The flight habits of the mosquito Culex tarsalis, a vector of viral encephalitis, were studied in the rice-growing area of Yolo County in the lower Sacramento Valley from 1959 through 1963. Females were captured in dry ice traps every night the traps were set and in all types of terrain. Apparently there are no real barriers to mosquito dispersal in the valley and the surrounding foothills.

The major flight of this mosquito occurs within two hours after sunset and most of the females appear to fly within 50 feet of the ground. When they emerge from their daytime resting places they tend to head into the wind, and at low wind velocities they disperse in all directions. Winds of 3 or 4 miles per hour catch the mosquitoes sooner or later and carry them downwind. Winds of 6 mph and more are apt to inhibit flight. We calculated actual flying speed over a short distance at 4.75 mph.

We have observed this mosquito in flight at temperatures between 55° and 92° F, but usually its activity is reduced below 65° and increased between 65° and 75°. Flight activity is governed by light intensity in relation to temperature and is affected also by wind and other factors.

We marked and released approximately 253,000 females of this species and recaptured 585 of them (0.23 per cent). In these experiments we recorded natural dispersal as far as 5 miles downwind on the night of release and 15.75 miles downwind two nights later. As the chances against recapturing any particular specimen are enormous and increase with distance, it seems certain that this mosquito spreads beyond any of the recovery sites. Probably it travels 8 or 10 miles in two evenings, and we believe that it can spread in one generation, with the prevailing SSE winds, at least 20 or 25 miles from its breeding areas.