Rapid thermal responses and thermal tolerance in adult codling moth Cydia pomonella (Lepidoptera: Tortricidae)
In order to preserve key activities or improve survival, insects facing variable and unfavourable thermal environments may employ physiological adjustments on a daily basis. Here, we investigate the survival of laboratory-reared adult Cydia pomonella at high or low temperatures and their responses to pre-treatments at sub-lethal temperatures over short time-scales. We also determined critical thermal limits (CTLs) of activity of C. pomonella and the effect of different rates of cooling or heating on CTLs to complement the survival assays. Temperature and duration of exposure significantly affected adult C. pomonella survival with more extreme temperatures and/or longer durations proving to be more lethal. Lethal temperatures, explored between -20°C to -5°C and 32°C to 47°C over 0.5, 1, 2, 3 and 4h exposures, for 50% of the population of adult C. pomonella were -12°C for 2h and 44°C for 2h. Investigation of rapid thermal responses (i.e. hardening) found limited low temperature responses but more pronounced high temperature responses. For example, C. pomonella pre-treated for 2h at 5°C improved survival at -9°C for 2h from 50% to 90% (p<0.001). At high temperatures, pre-treatment at 37°C for 1h markedly improved survival at 43°C for 2h from 20% to 90% (p<0.0001). We also examined cross-tolerance of thermal stressors. Here, low temperature pre-treatments did not improve high temperature survival, while high temperature pre-treatment (37°C for 1h) significantly improved low temperature survival (-9°C for 2h). Inducible cross-tolerance implicates a heat shock protein response. Critical thermal minima (CTmin) were not significantly affected by cooling at rates of 0.06, 0.12 and 0.25°Cmin-1 (CTmin range: 0.3-1.3°C). By contrast, critical thermal maxima (CTmax) were significantly affected by heating at these rates and ranged from 42.5 to 44.9°C. In sum, these results suggest pronounced plasticity of acute high temperature tolerance in adult C. pomonella, but limited acute low temperature responses. We discuss these results in the context of local agroecosystem microclimate recordings. These responses are significant to pest control programmes presently underway and have implications for understanding the evolution of thermal tolerance in these and other insects. © 2010 Elsevier Ltd.