Evaluation of trunk-injected emamectin benzoate as a potential management strategy for Kuroshio shot hole borer in avocado trees

The polyphagous shot hole borer (PSHB) and the Kuroshio shot hole borer (KSHB) are newly invasive ambrosia beetles in California. They are vectors of the plant pathogen Fusarium euwallaceae (S. Freeman, Z. Mendel, T. Aoki, K. O' Donnell), the causal agent of Fusarium dieback in a broad host range that includes commercial avocados, landscape trees, and native tree species in urban and wildland environments. Management of these beetles using contact insecticides is challenging because the beetles spend little time outside their hosts. Trunk injection of systemic insecticides has been proposed as an alternative to contact treatments because insecticides can more effectively target the vascular tissues where the beetles establish their colonies. In this study, several field trials were conducted to evaluate the efficacy of trunk injections of the systemic insecticide emamectin benzoate in avocado trees. The uptake and persistence of emamectin benzoate were determined by quantifying residues in wood cores sampled at various heights within the trees where beetles would likely target. In conjunction with the field trials, a series of bioassays was conducted with a KSHB colony using an avocado-based artificial diet infused with the insecticide. The bioassays showed a dose-dependent effect of emamectin benzoate on the survival and development of the beetle in diet. We derived a tentative working threshold of 300 ng/g insecticide from the bioassay data that we subsequently used as a guide in evaluating the efficacy of the trunk injections. Emamectin benzoate established quickly within trees at the threshold concentration in the areas most vulnerable to attack and colonization by KSHB. Injection of the insecticide in a more dilute form promoted both faster uptake and more rapid establishment of effective concentrations than the undiluted form, thereby providing potential options in how the material is injected based on the levels of infestation of groves.