RESISTANCE MONITORING TO BACILLUS-THURINGIENSIS INSECTICIDES FOR SOYBEAN LOOPERS (LEPIDOPTERA, NOCTUIDAE) COLLECTED FROM SOYBEAN AND TRANSGENIC BT-COTTON

One Bacillus thuringiensis variety aizawai and several B. thuringiensi s variety kurstaki insecticides were evaluated using dosage-mortality bioassays to determine toxicity against a reference soybean looper, Ps eudoplusia includens (Walker), strain. Costar and Design WSP (B. thuri ngiensis variety aizawai) had the lowest LC(50)s followed by Dipel ES, Condor XL, and Javelin WG. LC(50)s were highest for MVP LI. Mattch, M VP II; 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 t he development of discriminating concentrations for several B. thuring iensis products. Soybean loopers were collected from soybean and trans genic Bacillus thuringiensis cotton (Bt-cotton). Third to 5th instars were exposed to artificial diet overlaid with serial concentrations of Condor XL (B. thuringiensis variety kurslaki) to establish baseline d osage-mortality data. A discriminating concentration (130 ppm) was det ermined for Condor XL using a laboratory reference strain, and percent age of survival of the reference strain was compared with that of fiel d strains exposed to the same concentration. Field strains, collected from soybean and Bt-cotton, were less susceptible to Condor XL in dosa ge-mortality and discriminating concentration bioassays than the refer ence strain, and Bt-cotton strains were the least susceptible to Condo r XL. These data indicate reduced susceptibility of field soybean loop er strains compared to the reference strain exposed to Condor XL and d ecreased susceptibility of soybean loopers collected from Bt-cotton.