Susceptibility of Oriental fruit moth (Lepidoptera: Tortricidae) larvae to selected reduced-risk insecticides

To determine their baseline susceptibility to chlorantraniliprole, spinetoram, spinosad, and acetamiprid, oriental fruit moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae), neonates were placed on diet cubes containing a range of concentrations of each insecticide. Mortality was assessed after 96 h. Two populations-a long-term laboratory colony from Rutgers University and a colony established in 2007 from a southwestern Illinois (Calhoun County) field population-were tested. We used probit and logit analyses to compare the responses of Calhoun colony neonates from parents reared on 'Gala' apples (Malus spp.) with those of Calhoun colony neonates from parents reared on lima bean, Phaseolus lunatus L., diet. We also compared the responses of Calhoun colony neonates with those of Rutgers colony neonates (all from parents reared on apples). LC50s (ppm in diet) for Calhoun colony progeny of adults reared on apples were 0.08, 0.06, 0.41, and 0.30, respectively, for chlorantraniliprole, spinetoram, acetamiprid, and spinosad. Parental food source (apples versus lima bean diet) did not consistently influence the concentration-mortality relationships for neonates. Based on LC50s and toxicity ratio tests, Calhoun colony neonates were slightly but significantly less susceptible to spinetoram and acetamiprid than were Rutgers colony neonates. Similarly, LC90s and toxicity ratio tests indicated that Calhoun colony neonates were slightly but significantly less susceptible to chlorantraniliprole as well. However, toxicity ratios (Calhoun/Rutgers) were low in all instances, and the highest ratio was 1.73 at LC90 for chlorantraniliprole. Overall, the two colonies responded similarly to these insecticides. Results reported here provide baseline data for future monitoring of resistance development.