The effects of acclimation and rates of temperature change on critical thermal limits in Tenebrio molitor (Tenebrionidae) and Cyrtobagous salviniae (Curculionidae)

Critical thermal limits provide an indication of the range of temperatures across which organisms may survive, and the extent of the lability of these limits offers insights into the likely impacts of changing thermal environments on such survival. However, investigations of these limits may be affected by the circumstances under which trials are undertaken. Only a few studies have examined these effects, and typically not for beetles. This group has also not been considered in the context of the time courses of acclimation and its reversal, both of which are important for estimating the responses of species to transient temperature changes. Here we therefore examine the effects of rate of temperature change on critical thermal maxima (CTmax) and minima (CTmin), as well as the time course of the acclimation response and its reversal in two beetle species, Tenebrio molitor and Cyrtobagous salviniae. Increasing rates of temperature change had opposite effects on T. molitor and C. salviniae. In T. molitor, faster rates of change reduced both CTmax (c. 2 °C) and CTmin (c. 3 °C), while in C. salviniae faster rates of change increased both CTmax (c. 6 °C) and CTmin (c. 4 °C). CTmax in T. molitor showed little response to acclimation, while the response to acclimation of CTmin was most pronounced following exposure to 35 °C (from 25 °C) and was complete within 24 h. The time course of acclimation of CTmax in C. salviniae was 2 days when exposed to 36 °C (from c. 26 °C), while that of CTmin was less than 3 days when exposed to 18 °C. In T. molitor, the time course of reacclimation to 25 °C after treatments at 15 °C and 35 °C at 75% RH was longer than the time course of acclimation, and varied from 3–6 days for CTmax and 6 days for CTmin. In C. salviniae, little change in CTmax and CTmin (<0.5 °C) took place in all treatments suggesting that reacclimation may only occur after the 7 day period used in this study. These results indicate that both T. molitor and C. salviniae may be restricted in their ability to respond to transient temperature changes at short-time scales, and instead may have to rely on behavioral adjustments to avoid deleterious effects at high temperatures.