Mosquitoes are cold-blooded insects, meaning their body temperature and activity are directly tied to their environment, and they become inactive when temperatures drop below approximately 50°F (10°C). While it may seem like they vanish completely during the colder months in temperate climates, many species have evolved strategies to survive the winter. The ability to survive the cold varies significantly by species and local climate. For those in colder areas, survival involves entering a state of suspended animation until warmer conditions return, ensuring the next season’s population is ready to emerge immediately in the spring.
The Life Stages That Survive
Mosquito species employ different stages of their four-part life cycle—egg, larva, pupa, or adult—to withstand the winter.
Many common nuisance mosquitoes, such as those in the Aedes genus, survive the cold as desiccation-resistant eggs. The final generation of females in the fall lays these cold-hardy eggs in dried areas, such as ground depressions or containers that will later flood with snowmelt or spring rain. These embryos remain dormant and can tolerate sub-zero temperatures until warmth and moisture signal it is time to hatch.
Other widespread species, particularly those in the Culex genus, overwinter as mated adult females. These females seek sheltered locations to enter a hibernation-like state, ensuring they are ready to emerge, find a blood meal, and begin laying eggs as soon as the weather warms. A few species, like some Culiseta and Anopheles, may survive as semi-dormant larvae in water bodies that do not completely freeze, often buried in the mud of swamps.
Physiological Adaptations for Cold Survival
Mosquitoes that survive the winter rely on specific biological mechanisms to cope with freezing temperatures and suspended activity. The primary physiological change is entering diapause, a hormonally programmed state of suspended development. This state is triggered by environmental cues like shortening day length or decreasing temperatures in the fall, causing a reduction in metabolic rate and arrested reproductive development.
Once diapause is initiated, the mosquito undergoes biochemical changes to resist cold injury, primarily accumulating cryoprotectants. These compounds, which often include polyols like glycerol and sugars like trehalose, act as biological antifreeze. By concentrating these substances in their body fluids, the mosquito lowers the freezing point of its internal water, preventing the formation of ice crystals within its cells. This process, called freeze avoidance, combined with reduced metabolism and stored fat reserves, allows the mosquito to endure months of dormancy without feeding.
Finding Overwintering Hiding Spots
The physical locations chosen for overwintering are specific to the life stage that survives and must provide shelter from extreme cold. Adult female mosquitoes in diapause seek out dark, humid, and relatively frost-free locations.
Common overwintering sites include natural shelters like hollow logs, animal burrows, and rock crevices, or human-made structures. Within developed areas, adult females frequently utilize underground spaces such as storm drains, culverts, and abandoned sewer systems. They also find refuge in undisturbed parts of buildings, including basements, crawl spaces, and attics, where temperatures remain just above freezing.
For species that overwinter as eggs, the female lays them in places that are dry but guaranteed to flood later, such as the soil around dried-up ponds, old tires, or clogged gutters. These eggs can remain viable for months or even years, waiting for the optimal combination of water and warmth to signal the start of the next season.
Impact on Seasonal Disease Transmission
The overwintering population is the direct link between the previous season’s disease activity and the start of the next year’s outbreaks. Infected adult females, such as those in the Culex genus carrying West Nile Virus (WNV), can survive the winter with the virus still active in their system.
When these females emerge from diapause in the spring, they immediately seek a blood meal, potentially transmitting the pathogen to birds or other hosts and initiating the annual transmission cycle.
For species like Aedes, which transmit viruses such as Dengue and Zika, the virus can be passed directly from the infected female to her eggs, a process known as vertical transmission. When these infected, cold-hardy eggs hatch in the spring, the newly emerged adult mosquitoes are already capable of transmitting the virus. This mechanism allows the pathogen to survive the winter in a dormant state within the mosquito population, ensuring the reintroduction and circulation of diseases as soon as warm weather returns.