Population studies for determining seasonal fluctuations of N. vinitor indicated that the appearance or disappearance of populations within a short time can be attributed mainly to its highly migratory behavior. N. vinitor always migrated when host plants dried up, and sometimes also when they were still green. Continuous quantitative records of flight activity, made near the population source, showed that the seasonal migrations of N. vintor closely reflected its population curve. As populations increased and declined throughout the season, migrants were produced continuously, resulting in a fairly smooth succession of departing individuals which became particularly obvious when large numbers suddenly discharged which occurred when special weather conditions caused sudden excessive flights.