Body size and development time are two critical phenotypic traits that can be highly adaptive in insects. Recent population genetic analyses and crossing experiments with the mountain pine beetle (Dendroctonus ponderosae Hopkins) have described substantial levels of neutral molecular genetic differentiation, genetic differences in phenotypic traits, and reproductive isolation. To determine whether genetic differences in adaptive phenotypic traits exist that correspond to reproductive boundaries, we conducted a common garden experiment with seven D. ponderosae populations previously used to identify reproductive incompatibilities. Genetic differences in development time were striking between faster developing, and more synchronized, northern populations and slower developing, and less synchronized, southern populations. Additionally, genetic differences in average body size were found between many populations. Differences in these two traits, however, failed to clearly demarcate populations that exhibit reproductive incompatibilities. Our results suggest that local selection pressures likely drive divergence in these two traits that is largely independent of the evolution of reproductive isolation in D. ponderosae.