PYRETHROID RESISTANCE LEVELS IN SOYBEAN LOOPER (LEPIDOPTERA, NOCTUIDAE) IN MISSISSIPPI

Resistance to permethrin was monitored in soybean looper, Pseudoplusia includens (Walker), strains established with larvae collected from so ybean and cotton during the growing season in the central Delta area i n Mississippi in 1989 and 1991. Dose responses were tested topically u sing larvae. Populations (=strains) of first generation soybean looper collected on soybean in August in both years had LD50's [95% confiden ce limits (CL), expressed as mug/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 si gnificantly 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 tw o strains collected in September showed 3.31 and 2.52-fold increases i n resistance levels, respectively, when compared with the strains coll ected in August 1989 and 1991. Soybean looper strains collected on cot ton during September had levels of resistance similar to those collect ed on soybean in both years. However, a soybean looper strain collecte d on cotton in October (third generation) in 1989 showed a significant ly higher level of resistance than any other strain. Levels of soybean looper resistance to pyrethroid insecticides were low early in the se ason, but increased as the season progressed.