Pyrethroid Resistance Levels in Soybean Looper (Lepidoptera: Noctuidae) in Mississippi

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Resistance to permethrin was monitored in soybean looper, Pseudoplusia includens (Walker), strains established with larvae collected from soybean and cotton during the growing season in the central Delta area in Mississippi in 1989 and 1991. Dose responses were tested topically using larvae. Populations (= strains) of first generation soybean looper collected on soybean in August in both years had LD50's [95% confidence limits (CL), expressed as @mg/g larval weight] of 1.30 (0.80-2.30) and 0.77 (0.01-2.19) in 1989 and 1991, respectively, which were not significantly different from the LD50 (CL) of the susceptible strain [0.63 (0.47-0.87) in 1989 and 0.18 (0.13-0.23) in 1991]. Soybean looper strains (second generation on soybean) collected in September showed 6.8 to 10.8-fold increase in LD50 (CL) [4.30 (3.00-6.80) in 1989 and 1.94 (0.31-4.04) in 1991], compared with the susceptible strain. The two strains collected in September showed 3.31 and 2.52-fold increases in resistance levels, respectively, when compared with the strains collected in August 1989 and 1991. Soybean looper strains collected on cotton during September had levels of resistance similar to those collected on soybean in both years. However, a soybean looper strain collected on cotton in October (third generation) in 1989 showed a significantly higher level of resistance than any other strain. Levels of soybean looper resistance to pyrethroid insecticides were low early in the season, but increased as the season progressed.