Acaricide resistance of the two-spotted spider mite (Tetranychus urticae) in Pacific Northwest hops

Tara Gabrielle Piraneo

The two-spotted spider mite (Tetranychus urticae) is one of the greatest economic plantfeeding pests of agriculture in the world, attacking food crops, trees, and ornamentals, resulting in serious economic injury and occasionally plant death. In the Pacific Northwest, T. urticae is a common and severe pest in hops (Humulus lupulus), a perennial plant grown for its economically important strobile (or cone) that is used as a flavoring and stability agent in beer. Hops are a valuable commodity in Washington, Oregon, and Idaho, the most important hop growing regions in the U.S., accounting for nearly all of U.S. hop production. Severe infestations of T. urticae, however, can cause defoliation and dry, brittle cones, thereby reducing quantity of hop cones, as well as quantity and quality of alpha and beta acid constituents. Production of high quality hop plants and cones requires a rigorous integrated pest management program for T. urticae, and current control relies almost exclusively on the application of acaricidal pesticides. Unfortunately, T. urticae has a well-documented history of rapidly developing tolerance and/or resistance to most of the acaricides registered for their control. A greater understanding of the v mechanisms involved in developing resistant populations is expected to improve mite management strategies in hops. My current study investigated the response of T. urticae to acaricide exposure in three distinct experiments. First, I determined baseline toxicity of a naïve T. urticae laboratory population to the acaricides, abamectin, bifenazate, and bifenthrin. Second, I established mite colonies resistant to acaricides through artificial selection. Third, I tested field populations for their susceptibility to the three candidate acaricides. Baseline lethal concentrations (LC50) values were identified after direct exposure using a Potter spray tower. In the second test, I determined that T. urticae are capable of developing increased tolerance to abamectin, bifenazate, and bifenthrin. Following ten acaricide applications, the LC50 of the abamectin-resistant strain increased 26-fold, the bifenazate-resistant strain increased 14-fold, and the bifenthrin-resistant strain increased 5-fold. In a comprehensive survey of field populations I detected a wide range of responses to the acaricides. During the 2012 and 2013 hop seasons, T. urticae response to abamectin was calculated at 1.29-107-fold, while bifenazate was calculated at 2.29-96.3-fold greater than the naïve laboratory population.

Acaricide resistance of the two-spotted spider mite (Tetranychus urticae) in Pacific Northwest hops

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