Mechanism of resistance to spinosyn in the tobacco budworm, Heliothis virescens

Topical laboratory selection of tobacco budworm larvae, Heliothis virescens, with technical spinosad for multiple generations resulted in larvae 1068-fold resistant to topical applications of the insecticide and 316.6-fold resistant to insecticide treated diet as compared to the parental strain. The penetration of 2′-O-methyl[¹⁴C]spinosyn A across the cuticle of the susceptible (parental) and selected (resistant) tobacco budworms increased with time 3-12 h after application. A trend of reduced penetration in the resistant strain was found but the differences were not statistically significant. 2′-O-methyl[¹⁴C]spinosyn A when injected into the hemocoel was not metabolized 96 h after treatment in both the susceptible and resistant strain, suggesting that a change in metabolism was not the mechanism of resistance. Electrophysiological studies indicated that dose-dependent spinosyn A-induced currents occurred in neurons from spinosyn resistant and susceptible (adult) tobacco budworms. At both 10 and 100 nM spinosyn A, however, the amplitude of these currents in the resistant insects was significantly smaller than the amplitude of currents observed from neurons from susceptible tobacco budworm adults. This suggests that neurons from resistant insects have decreased sensitivity to spinosyn A. However, the reduced inward currents in the resistant strain may or may not be related to the mode of action of the spinosyns. No statistically significant cross-resistance was noted for the spinosad resistant tobacco budworms for topical applications of permethrin (Pounce^®), profenofos (Curacron^®), emamectin benzoate (Denim^®), or indoxacarb (Steward^®). A statistically significant reduction in susceptibility to acetamiprid (Mospilan^®) in artificial diet as determined from a resistance ratio of 0.482 was found.