Toxicity of selected insecticides to two laboratory strains of insecticide-resistant diamondback moth (Lepidoptera: Plutellidae) from Central Florida

Topical application and leaf residue bioassays were done to determine the toxicity of selected insecticides to two laboratory strains (A and B) of the diamondback moth, Plutella xylostella (L.), from central Florida. Both strains were started in the spring of 1987 with individuals from a field population. Strain A was isolated before insecticide application. Strain B was isolated after five weekly applications of fenvalerate. Both strains were maintained in the laboratory for 27 (strain B) and 29 (strain A) generations without exposure to insecticides. Results indicated that strain B was 38.3 times more resistant to fenvalerate than strain A based on topical application, but leaf residue bioassays indicated no difference. The toxicities of chlorpyrifos and methamidophos were similar in both strains based on topical application. Leaf residue bioassays with concentrations of insecticides representing field rates indicated both strains were resistant to fenvalerate and methomyl but were susceptible to chlorpyrifos, acephate, endosulfan, and thiodicarb