Enhanced Metabolism and Knockdown Resistance in a Field Versus a Laboratory Strain of the Soybean Looper (Lepidoptera: Noctuidae)

Abstract

Soybean looper, Pseudoplusia includens (Walker), larvae collected from a field where permethrin failed to provide adequate control and for which a reduction in susceptibility to permethrin had been demonstrated (3-fold at LC50) were compared with an established laboratory colony with respect to knockdown resistance ( kdr ) and in vitro metabolic capacity for a variety of substrates. The time necessary to achieve 50% knockdown of the field population (22.9 ± 1.3 min) after the topical application of 1 μg permethrin was significantly greater than that required for the laboratory colony (18.4 ± 1.0 min). Rates of metabolism for first-generation larvae from the field population were significantly greater than for larvae from the laboratory culture for substrates of glutathione transferase (l-chloro- 2,4-dinitrobenzene, 2.7-fold), monooxygenases (p-nitroanisole O-demethylase, l.8-fold), and hydrolases ( alpha -naphthyl acetate [1.5-fold], p-nitrophenyl acetate [1.5-fold], and permethrin [1.5-fold]). Significant differences between populations were not observed for NADPH cytochrome c reductase nor acephate hydrolysis. Results of the study indicate that a combination of target site insensitivity and increased activity of several enzymes involved in insecticide metabolism including a trans -permethrin hydrolase may be contributing to the reduced susceptibility of the field population relative to the laboratory colony.