Fall Armyworm (Lepidoptera: Noctuidae) Resistance Bioassays Using a Modified Pinto Bean Diet

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Abstract
A series of substandard diets modified from the regular pinto bean diet of Spodoptera frugiperda (J. E. Smith) were evaluated as bioassay candidates to be used in plant resistance phytochemical investigations. Larvae that were fed different concentrations of centipedegrass mixed in the regular pinto bean diet or the regular pinto bean diet without Torula yeast weighed significantly less than those fed on the same diets with similar concentrations of bermudagrass. Pinto bean diet without yeast plus bermudagrass was the only diet that produced an increase in larval weights with each increase in the amount of plant material added. As little as 2 g of centipedegrass per diet significantly inhibited larval growth. Larvae produced on the corn-soy flour-milk solids-based diet without yeast plus the grasses weighed significantly more than those produced on the pinto bean diet without yeast plus the grasses. The pinto bean diet containing either centipede or bermudagrass but without yeast produced larvae with weight differences large enough to separate the resistant and susceptible plant materials. Based on the evidence described here, two diets would be acceptable: the regular pinto bean diet or the substandard pinto bean diet without the yeast. We chose the substandard pinto bean diet without the Torula yeast as a bioassay vehicle for our future phytochemical investigations.