Heat hardening enhances mosquito heat tolerance in a species-specific and trait-specific manner

Abstract

Models predict that the distribution of ectotherms including mosquitoes will shift with climate change, but few incorporate adaptive capacity. Acclimation is one mechanism by which mosquitoes could adapt, allowing mosquitoes that have experienced sub-lethal stress previously to tolerate subsequent stressful environments. In this study we evaluated the heat tolerance of three vector mosquito species, Aedes aegypti, Ae. notoscriptus and Culex quinquefasciatus, after being previously exposed to heat hardening. Adult males and females were heat-hardened by exposure to 41{degrees}C for one hour and subsequently tested for heat survival and knockdown following one-hour heat shocks across a range of temperatures up to the lethal limit, ramping CTmax assays and static temperature knockdown time assays. The three species differed markedly in their heat tolerance across all assays, with Ae. aegypti being the most heat tolerant and Cx. quinquefasciatus being the least. Females from all three species were more heat tolerant than males in the one-hour heat shock assays, but effects of sex were absent or inconsistent for CTmax and heat knockdown time assays. A beneficial impact of heat hardening on subsequent heat shock knockdown was evident in both sexes of all three species. However, hardening effects differed substantially for survival 24 hr later, ranging from no effect of hardening in Cx. quinquefasciatus to a [~]1{degrees}C increase in LT50 in Ae. notoscriptus. In contrast, no effects of heat hardening were detected for CTmax or static knockdown time assays. An additional experiment in Ae. aegypti detected no benefits of heat shock exposure in female patents on the thermal tolerance of offspring. Our findings emphasize the need to consider effects of acclimation including heat hardening in models to predict the response of mosquitoes to climate warming. They also have implications for measuring thermal tolerance in mosquitoes more generally, given that both sex and hardening effects depend on the type of assay used and trait measured.