Monitoring for changes in tolerance and resistance to insecticides in bollworm/tobacco budworm in Mississippi, 1996-1999

Strains of the tobacco budworm, Heliothis virescens (F.), and bollworm, Helicoverpa zea (Boddie), collected in Mississippi in 1996 through 1999, were evaluated in bioassays to four classes of chemical insecticides and a biological product (Bt), Bacillus thuringensis Berliner. Stable, high frequencies of resistance in tobacco budworm were found to cypermethrin, methomyl, and thiodicarb; toxicity of Bt remained low but unchanged. Three years of tobacco budworm data against spinosad, representing a new class of insecticides, showed no reduction in susceptibility from one year to the next. Bollworm colonies showed little consistent change in tolerance for the 4-year period, but 1999 data indicated a slight but not significant decrease in susceptibility to cypermethrin and profenofos. Profenofos was equally as effective as spinosad against both species. Continued monitoring will be necessary to verify subsequent changes.