Mosquito populations are profoundly governed by temperature because these insects are ectotherms, meaning their internal body temperature mirrors the surrounding environment. This dependence dictates their survival limits, development rate, and capacity to transmit disease-causing pathogens. Understanding these thermal boundaries is central to predicting when and where mosquitoes will thrive.
The Critical Cold Threshold
The temperature at which an adult mosquito dies is not a single fixed point but a range known as the Lower Lethal Temperature (LLT), which depends on the species and the duration of exposure. Mosquitoes are “chill-susceptible,” meaning they die from indirect chilling injury at temperatures well above the actual freezing point of their body fluids. For many common species, like Aedes and non-diapausing Culex, death occurs rapidly when temperatures drop below 0°C (32°F) for even a few hours.
Exposure to temperatures between -2°C and -5°C (28.4°F and 23°F) without the protective layer of water can cause 100% mortality within 30 to 100 minutes. Although the true physiological freezing point (supercooling point) for species like Culex pipiens is around -16°C (3.2°F), they die long before reaching this point. The main cause of death at these low temperatures is often the inability to maintain internal water and ion balance, leading to organ failure rather than freezing.
Survival Strategies in Cold Weather
When temperatures fall below the threshold for adult activity but remain above the immediate lethal limit, mosquitoes employ diapause to survive the winter. This physiological adaptation is a state of arrested development, often triggered by a shortening of daylight hours (photoperiod). Diapause allows mosquitoes to survive for months without feeding, waiting for favorable spring conditions to return.
The specific life stage that enters diapause varies by species, which dictates their cold tolerance. For container-breeding mosquitoes like those in the Aedes genus, only the eggs enter diapause, becoming highly resistant to cold and desiccation while surviving on dried surfaces. Conversely, the common house mosquito, Culex pipiens, survives the cold as a non-feeding adult female. These females seek shelter in protected places like basements or culverts to enter a state of dormancy, tolerating low sub-freezing temperatures for longer periods.
Temperatures Governing Activity and Reproduction
Temperature governs mosquito daily activity and reproductive output, effectively defining the length of the “mosquito season.” The optimal temperature range for adult activity, including biting and flight, is between 15°C and 27°C (59°F and 81°F). Warmer temperatures, such as around 28°C (82°F), often correspond to maximum activity and faster feeding rates.
Below approximately 10°C (50°F), larval development ceases entirely for most species, establishing a baseline development threshold. As temperatures rise above this minimum, the entire life cycle accelerates. For example, the time from egg to adult emergence can be reduced from several weeks at 15°C to less than a week at 30°C, leading to larger mosquito populations.
Temperature also directly influences the speed at which pathogens, such as the viruses that cause West Nile or Dengue fever, develop inside the mosquito. This period, known as the extrinsic incubation period, shortens dramatically as temperatures increase. The time it takes for a Dengue virus to become transmissible in an Aedes mosquito can be halved when the ambient temperature rises from 25°C to 30°C, increasing the risk of disease transmission.
The Upper Lethal Limit (Heat Stress)
While cold is the primary seasonal constraint, high temperatures can also be lethal, defining the Upper Lethal Temperature (ULT) for mosquitoes. Sustained temperatures above 35°C (95°F) become stressful, and the upper thermal limit for many species is around 40°C (104°F). For Aedes aegypti, the 50% lethal temperature for eggs is approximately 36.5°C (97.7°F), with survival dropping sharply above this point.
Death from heat is often caused by a combination of high temperature and desiccation, especially for immature stages. Larvae in shallow water containers are vulnerable to rapid temperature increases, and the combination of heat and evaporation can quickly eliminate entire broods. Although adult mosquitoes may briefly tolerate temperatures up to 40°C, prolonged exposure significantly reduces their lifespan and fertility.