The fall armyworm (FAW) (Spodoptera frugiperda J.E. Smith) poses a considerable challenge to global corn production owing to its remarkable ability to develop resistance to synthetic insecticides and insecticides derived from microorganisms, thereby undermining conventional pest management strategies. A remarkable FAW adaptation to pesticides was detected in Puerto Rico (PR). This region has one of the most strategic nurseries in the world for seed augmentation and research. However, a combination of factors including more than ten FAW generations per year, a high pest infestation, isolated geographical conditions, and 29 insecticide sprays per season prompt FAW to develop quick resistance. This doctoral thesis comprises three detailed studies that address the FAW inheritance and metabolic mechanism of resistance to synthetic insecticides and derived insecticides from microorganisms Saccharopolyspora spinosa. Diet overlay bioassays were conducted in all experiments, employing third early instar larvae.In the initial study, I characterized the resistance of the PR strain of FAW to the diamide insecticides chlorantraniliprole and flubendiamide, uncovering high resistance ratios (RR50 = 96-fold for chlorantraniliprole and RR50 = 2,762-fold for flubendiamide). This resistance exhibits distinct inheritance patterns, with chlorantraniliprole showing autosomal inheritance and flubendiamide demonstrating an X-linked inheritance mode. Additionally, the resistant strains exhibited cross-resistance to cyantraniliprole and cyclaniliprole. Synergist bioassays using DEF, DEM, PBO and verapamil reveal crucial role of detoxification enzymes as glutathione S-transferases, ABC transporters and esterases in resistance to flubendiamide and reduced metabolism for chlorantraniliprole. High levels of resistance from the PR strain to pyrethroid insecticides esfenvalerate (62-fold) and deltamethrin (15-fold) were detected. The inheritance of resistance exhibited an incompletely dominant resistance trait. Cytochrome P450 monooxygenases, esterases, glutathione S-transferases, and ABC transporters had an important role in the detoxification of the esfenvalerate. For deltamethrin, significant resistance was attributed to glutathione S-transferases, with lesser roles for ABC transporters and minor contributions from cytochrome P450 monooxygenases and esterases. This sheds light on the complex interplay of detoxification enzymes in pyrethroid resistance. The final study investigates resistance mechanisms in the PR strain to spinosyn insecticides. Very high levels of resistance were found for spinosad (214-fold) and spinetoram (63-fold), revealing an autosomal inheritance of resistance for spinosad and an X-linked for spinetoram. A higher role of esterases and glutathione S-transferases, followed by ABC transporters, and cytochrome P450 monooxygenases was observed in the Puerto Rican strain. Conversely, resistance to spinetoram was primarily mediated through by glutathione S-transferases, followed by ABC transporters, and lesser contributions from cytochrome P450 monooxygenases and esterases. These findings underscore the critical roles of detoxification enzymes, both individually and collectively, in conferring resistance. The synergist bioassays, employing both diet-overlay and topical applications, validated the hypothesis regarding the involvement of detoxification enzymes in resistance to spinosyns in the PR strain. These studies analyze the genetic and biochemical factors behind insecticide resistance in FAW, highlighting the complexity and cross-resistance. They emphasize the need for new, holistic pest management methods and insecticide rotation. This research enhances understanding and provides crucial knowledge to manage resistance, potentially mitigating FAW's impact on agriculture worldwide.
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