The overall objective is to identify and quantify parameters that affect vertical (transovarial) or horizontal (amplification via vertebrates) transmission of La Crosse virus by the mosquito, Aedes triseriatus. Current emphasis on vertical parameters includes further refining of the range of estimated values for the maternal vertical transmission rate which involves the transovarial transmission rate, filial infection rate and the net replacement rate of infected as compared to uninfected females. Life table techniques will be used. To determine whether the current estimate of 1.2 annual generations of the vector, Aedes triseriatus, should be revised downward, studies will be conducted to determine the effect of competition and or toxic substances produced by 1st generation larvae upon 2nd generation larvae that are simultaneously in the treehole breeding sites. Current emphasis on horizontal parameters involves collection of bloodfed females in the field and identification of bloodmeal source using precipitin techniques, in order to determine the proportion of bloodmeals taken from amplifying hosts.