How Effective is Sharpshooter Control at Limiting Pierce's Disease Spread in California Vineyards?

Abstract

Pierce’s disease management in southern California vineyards hinges on chemical control of populations of the vector, the invasive glassy-winged sharpshooter (Homalodisca vitripennis), residing in citrus. Systemic insecticides (imidacloprid) are regularly applied to citrus, which is a preferred plant type for the sharpshooter, to reduce insect abundance before they move into vineyards. These treatment programs have been successful, reducing regional sharpshooter populations to a fraction of what they once were. Grape growers also frequently apply systemic insecticides in vineyards, but the efficacy of these treatments for disease management is not known. Over the last three years we conducted a series of surveys in treated and untreated vineyards in Temecula Valley to determine the relative economic value of within-vineyard chemical control for Pierces disease management. In each of the past three seasons we surveyed 34 vineyards in the Temecula Valley that differ in their use of systemic insecticides, and monitored regularly populations of sharpshooters and beneficial insects. Among the years overall Pierce’s disease prevalence was low; averaging approximately 1% based on visual symptoms. Prevalence differed slightly among fields of different treatment categories with the lowest infection rates in those vineyards that were either consistently or intermittently treated with imidacloprid. Based on sticky trap monitoring, consistently or intermittently treated vineyards also had lower catches of sharpshooters than untreated fields, but natural enemy catch did not differ appreciably among the three treatment categories. Finally, tap sampling results showed slightly lower natural enemy abundance in consistently treated sites, but the abundance of non-predatory arthropods was also substantially lower in those sites. Collectively, these results suggest that imidacloprid treatments may reduce slightly disease spread, at least in part due to reductions in vector pressure, but without any clear non-target effects on natural enemies that may lead to secondary pest outbreaks. However, it may not be critical to treat vineyards every year; at least not as long as regional vector populations continue to be reduced through areawide control programs.