Differential susceptibility of fall armyworm (Spodoptera frugiperda Smith) host-strains to selected Bt toxins

The fall armyworm (Spodoptera frugiperda Smith) is a polyphagous, multivoltine pest of several commercial crops including corn, cotton, sorghum, rice, and bermuda grass. Populations overwinter in the tropical regions of South and Central America, Mexico, and the Southern United States and migrate to more northern latitudes yearly. Fall armyworm is of particular concern in insect pest management as populations have developed resistance to the Cry1F toxin derived from the bacterium Bacillus thuringiensis (Bt) produced by transgenic corn (Zea mays L). Additionally, fall armyworm consists of two host-strains that are referred to as the corn and rice strains due to host preferences and the plants that they were originally collected from. The strains differ in wing morphometrics, but are more commonly distinguished using genetic markers. There is some evidence that the corn and rice strains differ in their tolerances to select insecticides; this extends to several Bt toxins, including Cry1Ab and Cry1Ac, though results are mixed. In this study, a series of diet-based bioassays were conducted with corn and rice strains of fall armyworm to test for differential susceptibility to Cry1Ab, Cry1Ac, and Cry1F. Mortality and growth inhibition data were collected, and used to calculate LC₅₀ and EC₅₀ values. Data on differential fitness between the host-strains of fall armyworm could aid in the development of regional pest and resistance management strategies in locations where each host-strain is known.