RADAR OBSERVATIONS OF ORIENTATION OF NOCTUIDS MIGRATING FROM CORN FIELDS IN THE LOWER RIO-GRANDE VALLEY

This paper describes measurements of orientation behavior of insects m igrating from corn fields located in the Lower Rio Grande Valley of no rtheastern Mexico and south Texas. Radar, biological, and meteorologic al data were collected during May and June of 1985 to 1990. The two sp ecies of Noctuidae (Lepidoptera) that dominated the radar echoes were corn earworm, Helicoverpa tea (Boddie), and fall armyworm, Spodoptera frugiperda (J.E. Smith). Common alignment of the insects' body axes oc curred during each night (n = 84). Crab angles (difference between ins ect alignment and wind displacement direction) were 30 degrees or grea ter during 85% of those nights when alignment occurred for more than 1 hr, and aligned insects occurred within a layer 200 m or greater in t hickness. Alignment was perpendicular to the wind direction when the w ind was from the SE. Minimum and maximum fallout areas were simulated by assuming various flight behaviors. These simulations indicate that collective orientation changes the mean displacement of migrants with respect to wind displacement and decreases fallout area when compared with random orientation. A smaller fallout area implies a greater conc entration of migrants in the fallout area. Thus, collective orientatio n can impact agricultural production and gene mixing. Also, significan ce of orientation behavior varies inversely with wind speed and size o f source area.