The strength of the factors favoring the evolution of resistance to insecticides in agriculture may vary in different production regions. The assumption that lower susceptibility to insecticides is to be observed in populations from areas with higher deployment of insecticides cannot be assumed. The main goal of this dissertation is to provide relevant information to better understand the impact that different pest management strategies have in the development of insecticide resistance in fall armyworm, specifically the use of synthetic insecticides alone or in combination with Bacillus thuringiensis (Bt) crop technology. Also, the potential capacity of fall armyworm to recover and complete development after long-term exposure to Bt proteins was addressed in laboratory experiments.Mexico and Puerto Rico have contrasting corn production systems, but fall armyworm is the number one pest in both territories. Mexico has a high diversity of corn production systems, varying from self-subsistence to commercial fields, where the amount of insecticides sprays against fall armyworm varies. Puerto Rico has a very intense corn production system, dominated by hybrid and seed research development, where there is virtually zero tolerance for pest damage. Therefore, fall armyworm is subjected to different levels of selection pressure by insecticides used in these two territories. While in Mexico, synthetic insecticides are the main control tool, in Puerto Rico, fall armyworm populations are managed with both synthetic insecticides and Bt corn. To determine the baseline susceptibility to synthetic insecticides and Bt proteins in field-collected fall armyworm populations from Mexico and Puerto Rico, bioassays were performed with insecticides of different modes of action. In general, fall armyworm from Mexico showed low resistance ratio values in comparison with a susceptible reference population. Only three Mexican populations displayed resistance ratios above 10-fold of the LC50 of the susceptible population: Sonora to chlorpyrifos, Sinaloa to flubendiamide and Oaxaca to permethrin. Fall armyworm from Puerto Rico showed the highest resistance ratios to flubendiamide, chlorantraniliprole, chlorpyrifos, thiodicarb, methomyl, triflumuron, spinetoram, permethrin, deltamethrin and zeta-cypermethrin. Only spinosad, abamectin and emamectin benzoate insecticides resulted in resistance ratios below 10-fold. Diet overlay bioassays were carried out with the Bt proteins Cry1F, Cry1A.105, Cry2Ab2 and Cry1Ac. All of the fall armyworm populations from Mexico were susceptible to Bt proteins. However, fall armyworm from Puerto Rico showed high resistance to Cry1F and Cry1Ac. Bioassays were performed with Cry1F and Cry1Ac on the susceptible reference strain and the population from Puerto Rico. Cry1F exposure in the fall armyworm susceptible population showed reduced survival with the highest concentration (31%). Both Cry1F and Cry1Ac caused asynchrony in development and adult emergence between the control and treatments in the susceptible population, but not in the Puerto Rico population. These results have serious implications for Bt resistance management because fall armyworm shows high capacity to recover after long Bt protein exposure.
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