PERSISTENCE OF AEDES TRISERIATUS DESPITE THE INVASION OF AEDES JAPONICUS IN WESTERN WISCONSIN.

For more than 40 yr, a multicounty vector control program that surrounded the epicenter of the outbreak of La Crosse virus (LACV) was in place to survey and control point sources for the virus transmission vector Aedes triseriatus. During that period, the number of cases of LACV disease declined to 1 and 2 cases in 2003 and 2004, respectively, and 0 reported cases within La Crosse County, WI, since 2005. We surveyed Ae. triseriatus populations in La Crosse and Trempealeau counties, WI, during the summers of 2020-2021 to determine whether the decline in LACV disease cases corresponded to a possible decline in Ae. triseriatus populations, and whether the invasive species, Ae. japonicus, is competing with and replacing Ae. triseriatus. We sampled containers for larvae and collected eggs using oviposition (OP) traps. We collected 606 and 20,020 mosquitoes in 2020 and 2021, respectively. Overall, we found a lower proportion of Ae. japonicas than Ae. triseriatus in natural oviposition sites in 2020 (n = 72 and 224 larvae, respectively; z = 8.78, P < 0.0001), in OP traps in 2020 (n = 5 and 199, respectively; z = 13.51, P < 0.0001), and in OP traps in 2021 (n = 358 and 19,099, respectively; z = 134.31, P < 0.001). Aedes japonicus, therefore, does not appear to be outcompeting native species in western Wisconsin forests. Although there were as few as 4 total cases of LACV disease reported in 2020 and 2021 in Wisconsin, we found ample evidence of Ae. triseriatus activity. These data point to the need for continued surveillance and control efforts in the interest of preventing vector-borne diseases.