Navel orangeworm (Amyelois transitella) detoxification mechanisms for various insecticides

The navel orangeworm, Amyelois transitella (Lepidoptera: Pyralidae), is a significant pest of almonds, pistachios, figs, and walnuts in California. The numerous insecticides registered for use in California include members of the pyrethroids, organophosphates, anthranilic diamides, and neonicotinoids. A current approach for managing navel orangeworm infestations in orchards is to use synthetic synergists in conjunction with registered pesticides. A significant problem with chemical control is that a possible overlap in the pathways used by the navel orangeworm to detoxify these chemicals could increase the likelihood of cross-resistance. We examined the effects of two synergists, piperonyl butoxide (PBO) and diethyl maleate (DEM), which inhibit detoxification of xenobiotics by cytochrome P450 monooxygenases (P450s) and glutathione-S-transferases (GSTs), respectively. The insecticides selected for use in bioassays included bifenthrin, chlorpyrifos, azinphos-methyl, chlorantraniliprole, and acetamiprid and represent member(s) of the different insecticide classes used to control the navel orangeworm. PBO synergized the toxicities of bifenthrin and acetamiprid when applied in combination with determined median lethal concentrations (LC₅₀). Antagonistic interactions were observed when mortality decreased with the application of PBO to the organophosphates chlorpyrifos and azinphos-methyl due to bioactivation by P450s. Synergism with DEM has only been observed with azinphos-methyl at this time. Our results highlight the involvement of P450 and GST detoxification systems in response to chemical treatments. In most insecticide classes, PBO use can interfere with the P450 enzymes responsible for insecticide metabolism; however, significant overlap in this mode of detoxification amongst these insecticides increases the likelihood of resistance occurring in navel orangeworms.