Malathion ulv at an aerial application rate of 12 oz. per acre gave excellent coverage for boll weevil control. Extended residual efficacy was found under differing Boll Weevil Eradication Program conditions in Texas and Mississippi. Applications were compared under drought and humid conditions through bioassay with lab reared boll weevils on collected cotton leaves. Mortality dropped from 100 % after 48 hours exposure to 90% at day 4 post treatment and to 35% on day 11 under drought conditions. Under humid conditions mortality remained at 100% through day 7 at which time a second application was made. With boll weevil mortality at 100% for several days the alkaline dew was concluded not to be an important factor in the decomposition of malathion ulv. Dew dripping from the leaves was shown to carry malathion ulv with it. However, rain was found to be the cause for reduced toxicity.