Critical thermal maxima of Polistes life stages from different climates, with a critical evaluation of methods.

Ambient temperature is a crucial abiotic factor for ectotherms. It strongly influences development, life and abundance, as well as success in colonizing new habitats. In the eusocial paper wasps Polistes sp., colony-forming insects with open nests, the larvae and pupae have limited options to influence their own body temperature in response to high environmental temperatures. They are dependent on measures taken by the adults to keep it at tolerable levels. We determined the upper thermal limits (CTmax) in field populations of three paper wasp species (Polistes dominula, P. gallicus, P. biglumis) from different climates (temperate, Mediterranean, alpine) for three life stages (larvae, pupae, adults). Due to morphological and physiological characteristics of the individual life stages, they did not show the same reactions to temperature rise and heat stress in terms of respiration and body movement. CTmax evaluation by established methods (mortal fall, short-term respiration patterns) was not possible, so we had to develop an adapted evaluation type based on long term respiration patterns. The most striking result was that the CTmax was similar in all populations and life stages, ranging from 47.6 to 48.8 °C in larvae and pupae, and from 47.1 to 47.9 °C in adults. P. dominula differed from P. gallicus and P. biglumis; the latter did not differ significantly (all stages). Tests in individual groups (populations, life stages) showed differences in one parameter or the other (population, life stage, mass). Overall, population (and thus climate as a related factor) and life stage, but not mass, had a significant effect on CTmax.s.