Fig. 2. The discrete time analysis by Thomas and Urena (2001) indicated that the important variable to track in the control of West Nile virus is the total number of mosquitoes as a function of time t in weeks, TM(t). The left inset shows that pesticide spraying to control mosquito populations is ineffective when the spray kill rate is below a threshold value. The right inset describes the effectiveness of spraying when the spray kill rate is higher than the threshold value.