Susceptibility of Helicoverpa armigera (Lepidoptera: Noctuidae) to Cyantraniliprole Determined From Topical and Ingestion Bioassays

The use of insect bioassay to establish baseline susceptibility and monitor changes in sensitivity to insecticides over time has been a key component of resistance management of Helicoverpa armigera Hubner in Australia for over 30 yr. Cyantraniliprole is a recently introduced insecticide, where toxicity is mediated at the ryanodine receptor. Baseline susceptibility of H. armigera to cyantraniliprole was determined in both topical and ingestion assays performed on field populations collected primarily from commercial farms across eastern Australia. Intraspecific variation in cyantraniliprole susceptibility amongst field strains was 9.3-fold in topical bioassays (n = 23 strains) and 2.6-fold in ingestion bioassays (n = 31 strains). The median lethal concentration in field strains was 28mg/liter in topical bioassays and 0.065mg/liter in ingestion bioassays, demonstrating that cyantraniliprole was >400-fold more toxic when administered orally than by contact. The narrow range of intraspecific tolerance, high slope values, goodness-of-fit to the probit binomial model, and enhanced toxicity in diet incorporation bioassays compared with topical bioassays suggest that delivery by ingestion is an effective laboratory method for measuring the dose-response of cyantraniliprole in H. armigera. A discriminating dose of 1.5mg of cyantraniliprole per liter of diet was calculated from diet incorporation bioassays, as a first step in resistance management of cyantraniliprole in Australia.