|DEMKOVICH, MARK - University Of Illinois|
|HIGBEE, BRADLEY - Paramount Farming Company, Inc|
|BERENBAUM, MAY - University Of Illinois|
Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2015
Publication Date: 4/23/2015
Citation: Demkovich, M., Siegel, J.P., Higbee, B.S., Berenbaum, M.R. 2015. Mechanism of resistance acquisition and potential associated fitness costs in navel orangeworm (Amyelois transitella) exposed to pyrethroid insecticides. Environmental Entomology. 44(3):855-863.
Interpretive Summary: The navel orangeworm, Amyelois transitella, is the most destructive pest of almonds and pistachios grown in California. These two crops are planted on more than 1.1 million acres and have a direct value to the grower of more than $7 billion. Insect damage is controlled by a combination of methods including field sanitation and insecticide use when necessary. As the value of these crops has increased, so has insecticide use, and resistance to a widely used family of insecticides known as pyrethroids has been reported in Kern County. We established a colony from these resistant insects and used this colony to test the degree of resistance relative to a susceptible strain. Our assay method consisted of feeding studies on artificial diet, to which were added insecticides and other chemicals to test how the larvae detoxified the chemicals present in their diet. Our studies revealed that two classes of enzymes, cytochrome P450 monooxygenases (P450s) and carboxylesterases (COEs), contribute to resistance in this population. Resistance is primarily metabolic and is not caused by a change in the target site for pyrethroids. We also found differences in development time between the resistant and susceptible strains. We conclude that resistance is inherited and has a fitness cost because resistant larvae are smaller than their susceptible counterparts. This may influence their spread into regions where pyrethroid use is less frequent.
Technical Abstract: The polyphagous navel orangeworm (Amyelois transitella) is the most destructive pest of nut crops, including almonds and pistachios, in California orchards. Management of this insect has typically been a combination of cultural controls and insecticide use, with the latter increasing substantially along with the value of these commodities. Possibly associated with increased insecticide use, apparent insecticide resistance has been observed recently in navel orangeworm populations in Kern County, CA. In studies characterizing a putatively pyrethroid-resistant strain (R347) of navel orangeworm, susceptibility to bifenthrin and ß-cyfluthrin was compared to that of an established colony of susceptible navel orangeworm (CPQ). Administration of piperonyl butoxide and S,S,S-tributyl phosphorotrithioate (DEF) in first instar feeding bioassays with the pyrethroids bifenthrin and ß-cyfluthrin produced synergistic effects and demonstrated that cytochrome P450 monooxygenases (P450s) and carboxylesterases (COEs) contribute to resistance in this population. Resistance is therefore primarily metabolic and likely the result of overexpression of specific P450 and COE genes. Resistance was assessed by median-lethal concentration (LC50) assays and maintained across nine generations in the laboratory. Life history trait comparisons between the resistant strain and susceptible strain revealed significantly lower pupal weights in resistant individuals reared on the same wheat bran-based artificial diet across six generations. Time to second instar was greater in the resistant strain than the susceptible strain, although overall development time was not significantly different between strains. Resistance was heritable and may have an associated fitness cost, which could influence the dispersal and expansion of resistant populations in nut-growing areas in California.