Selective Mortality in Field Trials Between Strains of Heliothis armigera (Lepidoptera: Noctuidae) Resistant and Susceptible to Pyrethroids: Functional Dominance of Resistance and Age Class

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Abstract
Field experiments with strains of Heliothis armigera (Hűbner), either resistant or susceptible to pyrethroids, were done to determine when selective mortality occurred. Eggs and larvae from neonates to 6 d old were placed on leaves collected from cotton fields 0-8 d after the crop had been sprayed with the pyrethroids cypermethrin, fenvalerate, deltamethrin, or cyhalothrin, by commercial application. In a second experiment, eggs of known phenotype were placed in fields just before a commercial application of a pyrethroid. After 48 h, high mortality rates were observed in 0-3-d-old larvae of both phenotypes when they were placed on leaves freshly sprayed with pyrethroids. Selective mortality occurred in older larvae when they were placed on freshly sprayed leaves and in all larvae tested when they were placed on leaves on which there had been dilution or decay of the insecticide. These experiments demonstrate that resistance alleles can be made recessive or neutral by spraying a sensitive life stage rather than by increasing the dose of insecticide.