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Thermal stress is a critical factor when assessing responses of organisms and populations to global warming. Understanding responses to upper lethal temperature is critical to predict the effect of heat waves on populations. On the other hand, it is important to understand physiological traits allowing populations to adapt to changes in temperature. Here we compare survival to high temperatures of two moth species: a Mediterranean species Thaumetopoea pityocampa (Denis et Schiffermuller) and the broadly distributed Helicoverpa armigera (Hubner). Larvae were exposed to thermal stress, mimicking heat waves with daily maximum temperature (T-max) up to 45 degrees C. Control groups were kept at 25 degrees C. The changes in two heat shock proteins HSP70 and HSP90 were further studied for the second and third larval stages of T. pityocampa and H. armigera, respectively. An increase in mortality was observed on T. pityocampa larvae for T-max above 38 degrees C. On the contrary, larvae of H. armigera survived well up to 44 degrees C T-max, but survival decreased abruptly at 45 degrees C T-max. Thus, the two species differ by 6 degrees C on their upper threshold. For H. armigera HSP70 and HSP90 were both inducible by temperatures above 38 degrees C, steeply increasing with temperature which may account for superior adaptation to upper temperatures. In T. pityocampa HSP70 and HSP90 occurred constitutively in the control at 25 degrees C and did not increase with temperature. Results evidence major differences in HSP expression between species, which might account for differences in larval survival at the upper thermal limits.