ABSTRACT

Bollgard^® transgenic cotton which contains CryIAc toxin of Bacillus thuringiensis (Bt) is being commercialized in Mexico and resistance monitoring programs have been implemented to detect any shift in response of key lepidopterous pests affected by the toxin. Bollgard II^® cotton with both CryIAc and Cry2Ab toxins is going to be available for growers soon and the high selection pressure imposed by Bt cotton is a concern. Tobacco budworm (TBW) is a key cotton pest that has been reduced by the CryIAc toxin, but now it is required to have data of both toxins, that may help to understand resistance processes in populations selected by these toxins. Bioassays were performed in field and laboratory TBW populations using the toxin overlay methodology. Results indicated that at the LC₅₀ level Cry2Ab was 21.72 X less toxic than CryIAc in a susceptible colony and 25.51X less toxic in a TBW field population collected in the Yaqui, Valley, but it was 44.55X less toxic in a field population from the Mexicali Valley. Monitoring of resistance to CryIAc is required to allow planting of transgenic cotton in Mexico. With the introduction of Bollgard II^® it will be necessary to have data on the response of both toxins to this and other target insect pests of transgenic Bt cotton. Monitoring of resistance should include besides diagnostic dosage bioassays, complete dosage mortality data which generates more information on the response of population under selection pressure at various concentrations.