Field-Evolved Resistance to Spinosyns in Fall Armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) from Puerto Rico: Consequences for Resistance Management

The fall armyworm (FAW), Spodoptera frugiperda, threatens corn production worldwide due to severe defoliation and rapid evolution of insecticide resistance. This study evaluated resistance mechanisms in a Puerto Rico strain (PPR) resistant to the spinosyn insecticides spinosad and spinetoram, compared with a susceptible strain (SUS). Using diet-overlay bioassays, PPR showed high resistance, with RR50 values of 214-fold for spinosad and 63.3-fold for spinetoram. Synergist assays with piperonyl butoxide (PBO), S,S,S-tributyl phosphorotrithioate (DEF), diethyl maleate (DEM), and verapamil hydrochloride (VER) increased susceptibility to the insecticides to varying degrees, providing indirect evidence that multiple detoxification and transport processes may contribute to resistance. For spinosad, the greatest synergistic effects were observed with DEM and DEF, whereas DEM and verapamil produced the strongest effects with spinetoram. These responses are consistent with the potential involvement of glutathione S-transferases, esterases, ATP-binding cassette transporters (ABC transporters) and cytochrome P450s; however, their specific roles require confirmation through biochemical and molecular analyses. Inheritance studies indicated that the degree of dominance was concentration-dependent and generally declined as insecticide concentrations increased. Resistance tended to be more dominant at lower concentrations, whereas the response became approximately additive at the highest concentrations tested, although the pattern varied between reciprocal crosses for spinetoram. These findings show that exposure to low or intermediate concentrations may favor the survival of heterozygotes and potentially contribute to the spread of resistance alleles. Overall, these findings support integrated resistance management through proper application rates and rotation insecticides with different modes of action, while identifying physiological processes that warrant further investigation.