FIGURE 4.

(a) Thermal performance is often measured by placing mosquitoes in different life stages and infection stages across a range of constant temperatures at a set relative humidity (typically between 70–90% RH). However, despite holding relative humidity constant, as temperatures warm there will be a corresponding increase in the vapour pressure deficit (VPD) and the amount of water stress mosquitoes experience. Overlaying these relationships (from Figure 1) on a given temperature-trait relationship demonstrates that the sensitivity of trait performance to variation in relative humidity should be highest on the descending limb of this relationship. Es = saturation vapour pressure, which increases exponentially with temperature and is estimated as a function of temperature using the Tetens equation. Ea = vapour pressure, meaning the actual amount of water in the air and can be derived from relative humidity (RH) as Ea = RH/100 * Es. (B–D) represent the hypothetical responses of three temperature-trait relationships to variation in relative humidity. These shifts are predicted to both decrease the thermal optimum and maximum for some traits (e.g., (b) lifespan and (d) vector competence) or increase them for others (e.g., (c) per capita biting rate).