Variation in postdiapause emergence patterns of Colorado potato beetle adults over six consecutive years at a single site was only partially explained by cumulative soil heat units. However, soil moisture measured at a depth of 15 cm and daily rainfall accounted for much of the unexplained variation in yearly emergence patterns. We hypothesized that emergence of overwintered beefles was a two-stage process consisting of thermally regulated postdiapause development followed by moisture-driven emergence from the soil. To test this hypothesis we constructed two simulation models that mirrored these processes. One model used soil moisture measured at a depth of 15 cm to drive emergence, while the other used daily rainfall for the same purpose. Both models were able to represent the field data; however, the best congruence was achieved when rain was an independent factor. Because rainfall was a better determinant of moisture, the dynamics of moisture at the soil surface may be of great importance to the beetle. Greater precision in model simulations could probably be achieved by improving the way in which soil moisture dynamics are depicted.