Sphenoptera jugoslavica negatively influences diffuse knapweed populations, but the influence is inconsistent in space and time. In spring 1998, a 3-yr experiment was established to determine whether low rates of picloram or clopyralid would increase the percentage of plants infested with S. jugoslavica, thereby potentially enhancing its capacity to control diffuse knapweed. The experiment was conducted at three sites in Colorado, where S. jugoslavica was released in 1994. Picloram and clopyralid were applied separately to plots at 35, 70, or 140 g ha⁻¹ during June or September, and a non–herbicide-treated control was added. Density and cover measurements were collected in permanent quadrats three times during each growing season to determine whether herbicides influenced diffuse knapweed growth and population dynamics. Diffuse knapweed plants were harvested outside the permanent quadrats to determine the percentage of plants bearing S. jugoslavica larvae. During the spring after herbicide application, all picloram rates and the 35-g ha⁻¹ rate of clopyralid applied the previous June increased the percentage of plants infested by S. jugoslavica approximately 25% compared with the nonsprayed control. None of the herbicide treatments increased the percentage of plants infested by S. jugoslavica 2 yr after application, indicating that the herbicides' positive effects on the percentage of plants infested by S. jugoslavica lasted only 1 yr. Results indicate that combining S. jugoslavica with low rates of picloram or clopyralid applied in June can improve diffuse knapweed control compared with using S. jugoslavica alone.

Nomenclature: Clopyralid; picloram; diffuse knapweed, Centaurea diffusa Lam. CENDI; Sphenoptera jugoslavica Oben.