The study investigates the heat tolerance and acclimation capacity of six bee species from Lesvos, Greece, which vary in body size, nesting habits, and sociality. The critical thermal maximum (CTMax) was measured to assess acclimation responses, and the influence of body size and CTMax on acclimation capacity was evaluated. Bees' CTMax increased with body size but did not significantly change in response to short-term acclimation or acute heat exposure. The average acclimation response ratio (ARR) was 9%, and it was not associated with body size or CTMax. Acute heat exposure did not increase CTMax. These findings suggest that bees have limited capacity to enhance heat tolerance through acclimation, making them physiologically sensitive to rapid temperature changes during extreme weather events. The results highlight the importance of behavioral thermoregulation and nest selection for bees' survival in a changing climate.The study investigates the heat tolerance and acclimation capacity of six bee species from Lesvos, Greece, which vary in body size, nesting habits, and sociality. The critical thermal maximum (CTMax) was measured to assess acclimation responses, and the influence of body size and CTMax on acclimation capacity was evaluated. Bees' CTMax increased with body size but did not significantly change in response to short-term acclimation or acute heat exposure. The average acclimation response ratio (ARR) was 9%, and it was not associated with body size or CTMax. Acute heat exposure did not increase CTMax. These findings suggest that bees have limited capacity to enhance heat tolerance through acclimation, making them physiologically sensitive to rapid temperature changes during extreme weather events. The results highlight the importance of behavioral thermoregulation and nest selection for bees' survival in a changing climate.