Spinosad selection of a laboratory strain of the tobacco budworm, Heliothis virescens (Lepidoptera: Noctuidae), and characterization of resistance

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The potential for insect resistance to the spinosyns, a novel class of insecticide chemistry, was examined using a laboratory strain of the tobacco budworm, Heliothis virescens (F.), originally collected from tobacco at sites in North Carolina. Technical grade spinosad (spinosyns A and D), was topically applied to third instars. Initially 533 third instars were used but one to two thousand larvae were treated per generation thereafter. Initially mortality ranged from 75% to 85% with doses of 0.044–0.088μg per larva, until the fifth generation (G5) when mortality decreased. The selection dose was subsequently increased every generation from G5 to G11 in an attempt to restore mortality to >70%. After six generations of selection, the LD50 of the selected budworms was 1.68-times that of the parental generation (G1) as estimated 15d after treatment. By G14, the topical LD50 of the selected insects was 1068-fold greater than the parental generation. Four additional populations of the budworm from the southeastern US demonstrated similar LD50s to spinosad as our parental strain, suggesting that the parental budworms from North Carolina were representative of field populations elsewhere. The resistance ratio determined with spinosad (formulated as Tracer®) in heliothine diet was 314-fold at 15d after the start of exposure. Injection of spinosad into the larval hemocoel resulted in a >163-fold resistance ratio 15d after injection, indicating that resistance could not be explained simply by altered penetration alone. Mortality was delayed in the resistant relative to the parental generation regardless of whether third instars were topically treated or exposed to treated diet. Spinosad resistance was also expressed in G14 adults, indicating that an adult vial test would be feasible for monitoring resistance. A feeding disruption assay was developed to monitor larval resistance in the field.