The flights of adult Mexican bean beetles from experimental plots of lima beans and soybeans were monitored to determine the proximate cues to which beetles respond when initiating flight. Most beetles, including newly eclosed individuals, flew out of the plots within 1 week of their release. Two indices of activity relating to flight frequency and flight intensity varied similarly in response to weather conditions, and indicated that flight was related most strongly to temperature and wind speed. Flight intensity was then modeled as a nonlinear function of these two interacting weather variables. The model accounts for the observed bimodal pattern of daily flight activity.