Development of navel orangeworm (Amyelois transitella) resistance to pyrethroid insecticides: Comparisons between a resistant and a susceptible strain

The polyphagous navel orangeworm (Amyelois transitella) is considered the most destructive pest of introduced nut crops, including almonds and pistachios in California orchards. Management of this pest has typically been a combination of cultural controls (including removal of unharvested fruits), but insecticide use has increased substantially along with the value of these commodities. Applicators assume that rotation of insecticides with different modes of action, applied either individually or in combination, will delay resistance acquisition; however, navel orangeworm resistance to bifenthrin, a pyrethroid insecticide, has recently been reported in some almond orchards. Navel orangeworm eggs from resistant adults were shipped to the University of Illinois Urbana-Champaign to characterize this resistant strain and compare it with an established susceptible navel orangeworm colony. Administration of piperonyl butoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF) in bioassays with the pyrethroids bifenthrin and beta-cyfluthrin produced synergistic effects and demonstrated that cytochrome P450 monooxygenases (P450s) and esterases are contributing toward resistance in this navel orangeworm population. Resistance is therefore metabolic and likely occurs through overexpression of specific P450 and esterase genes.  Results from median-lethal concentration (LC₅₀) assays revealed that resistance has been maintained across seven generations. Life table experiments that compare survivorship, development time, and pupal weights did not detect any significant life history differences between the susceptible and resistant strains. Results from these experiments indicate that resistance is heritable and may not have an associated fitness cost, which may facilitate the dispersal and expansion of resistant populations.