Factors Influencing Pyrethroid Toxicity in Pink Bollworm (Lepidoptera: Gelechiidae): Implications for Resistance Management

Abstract

Toxicities of fenvalerate and permethrin were measured by three bioassay methods (topical application, vial-discriminating dose, and sticky-trap attracticide) at different temperatures and exposure times on a laboratory strain of adult male pink bollworm, Pectinophora gossypiella (Saunders). Variations in toxicity and slopes of dose response regressions for each bioassay were dependent on the temperature at which the bioassay was performed and the total time of exposure to a concentration. More reliable regression lines were obtained for fenvalerate than for permethrin using similar protocols. Control mortality was lowest for the vial-discriminating dose of the three bioassays. Slope shifts for the vial bioassay indicated that toxicity of permethrin, but not fenvalerate, was independent of temperature. Shifts in slope values of sticky-trap and topical bioassays indicated temperature dependence for both pyrethroids. Based on a discriminating dose, the vial bioassay was a good predictor of possible resistance. Data from the attracticide bioassay fit the probit model well and gave a precise estimate of a discriminating dose. Lower temperatures and longer exposure times gave less heterogeneous probit lines for all methods. The hypothesis of equality was rejected for permethrin and fenvalerate in all three bioassays except for fenvalerate in the vial bioassay at low temperatures and long exposure times. Different LCs from different bioassays indicate that the monitoring bioassay used in resistance management must be chosen carefully.