Fall Armyworm Symposium: Fall Armyworm (Lepidoptera: Noctuidae) Outbreak Originating in the Lower Rio Grande Valley, 1989

Abstract

Ecological, meteorological, and radar evidence indicated that 200,000 ha of irrigated corn grown in the Lower Rio Grande Valley (LRGV) of Texas and northeastern Mexico served as a primary source of migrant fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), that severely impacted crops at Corpus Christi and on the High Plains of Texas during the summer of 1989. Based on soil samples taken in 100 corn fields in the region which yielded an average of 3.97 pupae/m^2 of soil, an estimated 7.94 billion FAW adults were available for transport from the LRGV during late June and early July. A similar study conducted in 40 fields of 16,200 ha of corn grown 400 km NNW of the LRGV near Uvalde, Texas yielded populations averaging 0.39 pupae/m@^2 which likely produced approximately 0.06 billion FAW adults. In the LRGV 15.9% of the adults had emerged during June 5-13 while no FAW adults had emerged at Uvalde June 19-21, thus indicating that the major FAW emergence events in the LRGV preceded those at Uvalde. Large numbers of FAW were captured in pheromone traps at Uvalde on the night of June 21 in the absence of local emergence, suggesting an influx of migrants from the LRGV. Concurrently, pheromone traps indicated large influxes of FAW on the Texas High Plains. Subsequent reports indicated unusually early FAW infestations at Ankeny, Iowa and at Columbia, Missouri. Ground-based and airborne RADARS detected the maximum aerial density of insects emanating from the LRGV on the night of June 20. Calculated trajectories indicated that weather transport systems were available for displacement of passive objects to the High Plains (1000 km) and to Ankeny, Iowa (1900 km). This study constitutes the first reported evidence of long-distance FAW migration from a defined source area to directly impact crops in remote areas. Mature corn in the LRGV is proposed as a major contributory source of migrant FAW in the Central U.S.