How Do Mosquitoes Survive the Winter?

Most of the mosquitoes biting you in late summer die once the cold sets in, but their species doesn’t disappear. Each kind hands winter off to one tough, sleeping stage of its life, whether egg, larva, or adult female. These sleepers shut down weeks before the first frost, reading the season from day length rather than waiting for the cold to arrive.1Oxford Academic (J. Med. Entomol.). Cold Nights, City Lights: Artificial Light at Night Reduces Photoperiodically Induced Diapause in Urban and Rural Populations of Aedes albopictus (Diptera: Culicidae) That means much of next spring’s mosquito population is already in place around your yard and home before winter begins.

Mosquitoes Go Into A Planned Winter Shutdown

In winter dormancy, development or reproduction is paused on purpose and stays paused even if a warm day comes along.2Oxford Academic (J. Med. Entomol.). Cold Nights, City Lights: Artificial Light at Night Reduces Photoperiodically Induced Diapause in Urban and Rural Populations of Aedes albopictus (Diptera: Culicidae) The ability seems to have evolved separately many times among mosquitoes, so even closely related species sleep through winter at different life stages.3PubMed. Mosquito diapause Within any one species, though, dormancy usually happens at just one stage of life.4PubMed Central. Diapause and quiescence: dormancy mechanisms that contribute to the geographical expansion of mosquitoes and their evolutionary success

This is different from being cold and sluggish. A merely cold insect revives as soon as it warms up, while a dormant one stays shut down until its program ends, which is why a single warm winter afternoon doesn’t hatch dormant eggs early. Dormancy also comes with body changes that protect against cold.

Females preparing to overwinter also change what they eat. In the common house mosquito, they stop seeking blood and switch to gorging on sugar from nectar and fruit.5PubMed Central. Diapause in the mosquito Culex pipiens evokes a metabolic switch from blood feeding to sugar gluttony Their bodies turn down the genes needed to digest blood and turn up the ones that build fat reserves.6PubMed Central. Diapause in the mosquito Culex pipiens evokes a metabolic switch from blood feeding to sugar gluttony

The fat matters because a female may need to survive months with no meals at all. What she stocks in fall is her only fuel until spring, and if she runs out early, she dies before she can lay eggs.

The Three Ways Mosquitoes Spend The Winter

Overwintering eggs are laid on surfaces such as soil, plastic containers and plants.10Cary Institute of Ecosystem Studies. West Nile virus beats winter They hatch when the longer, wetter days of spring arrive, and the first wave of biting mosquitoes follows soon after.11Cary Institute of Ecosystem Studies. West Nile virus beats winter

Dormant eggs are tiny and dark, often stuck just above a waterline, so you won’t spot them. They sit dry on container walls, tires and soil through winter and need water plus the right season to hatch.

Species that overwinter as adults hide in animal burrows, basements and sewer tunnels.12Cary Institute of Ecosystem Studies. West Nile virus beats winter Tree hollows and other human structures also serve as winter shelters.13Candor Pest Control. Do Mosquitoes Really Die in Winter? When temperatures rise, even briefly, these females may come out looking for food.14Cary Institute of Ecosystem Studies. West Nile virus beats winter

That is why you may see a mosquito in a basement or garage in midwinter. It is most likely an overwintering female of a house-mosquito type, not a new hatch. While truly dormant, she is sluggish and rarely bites, but a warm spell or a heated room can wake her. Male mosquitoes generally die in fall, so the winter survivors are mostly females.

How Eggs And Larvae Survive Ice

In the Asian tiger mosquito, dormant eggs don’t survive cold by growing a thicker shell or wax coat.15PubMed. Effects of diapause and cold acclimation on egg ultrastructure: new insights into the cold hardiness mechanisms of the Asian tiger mosquito Aedes (Stegomyia) albopictus Instead, cold exposure separates an inner membrane from the shell, which may trap ice before it reaches the embryo.16PubMed. Effects of diapause and cold acclimation on egg ultrastructure: new insights into the cold hardiness mechanisms of the Asian tiger mosquito Aedes (Stegomyia) albopictus

Water-dwelling stages face a harder problem. Most insects that live in water can only stay unfrozen down to about −3 to −7°C (27 to 19°F), because ice in the surrounding water can push into their bodies through tiny openings in their skin.17American Entomologist. Surviving the Big Chill: Overwintering Strategies of Aquatic and Terrestrial Insects The few water insects that can survive freezing belong to the fly group that includes mosquitoes.18American Entomologist. Surviving the Big Chill: Overwintering Strategies of Aquatic and Terrestrial Insects Tree-hole mosquito larvae collected in fall survived being frozen, while summer larvae were killed by it.19Canadian Journal of Zoology. Cold hardiness of tree-hole mosquitoes in the Great Lakes region of the United States Some of the fall larvae survived freezing at −25°C (−13°F), the lowest temperature known to be survived by a water animal in ice.20Canadian Journal of Zoology. Cold hardiness of tree-hole mosquitoes in the Great Lakes region of the United States They need both shortening days and a period of chilling beforehand to build that hardiness.21Canadian Journal of Zoology. Cold hardiness of tree-hole mosquitoes in the Great Lakes region of the United States

A tree hole or stump cavity makes a good winter home. These holes collect rain and leaves year after year, and rotting tree-hole water helped larvae tolerate cold better than plain water. A hard freeze kills active adults, but not prepared eggs, larvae or sheltered females, so a cold winter doesn’t guarantee a mosquito-free summer.

Natural Antifreeze And Supercooling In Adults

Some mosquitoes build up a sugar-alcohol in their bodies that acts as natural antifreeze.22Geophysical Institute, University of Alaska Fairbanks. How Mosquitoes Overwinter in Alaska It helps protect body cells as temperatures reach freezing.23Geophysical Institute, University of Alaska Fairbanks. How Mosquitoes Overwinter in Alaska

Adults resist ice better than the young stages. In one house-mosquito type, adults stayed unfrozen down to about −18°C (0°F), while larvae and pupae of the same species froze at higher temperatures, from about −8°C (18°F).24Journal of Entomological Science. Supercooling Point of Selected Developmental Stages ofCulex pipiens pallens(Diptera: Culicidae) Males and females tolerated cold about equally in the lab.25Journal of Entomological Science. Supercooling Point of Selected Developmental Stages ofCulex pipiens pallens(Diptera: Culicidae)

The adults stay unfrozen by staying liquid below freezing. Liquids can drop below freezing without turning to ice if nothing triggers crystals; a bottle of water in a freezer can stay liquid, then freeze suddenly. Mosquitoes avoid the triggers and stay liquid inside at subfreezing temperatures. Once ice forms inside, most die, so avoiding it is everything. Even with natural antifreeze, protection has limits, and sheltered spots like basements and tunnels rarely get as cold as open air.

Where Mosquitoes Hide Around Homes And Cities

Overwintering house-mosquito females seem to do best around 0 to 8°C (32 to 46°F).26PubMed Central. Mosquitoes in the Dark—Highlighting the Importance of Genetically Identifying Mosquito Populations in Subterranean Environments of Central Europe Warming, predators or human activity wake them, which burns energy and raises deaths.27PubMed Central. Mosquitoes in the Dark—Highlighting the Importance of Genetically Identifying Mosquito Populations in Subterranean Environments of Central Europe A spot that is too warm keeps females half-awake and hungry, and they may try to bite people in winter. In caves, most mosquitoes rest just past the entrance rather than in the deepest dark.28PubMed Central. Mosquitoes in the Dark—Highlighting the Importance of Genetically Identifying Mosquito Populations in Subterranean Environments of Central Europe They usually enter such shelters in early autumn, though some use them most of the year.29PubMed Central. DNA-aided identification of Culex mosquitoes (Diptera: Culicidae) reveals unexpected diversity in underground cavities in Austria

City infrastructure can shelter mosquitoes too. Underground spaces in cities offer a climate-buffered refuge for house mosquitoes.30Bharati International Journal of Multidisciplinary Research & Development. The Urban Underground: Subterranean Evolution and Pathogen Dynamics of Disease Vectors Storm drains are less reliable: in Southern California, winter larvae developed only at low rates, so winter storm drains there added little to invasive mosquito numbers.31PubMed Central. Impact of underground storm drain systems on larval ecology of Culex and Aedes species in urban environments of Southern California Dry, acidic underground spots also held fewer mosquitoes in midwinter.32PubMed Central. An investigation of hibernating members from the Culex pipiens complex (Diptera, Culicidae) in subterranean habitats of central Germany

To check your own property, look in crawl spaces, unheated cellars, sheds, and around sump pits. Mosquitoes resting on walls or ceilings in winter are often dormant females. Sealing gaps in fall keeps them from moving in, while heating a space or disturbing them can cost them their fat reserves.

Can Mosquitoes Carry Disease Through Winter

An infected overwintering female can pass West Nile virus to the next animal or person she bites after waking.33Cary Institute of Ecosystem Studies. West Nile virus beats winter The virus has been found overwintering in mosquitoes in Germany, but only about 0.02 to 0.2% of those overwintering mosquitoes were infected.34PubMed Central. First Evidence of West Nile Virus Overwintering in Mosquitoes in Germany Even so, winter carryover can give the virus an early start to the next season.35PubMed Central. First Evidence of West Nile Virus Overwintering in Mosquitoes in Germany

The main source of West Nile each summer is still the cycle between mosquitoes and wild birds, which restarts as mosquitoes become active and feed on infected birds. Because only a tiny fraction carry the virus, a basement or garage full of sleepy mosquitoes is not a sign that disease is waiting there.

Do Mild Winters Mean More Mosquitoes

The effects of warmth differ. Tiger mosquito eggs that experienced winter heatwaves had about 10% higher survival, but a longer fall had no effect on their winter survival.36PubMed Central. Contrasting effects of an extended fall period and winter heatwaves on the overwintering fitness of diapausing disease vector, Aedes albopictus Warming in the UK is predicted to raise peak mosquito numbers,37PubMed Central. Modelling the effect of temperature on the seasonal population dynamics of temperate mosquitoes while one model predicts fewer active house mosquitoes in some future climates.38PubMed Central. Population of the temperate mosquito, Culex pipiens, decreases in response to habitat climatological changes in future

Lights and local adaptation matter too. Artificial light at night cut dormancy in tiger mosquitoes by as much as 40%,39Oxford Academic (J. Med. Entomol.). Cold Nights, City Lights: Artificial Light at Night Reduces Photoperiodically Induced Diapause in Urban and Rural Populations of Aedes albopictus (Diptera: Culicidae) probably because light can make shortening days look longer. Northern tiger mosquito eggs survived a long winter better, about 93% versus 86% for southern ones, so mosquitoes moving north seem to adapt quickly to colder winters.40PubMed Central. Rapid adaptive evolution of the diapause program during range expansion of an invasive mosquito

One mild winter doesn’t predict the summer. Spring rain largely decides when eggs hatch.

What You Can Do In Fall And Winter

Each overwintering stage has a matching chore. Scrub or store buckets, tires and planters, since dormant eggs cling to their surfaces. Fill or drain tree holes and stump cavities that shelter larvae. Seal crawl spaces and basement gaps in early autumn, when females start seeking shelter. Clear gutters and drains so water doesn’t pool when spring rain arrives. Late fall to winter is ideal for scrubbing containers before spring hatch, and early spring checks catch any water that collected over winter.

Spraying in midwinter does little, as dormant stages are hidden. Dumping water alone leaves eggs behind on container walls.

Scrub the band just above the waterline with a stiff brush and soapy water, then rinse. After scrubbing, dry the container or turn it upside down so it can’t collect rain, since eggs left behind can hatch when water returns, even after months of being dry. Containers that are hard to scrub, like old tires or cracked pots, are better stored under cover or thrown away, since they hold water and keep eggs in crevices.