La Crosse (LAC) virus was isolated from 3 suspensions of Aedes triseriatus larvae and from 1 suspension of male mosquitoes reared from larvae collected from basal tree-holes in Wisconsin during March, April, May, and June of 1973. Isolations of this virus were also obtained from 5 suspensions of A. triseriatus larvae and 4 suspensions of male mosquitoes originating from old automobile tires. Virus transmission to suckling mice was accomplished by female A. triseriatus reared from larvae collected from the tires.
Only two tree-holes and the tires yielded LAC virus-infected larvae and adults. The minimum field infection rate (MFIR) for these tree-holes and the tires, based on one infected larva per pool, was 1/110 larvae from the tree-holes and 1/20 larvae from the tires. The MFIR for adults was 1/41 male mosquitoes originating from one of the tree-holes and 1/27 for male mosquitoes originating from the tires. The MFIR for female mosquitoes reared from larvae collected from the tires was 1/110. No virus was isolated from 255 female mosquitoes reared from larvae collected from the tree-holes. These findings strongly suggest that A. triseriatus is the reservoir of LAC virus and that transovarial transmission of the virus in A. triseriatus is the mechanism responsible for the survival of this arbovirus during the winter season in the north-central United States.