Abiotic factors, such as temperature, are important in the activity and performance of insecticidal compounds, as they influence biochemical reactions that may either enhance or limit the insecticide effectiveness. The influence of these temperature-mediated factors on the toxicity of insecticides in red and green color morphs of the tobacco-adapted form of the green peach aphid, Myzus persicae (Sulzer), was evaluated using leaf-dip bioassays in laboratory incubators. Postexposure temperatures of 15, 20, and 25°C were evaluated for 4 classes of insecticides: organophosphate (acephate), carbamate (methomyl), pyrethroid (lambda-cyhalothrin), and neonicotinoid (imidacloprid). Except for lambda-cyhalothrin, all the insecticides had positive temperature coefficients that indicated increased toxicity to M. persicae at both 5 (15 - 20 and 20 - 25°C) and 10°C (15 - 25°C) temperature ranges. Postexposure temperature had similar effects on insecticide toxicity to both color morphs. A temperature increase of 5°C, from 15 - 20°C and 20 - 25°C, caused 1.3- to 3-fold increases in toxicity for methomyl, acephate, and imidacloprid in both color morphs. A change of 10°C (15 - 25°C) increased the toxicity of the three chemicals from 2.9- to 6.0-fold. In contrast, the toxicity of lambda-cyhalothrin decreased as the temperature increased, showing a negative temperature coefficient. Because laboratory bioassays are typically used for monitoring insecticide resistance, this study confirms that using standardized temperatures is necessary for diagnosing problems and making recommendations for resistance management programs in aphids.