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 (meth yl parathion) were determined with third instars of two Helicoverpa ze a (Boddie) strains that were established from field collections in com and cotton where-control difficulties with pyrethroids were experienc ed. The data were compared with those of an insecticide-susceptible la boratory strain. Resistance ratios (RR at LD50 and LD90) indicated tha t Illinois-strain larvae were resistant to permethrin (5.2- and 10.8-f old), cypermethrin (3.0- and 18.0-fold), and carbaryl (2.6- and 2.7-fo ld), whereas Arkansas-strain larvae were resistant to permethrin (5.7- and 8.8-fold), cypermethrin (5.0- and 10.0-fold), fenvalerate (2.2- a nd 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 toxi city of cypermethrin to larvae of the Arkansas strain were inconclusiv e; however, they suggested that both metabolic and nonmetabolic (targe t site) resistance mechanisms could be involved.