The effects of acclimation and rates of temperature change on critical thermal limits in Tenebrio molitor (Tenebrionidae) and Cyrtobagous salviniae (Curculionidae) Jessica L. Allen, Susana Clusella-Trullas, Steven L. Chown ⇑ Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa article info Article history: Received 19 November 2011 Received in revised form 28 January 2012 Accepted 30 January 2012 Available online 8 February 2012 Keywords: Acclimation Biocontrol Critical thermal limits Thermal tolerance Rate of change abstract 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 ther- mal environments on such survival. However, investigations of these limits may be affected by the circum- stances 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 temper- ature changes. Here we therefore examine the effects of rate of temperature change on critical thermal maxima (CT max ) and minima (CT min ), 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 CT max (c. 2 °C) and CT min (c. 3 °C), while in C. salviniae faster rates of change increased both CT max (c. 6 °C) and CT min (c. 4 °C). CT max in T. molitor showed little response to acclimation, while the response to acclimation of CT min was most pronounced following exposure to 35 °C (from 25 °C) and was complete within 24 h. The time course of acclimation of CT max in C. salviniae was 2 days when exposed to 36 °C (from c. 26 °C), while that of CT min 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 CT max and 6 days for CT min . In C. salviniae, little change in CT max and CT min (<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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The investigation of critical thermal limits provides important insights into how the physiology, distribution and ecology of species are influenced by climate (Lutterschmidt and Hutchison, 1997; Chown and Terblanche, 2007). These limits constitute the endpoints of thermal performance curves, which form one of the major means in which the effects of climate on animal and plant performance, and the short- and long-term responses of animals to such climate effects may be investigated (Huey and Stevenson, 1979; Chown et al., 2010). The impetus for investigating these lim- its, the extent to which they show phenotypic plasticity, and the rates at which both trait means and extent of plasticity might evolve has grown recently. Most prominent among the reasons for the situation is the realization that plasticity or evolutionary lability may be limited for some traits (Chown, 2001; Hoffmann et al., 2003a; Stillman, 2003; Kellermann et al., 2009), which may mean reduced scope for some organisms to respond to on-going climate change. The implications thereof are important for several reasons, of which two are perhaps most significant. First, it appears that tropical or subtropical terrestrial ectotherms, and perhaps oth- ers, have narrow thermal safety margins (Deutsch et al., 2008; Hoff- mann, 2010; Clusella-Trullas et al., 2011) and may therefore face extinction under continuing climate change (Sinervo et al., 2010). Second, forecasts indicate that climate change will bring with it an increase in the frequency of extreme temperatures, both for rea- sons of a straightforward shift in the distribution of temperatures overall (IPCC, 2007), and because extreme events are likely to be- come more common for other reasons (Easterling et al., 2000; Knapp et al., 2008; Rahmstorf, 2008). The effects of the latter are complicated by their highly asynchronous patterns of occurrence (Walther et al., 2002). Thus, the absence of phenotypic plasticity or a slow rate of response may have profound effects on organisms that are exposed to such extremes (Helmuth et al., 2006; Chown et al., 2010). In consequence, predicting the possible impacts of 0022-1910/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jinsphys.2012.01.016 ⇑ Corresponding author. E-mail address: slchown@sun.ac.za (S.L. Chown). Journal of Insect Physiology 58 (2012) 669–678 Contents lists available at SciVerse ScienceDirect Journal of Insect Physiology journal homepage: www.elsevier.com/locate/jinsphys