PYRETHROID RESISTANCE IN 2 FIELD STRAINS OF HELICOVERPA-ZEA (LEPIDOPTERA, NOCTUIDAE)

Topical toxicities of three pyrethroids (permethrin, cypermethrin, and fenvalerate), one carbamate (carbaryl), and one organophosphate (methyl parathion) were determined with third instars of two Helicoverpa zea (Boddie) strains that were established from field collections in com and cotton where-control difficulties with pyrethroids were experienced. The data were compared with those of an insecticide-susceptible laboratory strain. Resistance ratios (RR at LD50 and LD90) indicated that Illinois-strain larvae were resistant to permethrin (5.2- and 10.8-fold), cypermethrin (3.0- and 18.0-fold), and carbaryl (2.6- and 2.7-fold), whereas Arkansas-strain larvae were resistant to permethrin (5.7- and 8.8-fold), cypermethrin (5.0- and 10.0-fold), fenvalerate (2.2- and 2.7-fold), methyl parathion (3.2- and 3.2-fold), and carbaryl (3.1- and 1.9-fold). Results of studies of the effect of synergists on toxicity of cypermethrin to larvae of the Arkansas strain were inconclusive; however, they suggested that both metabolic and nonmetabolic (target site) resistance mechanisms could be involved.