A plant-cage study was conducted in the field to measure the effects of ultra-low volume and conventional oil formulations on the residual toxicities of three insecticides to the boll weevil. Compared to aqueous sprays, ULV and conventional oil sprays enhanced toxicity of the carbamate, oxamyl, and the pyrethroid, cyfluthrin, immediately after application (i.e., O-h residue). The toxicity of the organophosphate, azinphosmethyl, was not significantly affected by oil at O-h posttreatment. ULV/oil sprays of azinphosmethyl and cyfluthrin had longer residual efficacy against the boll weevil, compared to conventional/water-oil and conventional/water sprays. However, bollweevil toxicity of oxamyl was not prolonged by oil in either formulation.