated to date. Our study investigated the effect of two systemic insecticide treatments, azadirachtin and emamectin benzoate, on levels of ash seed weevil infestation, seed germination ability, and seed germination time of seeds collected from boulevard trees of green ash (F. pennsylvanica Marsh.) in cities in Minnesota from 2017 to 2019. Weevil infestation levels were similar between untreated and treated trees in 2017 and 2018. In 2019, the weevil prevalence in untreated trees was on average 17% and 30% higher than in azadirachtin and emamectin benzoate-treated trees respectively. Weevil infestation data suggests that repeated insecticide treatments at labelled rates can reduce seed weevils that target germplasm. Additionally, insecticide treatments did not affect ash seed germination rates between treatments. These results suggest that systemic insecticides may be effective at conserving the seed resource in addition to known benefits such as canopy preservation.