A temperature model for predicting the timing of increased risk of West Nile Virus

In Illinois, two mosquito species, Cx. pipiens and Cx. restuans, carry the West Nile Virus (WNV). Culex pipiens is the primary vector for transmission of West Nile Virus because it does bite humans while Cx. restuans does not. Cx. restuans is dominant early in the season while Cx. pipiens becomes dominant later in the season. The change in population dominance from Cx. restuans to Cx. pipiens, called crossover, signals a time of increased risk of WNV transmission. We have developed a model that uses temperature data to predict the timing of crossover. In the Champaign-Urbana area, the average time of crossover is around the second week of August, but this can vary by several weeks. Using data collected since the late 1980s, we investigated the dependence of crossover timing on temperature conditions. Models were developed, relating crossover timing to the number of days with daily maximum temperature exceeding 83°F and degree day accumulations for a base temperature of 63°F. These models explain over 60% of the interannual variance in crossover date. The models were applied operationally for the first time in 2005. The models were run weekly and a web site provided probabilistic forecasts of crossover date. The model details and 2005 performance will be described.