CHANGES IN RESISTANCE TO INSECTICIDES IN TOBACCO BUDWORM (LEPIDOPTERA, NOCTUIDAE) POPULATIONS IN MISSISSIPPI, 1993-1995

Strains of the tobacco budworm, Heliothis virescens (F.), collected in Mississippi in 1993 through 1995 were evaluated in bioassays to four classes of insecticides and Bacillus thuringiensis Berliner. High freq uencies of resistance were found to cypermethrin, methomyl, and thiodi carb. Resistance to the organophosphorus insecticide profenofos was fo und in several strains. Significant resistance to B. thuringiensis Ber liner was observed in one strain. Possible synergism of a pyrethroid i n combination with the synergist piperonyl butoxide was observed. Imid acloprid, representing a new class of insecticide, was found to have o vicidal and larvacidal activity on H. virescens. Tolerance to imidaclo prid was present in a field population resistant to carbamates but not to organophosphorus insecticides, suggesting the possibility of cross -resistance between carbamates and imidacloprid. Resistance to carbama tes was clearly present in some populations susceptible to profenofos, indicating the presence of different mechanisms of resistance for the two classes of insecticides. Resistance to pyrethroids appears to be stabilized at a high level.