Evaluation of tolerance to Bacillus thuringiensis toxins among laboratory-reared western bean cutworm (Lepidoptera: Noctuidae)

The western bean cutworm, Striacosta albicosta (Smith) (Lepidoptera: Noctuidae), is a destructive insect pest of dry beans and corn within its native range of western Nebraska and eastern Colorado. However, since the initiation of an eastward range expansion of S. albicosta in the late 1990s, economic damage has been observed in the Midwest, and the species has now reached the Atlantic Coast and Quebec. Economic damage to corn occurs by larval feeding on ears, which is not controlled by commercial transgenic hybrids that express Bacillus thuringiensis (Bt) Cry1Ab, but partial control is observed by corn varieties that express Cry1 F toxins. Inadequate protocols for laboratory rearing of S. albicosta have hindered controlled efficacy experimentation in the laboratory and field. We report an S. albicosta rearing methodology used to maintain a laboratory colony for 12 continuous generations. Rearing procedures were adapted for Bt toxin diet-overlay assays, revealing that S. albicosta larvae exposed to Bt toxin for 14 d were insensitive to Cry1Ab concentrations up to 25,000 ng/cm2. In contrast, neonates exposed to Cry1 F toxin at ≥250 ng/cm2, showed reduced developmental rates, with estimated effective concentrations of EC50 = 1,187.5 ng/cm2 and EC95 = 10,000.5 ng/cm2. The ability to mass produce this pest insect will enhance fundamental research, including evaluation of control tactics and toxin susceptibility.