Dormant-season applications of copper-containing fungicides and organophosphate insecticides are common in Prunus spp. orchards, and these compounds are often mixed into a single application. However, copper(II) catalyzes the hydrolytic breakdown of many organophosphate insecticides. We measured the impact of tank-mixing these products using field efficacy data collected from 2002 to 2004 coupled with active ingredient degradation studies in the laboratory. Formulations of the organophosphate insecticides chlorpyrifos and diazinon were applied with and without the fungicide Cu(OH)₂ in peach orchards, and the resulting peach twig borer (Anarsia lineatella Zeller) damage was measured. Laboratory experiments to quantify the degradation of the active organophosphate ingredient in various dry deposits from treatment solutions showed that addition of Cu(OH)₂ to laboratory organophosphate solutions resulted in more rapid degradation of the active organophosphate ingredient in dry deposits stored at 100% relative humidity at room temperature. Unsurprisingly, there was significantly more peach twig borer field damage in chlorpyrifos treatments containing copper than those that excluded copper. However, this was not observed with diazinon. Because copper(II) catalyzes the breakdown of various organophosphate insecticides across a range of pH values, caution should be used with the simultaneous application of copper(II)-containing fungicides/bactericides and organophosphate insecticides.