Field studies were conducted to evaluate the acute effects of spring applications of 2,4-D ((2,4-dichlorophenoxy) acetic acid), picloram (4-amino-3,5,6-trichloro-2- pyridinecarboxylic acid), and a 2,4-D + picloram mixture on overwintering larvae ofUrophora affinis(Frauenfeld) andU. quadrifasciata(Meigen) (Diptera: Tephritidae) associated with spotted knapweed,Centaurea maculosaLamarck (Compositae: Cynareae). Also, the chronic effects of picloram on the re-establishment of the flies into previously treated spotted knapweed sites were evaluated. Spring applications of 2,4-D, picloram, or their combination did not significantly affect larval-pupal mortality rates ofU. affinis.Picloram and a picloram- 2,4-D mixture did not significantly affectU. quadrifasciatamortality. There were significant differences, albeit conflicting, between the high and low rates of 2,4-D in the two plots studied. Flies from one plot exhibited a higher rate of mortality with the high rate of 2,4-D, whereas flies from the second plot displayed higher mortality with the lower rate of 2,4-D. No long-term impacts of picloram on the re-establishment ofUrophoraspp. into previously treated sites were found. Infestation rates (number of galls per capitulum and percent capitula infested) for the two fly species in treated plots were not significantly different from those in control plots. Larval-pupal mortality rates for both species were also not significantly affected. The results of these studies suggest that applications of 2,4-D and picloram are generally compatible with the utilization of these two biological control agents for the management of spotted knapweed in northern Idaho.