Susceptibility of Cry1Ab-resistant and -susceptible sugarcane borer (Lepidoptera: Crambidae) to four Bacillus thuringiensis toxins

Sugarcane borer, Diatraea saccharalis (F.), is a primary corn stalk borer pest targeted by transgenic corn expressing Bacillus thuringiensis (Bt) in some areas of the mid-southern region. Recently, genes encoding for Cry1A.105 and Cry2Ab2 Bt proteins were transferred into corn plants using Vector-Stack Transformation. This new generation of Bt corn with stacked-genes of Cry1A.105, and Cry2Ab2 is expected to become commercially available for managing lepidopteran corn pests in 2009. In this study, larval mortality and growth inhibition of Cry1Ab-susceptible and -resistant strains of D. saccharalis were evaluated on four selected Bt proteins including Cry1Aa, Cry1Ac, Cry1A.105, and Cry2Ab2. The Cry1Ab-resistant strain of D. saccharalis is capable of completing larval development on commercial Cy1Ab corn plants. Neonates of D. saccharalis were assayed on a meridic diet containing one of the four Cry proteins at concentrations ranging from 0.016 to 128 μg/g. Larval mortality, body weight, and number of surviving larvae that did not gain significant weight (< 0.1 mg per larva) were recorded at 7 d after inoculation. Cry1Aa was the most effective protein against both insect strains, followed in decreasing potency by Cry1A.105, Cry1Ac, and Cry2Ab2. Using practical mortality (larvae either died or no significant weight gain after 7 d), the median lethal concentration (LC₅₀) of the Cry1Ab-resistant strain was estimated to be >80- , 45-, 4.1-, and 0.5-fold greater than that of the susceptible strain to Cry1Aa, Cry1Ac, Cry1A.105 and Cry2Ab2 proteins, respectively. This information should be useful to support the commercialization of the novel Bt corn technologies across the mid-southern region of the United States.