Variation in susceptibility of Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren) (Lepidoptera: Noctuidae) in Australia to two Bacillus thuringiensis toxins

Intra-specific variation in susceptibility of Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren) in Australia to the Cry1Ac and Cry2Ab δ-endotoxins from Bacillus thuringiensis (Berliner) (Bt) was determined to establish a baseline for monitoring changes that might occur with the use of Bt cotton. Strains of H. armigera and H. punctigera were established from populations collected primarily from commercial farms throughout the Australian cotton belts. Strains were evaluated for susceptibility using two bioassay methods (surface treatment and diet incorporation) by measuring the dose response for mortality (LC₅₀) and growth inhibition (IC₅₀). The variation in LC₅₀ among H. armigera (n = 17 strains) and H. punctigera (n = 12 strains) in response to Cry1Ac was 4.6- and 3.2-fold, respectively. The variation in LC₅₀ among H. armigera (n = 19 strains) and H. punctigera (n = 12 strains) to Cry2Ab was 6.6- and 3.5-fold, respectively. The range of Cry1Ac induced growth inhibition from the 3rd to 4th instar in H. armigera (n = 15 strains) was 3.6-fold and in H. punctigera (n = 13 strains) was 2.6-fold, while the range of Cry2Ab induced growth inhibition from neonate to 3rd instar in H. armigera (n = 13 strains) was 4.3-fold and in H. punctigera (n = 12 strains) was 6.1-fold. Variation in susceptibility was also evaluated for two age classes (neonates and 3rd instars) in laboratory strains of H. armigera and H. punctigera. Neonates of H. punctigera had the same or higher sensitivity to Bt than 3rd instars. Neonates of H. armigera were more sensitive to Cry2Ab than 3rd instars, while being less sensitive to Cry1Ac than 3rd instars. Differences in the two methods of bioassay used affected relative sensitivity of species to Bt toxins, highlighting the need to standardize bioassay protocols.