Seasonal distribution of fall armyworm (Lepidoptera: Noctuidae) host strains in agricultural and turf grass habitats

Male fall armyworm moths [ Spodoptera frugiperda (J. E. Smith)] were captured in pheromone traps over a 16- to 24-mo period in selected sites in southern Florida. Molecular markers were used to determine whether individuals were of one of two host strains (historically designated rice-strain and corn-strain). Traps placed in agricultural areas showed a population peak in the spring (March-May) and fall (October-December), with a prolonged decline in numbers in summer (July-October) and a smaller reduction in mid-winter (January). The host strain distribution during these periods varied significantly, suggesting strain-specific and seasonal population patterns. Both strains were captured in substantial numbers during the spring peak, but surprisingly, only the rice-strain showed an increase in capture rates during the fall, despite the presence of sweet corn throughout this period. Trap captures in a sod (turfgrass) farm were composed almost entirely of the rice-strain and showed a bimodal seasonal distribution similar to that seen in the agricultural areas, with peaks in the spring and fall. These results represent the first indication that the two host strains might have substantially different population dynamics in the overwintering agricultural areas of Florida and suggest that the rice-strain is the predominant fall armyworm pest during the fall and winter growing periods. It further indicates that the two strains can display a markedly different response to seasonal environmental cues. The implications of these findings on our understanding of fall armyworm) migration are discussed.