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Field Observations on the Biting Habits of Culex Tarsalis at Mitchell, Nebraska, and Logan, Utah

Systematic man-biting collections of mosquitoes were made at an urban and a rural site at Mitchell, Nebraska, in 1953, and at Logan, Utah, in 1954. Culex tarsalis ranked first in biting at the rural site in Mitchell (43% of 8,032 mosquitoes) and at the urban site in Logan (50% of 1,330). It ranked second to Aedes vexans at the urban site in Mitchell (24% of 4,024) and at the farmstead in Logan (25% of 10,779).

C. tarsalis was found to have a strong affinity for man, also for other hosts such as wild birds, chickens, and cattle, during the period from June until early September. Availability of host seemed to be the main criterion. With the advent of cool weather in the fall, C. tarsalis was found to feed reluctantly on man and other vertebrate hosts.

In both areas (42° N. Lat.), the normal peak activity started 30 minutes after sunset and lasted for 1 hour. The largest catches were made during the fourth 15-minute period following sunset.

Large catches of C. tarsalis were made at temperatures ranging from 58°F to 78° at Mitchell and from 56° to 78° at Logan. The mean temperature for the fourth quarter-hour period following sunset was 68.8° at Mitchell and 63.5° at Logan.

In Mitchell, the largest catches of C. tarsalis were made between July 25 and August 26; in Logan between July 11 and August 12. In biting intensity, the mean numbers of C. tarsalis per man/hour of exposure (quarter-hour periods 3 through 6) for the months of July and August varied from 13 to 79 at Mitchell, and 26 to 67 at Logan.

These studies suggest that avoidance of human exposure to mosquito bites during the first hour of summer darkness might serve as a practical prophylactic measure from the standpoint of encephalitis transmission. This same hour also would seem to be an excellent period for intensification of control efforts directed against adult C. tarsalis populations.