The range of the spruce budworm, Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae) is coincident with the boreal forest in western Canada. Spruce budworm is also found in disjunct white spruce stands south of the boreal forest zone in Alberta. This range spans 15 degrees of latitude, from the Cypress Hills in southern Alberta to the Arctic Circle. Observed regional differences in budworm phenology led to an investigation of intrinsic development rates for populations across this gradient of latitude. Individuals from six outbreak populations were reared at ten constant temperatures from 9 to 33$\sp\circ$C, under a photoperiod of 16L:8D. When development rates were described by a nonlinear equation, rate parameters were consistent between populations. Thus, differences in temperature-dependent development rates are not responsible for differences in phenology between budworm populations at different latitudes. The day-degrees required for spruce budworm to complete development are variable between geographic locations and between years. Bias in calculating day-degrees from daily versus hourly temperatures also varied with respect to location and year, but could not account for differences in thermal constants. Deviation of development rates from linearity at temperature extremes also fails to account for regional differences in thermal constants. A nonlinear physiological time model and a linear day-degree model produce similar development profiles for annual temperature regimes at northern and southern locations. These results reveal limitations of phenological models based solely on ambient temperature.