Decreased insecticide efficacy against cotton aphid was documented during the 1993 growing season in eastern Tulare County, CA. Dose-response curves generated from aphids within this area indicated that these aphids were much less susceptible to insecticides than were known susceptible aphids. However, discriminating dose tests performed with Kern County aphid populations and later in the growing season with Tulare County aphids indicated that reduced cotton aphid insecticide susceptibility is not geographically or temporally stable. Also, reduced insecticide susceptibility does not seem to completely explain the decreased efficacy of RH 7988 in Tulare County. In an attempt to explain this reduced efficacy, we designed a series of experiments to examine the effects of plant water stress, PIX application, and planting date on cotton aphid insecticide efficacy. PIX application and plant water status were shown to significantly affect RH 7988 cotton aphid control and residual toxicity, which suggests that certain cultural practices may in some way alter the efficacy of this compound.