The efficacy of malathion ulv (ultra low volume) was tested on laboratory reared boll weevils under simulated temperature regimes. The treatment rates were one, three and five (29 micrograms per droplet) droplets applied to the laboratory reared adult boll weevils. Temperature regimes of 55 and 35, 60 and 40, and 65 and 45 degrees F. were compared to a constant 85 degree F. temperature. These temperature regimes were for a high temperature for 20 hours and then the low for 4 hours. This was reversed for the same temperatures but with the low temperature for 20 hours and high for 4 hours. These temperature regimes with 3 dosage rates resulted in 21 treatments. Each treatment was composed of at least 12 replicates of 10 boll weevils each. Untreated replicates were held for the same periods. Malathion ulv efficacy on the boll weevil was significantly different between several of the lower temperature rates at one droplet and the constant after 48 hours but all had 70% or more mortality with the constant at 99.7%.