Pyrethroid Resistance in the Tobacco Budworm (Lepidoptera: Noctuidae): Insecticide Bioassays and Field Monitoring

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Abstract
Resistance to pyrethroid insecticides in the tobacco budworm, Heliothis virescens (F.), represents a problem in Texas and several other cotton production areas in the United States. To test for resistance, a technique was developed in which tobacco bud worm first instars were exposed to insecticide residues in 20-ml glass vials. Bioassays demonstrated resistance to cypermethrin and fenvalerate in all pyrethroid-resistant strains; low or no tolerance to the organophosphates profenofos, acephate, and methyl parathion; and resistance to the oxime carbamate thiodicarb in one resistant strain. Chlordimeform synergized all insecticides tested, even though the synergism was variable for different insecticides against different strains. Cypermethrin combined with chlordimeform was usually as toxic to resistant larvae as cypermethrin alone to susceptible larvae. The organophosphates (alone or combined with chlordimeform), the carbamate thiodicarb plus chlordimeform, and cypermethrin plus thiodicarb plus chlordimeform are possible alternate toxicants for control of resistant tobacco bud worm. Differences in tolerance to cypermethrin observed for resistant males and first instars showed that resistance mechanisms are not manifested equally in all developmental stages of tobacco budworm. Data on resistance gene frequency based on bioassays with cypermethrin conducted with males collected season-long in the Brazos Valley, College Station, Tex., indicated that resistance is not constant but fluctuates over time. Peaks in resistance gene frequency lag approximately 20 d after peaks in moth numbers. Based on the information obtained, we propose a general approach for resistance management.