Integrated effects of host resistance and insecticide concentration on survival of and turfgrass damage by the fall armyworm (Lepidoptera: Noctuidae)

The residual activity of six concentrations of chlorpyrifos, spinosad, and halofenozide on fall armyworm, Spodoptera frugiperda (J. E. Smith), as mediated by five warm-season turfgrass cultivars expressing varying levels of genetic resistance was evaluated in greenhouse trials. Similarly, varying concentrations of halofenozide were applied to six turfgrass cultivars in the field; mortality of neonate and third-instar fall armyworms was assessed. Reduced rates of chlorpyrifos resulted in lower fall armyworm survival on resistant zoysiagrass cultivars relative to that on bermudagrass or paspalum. In a separate trial when treated with spinosad, survival on the same zoysiagrasses was equal to or greater than that on more susceptible bermuda or paspalum. Reduced rates of halofenozide in another greenhouse trial resulted in lower survival on resistant zoysiagrasses at some concentrations at 7, but not at 14, days exposure compared to more susceptible grasses. In the field, at the full labeled rate of halofenozide, 100% mortality was observed regardless of turfgrass cultivar. Larval survival on the most susceptible turf, "TifEagle", was higher than that on the remaining turf cultivars at the intermediate rate applied. Larvae exposed to treated turf as third instars displayed a trend toward greater survival at intermediate rates on the two paspalums, "Sea Isle 1" and 561-79, while a trend toward lower survival was observed on "Palisades" and "Cavalier" zoysiagrasses. Factors potentially contributing to the variation in responses observed in the present study include different modes of action of insecticides, host plant resistance mechanisms, differential foliar consumption rates and insecticide dose in relation to body weight. Development of management guidelines for pest management practitioners must address the complexity of potential interactions and may require "case by case" evaluation.