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In short‐term field trials at combinations of ambient temperature (°C) and insolation (W·m−2), larval Colorado potato beetles (Leptinotarsa decemlineata [Say] [Coleoptera: Chrysomelidae]) were observed after their release on the adaxial surface of leaflets on potato plants (Solanum tuberosum L. Solanaceae). The larvae either began feeding or moved under the leaflet; mean interval from release to expression of these behaviors (2.9 ± 0.05 min [n = 358]) was independent of air temperature and insolation. Proportion of larvae moving under the leaflet increased logistically with both air temperature and insolation. A 1 W·m−2 change in insolation (P) evoked the same effect on this proportion as a 0.0838 °C change in air temperature (Ta), so the two quantities were combined as T* = Ta + P · 0.0838 °C/(W·m−2), which has units of °C. The proportion of larvae moving under the leaflet increased logistically with T*. In 1‐day field trials we monitored air temperature, insolation and proportion of larvae under the leaflet, and compared the latter to predictions from the logistic regression derived from the short‐term trials. Consistently more larvae occurred under leaflets than predicted from the logistic regression; this bias diminished as T* increased until at T* > 40 °C, observed and predicted proportions were equal. This pattern of deviation from the predictions of the logistic regression is consistent with a thermoregulatory strategy in which larvae move away from hostile conditions, rather than seek optimal conditions.