The effect of (*a*) immunity (cumulative incidence; coeff. = -2.05, *F*_{1,300} = 96.42, *p* < 0.001), (*b*) precipitation (coeff. = -0.0009, *F*_{1,161} = 2.20, *p* = 0.14), (*c*) drought (coeff. = -0.14, *F*_{1,274} = 27.01, *p* < 0.001), (*d*) winter severity (coeff. = -0.05, *F*_{1,34} = 2.95, *p* = 0.09), (*e*) temperature (PIP: coeff. = 0.06, *F*_{1,276} = 2.58, *p* = 0.10; TAR: coeff. = 0.22, *F*_{1,144} = 53.59, *p* < 0.001; QUI: coeff. = 0.002, *F*_{1,104} = 0.0005, *p* = 0.98) and (*f*) temperature modelled as the relative *R*_{0} value at a given temperature (coeff. = 1.66, *F*_{1,121} = 17.33, *p* < 0.001) on the total logged number of WNND cases (adjusted for state random effects) in a given state and year (1999–2013). In (*a*–*d*,*f*), the filled red points and fitted lines are univariate regressions for states in which that predictor was significant (*a* < 0.05), while open black points depict states in which the predictor was not significant. In (*e*), green crosses, blue circles and green triangles denote states where *C. tarsalis*, *C. pipiens* and *C. quinquefasciatus*, respectively, dominate transmission and the relationship is only significant for *C. tarsalis*. (Online version in colour.) |