A plant-cage study was conducted on cotton to compare mineral oil, mineral oil + water, and water spray formulations of cyfluthrin for residual toxicity to the boll weevil. Oil formulations, particularly mineral oil without water, prolonged the residual -toxicity of cyfluthrin against boll weevils (i.e., compared to the water formulation). Mineral oil synergized cyfluthrin, as 0.019 lb/ac cyfluthrin in mineral oil killed significantly more weevils than 0.033 lb/ac cyfluthrin in water. Mineral oil also enhanced the repellent or antifeedant effect of cyfluthrin on boll weevils.