Monitoring Pyrethroid Resistance in Bollworm (Lepidoptera: Noctuidae) Moths in Missouri, 1988 to 1994

From 1988 to 1994, adult vial bioassays were conducted on bollworms, Helicoverpa zea (Boddie), collected from pheromone traps in Missouri to determine their susceptibility to pyrethroids. Although most moths were susceptible to cypermethrin, many assays contained individuals that survived concentrations of 5 and 10 μg per vial. The number of individuals that survived these concentrations increased each of the first 3 yrs, and then fluctuated from year to year. In some cases, moths with increased tolerance to cypermethrin occurred in locations where little or no pyrethroid insecticides were used for bollworm control. A likely explanation for tolerant bollworms in Missouri is immigration from more southerly locations, and evidence for long range dispersal of these insects is presented. Implications for regional resistance monitoring also are discussed.