Resistance Monitoring to Bacillus thuringiensis Insecticides for Soybean Loopers (Lepidoptera: Noctuidae) Collected from Soybean and Transgenic Bt-Cotton

Abstract

One Bacillus thuringiensis variety aizawai and several B. thuringiensis variety kurstaki insecticides were evaluated using dosage-mortality bioassays to determine toxicity against a reference soybean looper, Pseudoplusia includens (Walker), strain. Costar and Design WSP (B. thuringensis variety aizawai) had the lowest LC50s followed by Dipel ES, Condor XL, and Javelin WG. LC50s were highest for MVPII. Mattch, MVPII; and Xentari required significantly more time to cause mortality than the other B. thuringiensis insecticides. Variability of response of soybean looper to B. thuringiensis insecticides will necessitate the development of discriminating concentrations for several B. thuringiensis products. Soybean loopers were collected from soybean and transgenic Bacillus thuringiensis cotton (Bt-cotton). Third to 5th instars were exposed to artificial diet overlaid with serial concentrations of Condor XL (B. thuringiensis variety kurstaki) to establish baseline dosage-mortality data. A discriminating concentration (130 ppm) was determined for Condor XL using a laboratory reference strain, and percentage of survival of the reference strain was compared with that of field strains exposed to the same concentration. Field strains, collected from soybean and Bt-cotton, were less susceptible to Condor XL in dosage-mortality and discriminating concentration bioassays than the reference strain, and Bt-cotton strains were the least susceptible to Condor XL. These data indicate reduced susceptibility of field soybean looper strains compared to the reference strain exposed to Condor XL and decreased susceptibility of soybean loopers collected from Bt-cotton.