Few studies have examined the combined effect of herbicide-induced stress and arthropod herbivory to reduce weed fitness. The purpose of this study was to quantify the effect of arthropod herbivory on the herbicide dose–response of a perennial weed. Fluroxypyr dose–response bioassays using volunteer potato were conducted in the presence and absence of Colorado potato beetle (CPB) herbivory. Logistic model parameter estimates for leaf area, shoot biomass, tuber number, and tuber biomass were often lower with herbivory, compared with no herbivory. Greater variance of parameter estimates within herbivory plots was attributed largely to differential feeding because CPB density was not manipulated in the field. Results from short-season field studies (1,000 growing degree days [GDD] after postemergence [POST] herbicide application) indicated that herbivory had the most effect on potato during a period that coincided with high CPB density and optimal temperatures for CPB development. Season-long bioassays (> 3,100 GDD after POST) revealed that addition of herbivory reduced herbicide use 65 to > 85%, compared with the dose needed to achieve the same reduction in tuber production in the absence of herbivory. Integrated weed management systems targeting volunteer potato are more effective when fluroxypyr applications are made before periods of high herbivory. Moreover, this article describes an experimental approach contributing to optimization of combined effects of arthropod herbivory and reduced herbicide doses.

Nomenclature: Fluroxypyr; volunteer potato, Solanum tuberosum L.