Mosquito thermal tolerance is remarkably constrained across a large climatic range

Abstract

How mosquitoes may respond to rapid climate warming remains unknown for most species, but will have major consequences for their future distributions, with cascading impacts on human well-being, biodiversity and ecosystem function. We investigated the adaptive potential of a wide-ranging mosquito species, Aedes sierrensis, across a large climatic gradient by conducting a common garden experiment measuring the thermal limits of mosquito life-history traits. Although field-collected populations originated from vastly different thermal environments that spanned over 1200 km, we found limited variation in upper thermal tolerance between populations. In particular, the upper thermal limits of all life-history traits varied by less than 3°C across the species range and, for most traits, did not differ significantly between populations. For one life-history trait-pupal development rate-we did detect significant variation in upper thermal limits between populations, and this variation was strongly correlated with source temperatures, providing evidence of local thermal adaptation for pupal development. However, we found that maximum environmental temperatures across most of the species' range already regularly exceed the highest upper thermal limits estimated under constant temperatures. This result suggests that strategies for coping with and/or avoiding thermal extremes are likely key components of current and future mosquito thermal tolerance.

Keywords: climate adaptation; common garden experiment; mosquito; thermal tolerance.

Figure 1.

Sample collection locations and experimental design used to measure mosquito thermal performance. Ten populations were collected as larvae from tree holes across the Western USA, reared in the laboratory under common conditions for one generation, then randomly designated into one of six temperature treatments. The total number of larvae assigned to each treatment is noted above (n_total) as is the range of larvae from each population (n_pop); electronic supplementary material, table S2 indicates the full breakdown of larvae per population and treatment. Individuals were checked daily for life stage transitions (e.g. larvae to pupae, pupae to adult) or death. Map colours denote the average maximum annual temperature (°C) from 1991 to 2020 from PRISM data. Electronic supplementary material, figure S1 shows the average minimum and mean temperature across this same extent. Population metadata, including full site names, latitude, longitude and elevation are provided in electronic supplementary material, table S1.

Figure 2.

For most life-history traits, thermal performance varies minimally between populations. Each curve denotes the average thermal performance for one population for a given trait. Populations are coloured and ordered in the legend by their latitude of collection.

Figure 3.

Thermal limits and optima vary between life-history traits, but minimally between populations. Lower thermal limits, thermal optima and upper thermal limits for each life-history trait and population (left, middle and right points and error bars in each panel, respectively). Thermal performance parameter estimates are derived from the thermal performance curves for traits for which the means are depicted in figure 2. Points and error bars denote the mean and 95% credible intervals for each parameter, respectively. Populations (listed on the right) are coloured and ordered by latitude of collection. Units of development rates and lifespan are 1/days and days, respectively. Note that survival probability curves that are truncated at one have very uncertain optimal temperatures because a wide range of temperatures have similarly high survival probability.

Figure 4.

Evidence of local thermal adaptation. Correlations between the source thermal environment and population thermal performance provide evidence of local thermal adaptation (right). Statistically significant Pearson's correlations (r; p < 0.05) are denoted with (*). Note that correlations were only examined for traits with significant between-population variation. The relationship between upper thermal limits for pupal development rate and the number of days with temperatures exceeding 35°C (one of the significant correlations noted in the table) is visualized in the plot on the left.