How plastic are the critical thermal limits of insects? Tsetse (Glossina spp.) as a case study for investigating upper thermal limits and plasticity

Animals vary in thermal tolerance, which set the limits at which survival and reproduction can occur. Thermal plasticity, or the ability to respond to temperature exposure via phenotypic changes, can alter thermal limits, allowing animals to tolerate more extreme temperatures. We investigated the response of insects - significant as ecosystem service providers, vectors of disease, and crop pests - to warming temperatures using comparative meta-analyses across over 100 insect species and detailed experimental work on tsetse flies (Glossina spp.), vectors of human and animal African trypanosomiasis. I find that plasticity of insect thermal tolerance is generally weak, especially upper thermal limits, indicating physiological and evolutionary limits at high temperatures. Weak plasticity of upper thermal limits was mirrored in tsetse, which show limited or non-existent adult and between-generation thermal plasticity. Considerable variation was found in the level of thermal plasticity among insects generally, and among tsetse species, but trends in tolerance remained obscure. Thermal fertility limits, the temperature at which reproduction is prevented, was found to occur at lower temperatures than those which kill tsetse, but, in contrast to studies on other insect species, female fertility as temperature sensitive as male fertility. These differences indicate that a diversity of species should be examined to ensure generalisations are relevant across insect species. Finally, it was found that body size was important in shaping thermal tolerance limits, with high developmental temperature leading to small adult body size and, in consequence, reduced upper thermal tolerance and survival. These data support predictions of range contractions in tsetse species in response to climate change. More broadly, findings highlight grave consequences of warming temperature for insect populations and the need for detailed experimental work on further understudied groups of insects.