Gnats don’t enjoy the cold, and outdoors most of them simply shut down for winter instead of staying active. The gnats people notice in winter are usually indoors, where cold never reaches them.1Colorado State University Extension. Houseplant Pests
Why You Still See Gnats Indoors in Winter
The small dark flies hopping over houseplant soil and gathering at windows are usually fungus gnats.2Colorado State University Extension. Houseplant Pests Their larvae develop in potting soil, so almost any houseplant can host them.3Colorado State University Extension. Houseplant Pests Problems are most common in winter and early spring.4Colorado State University Extension. Houseplant Pests The plants themselves take little or no harm, so the trouble is mostly a nuisance.5Colorado State University Extension. Houseplant Pests
Fungus gnats are easy to confuse with fruit flies, but they behave differently. Fungus gnats are dark and slender with long legs, and they hover near plant soil and windows. Fruit flies are tan with red eyes and gather around ripe fruit and drains.
Room temperature lets fungus gnats breed all year. At 65 to 75°F, one goes from egg to adult in about three to four weeks, so indoor plants can support continuous breeding.6Colorado State University Extension. Fungus Gnats as Houseplant and Indoor Pests Eggs can also arrive in bagged potting soil, even soil that looks dry.7lovethatleaf. Do Gnats Like the Cold? How They Survive Winter
Winter tends to make things worse, not better. Plants use less water when there’s less light and growth, so the soil stays wet longer, yet many people keep watering on a summer schedule. That leaves the soggy soil gnats need. Adults fly toward window light, so they’re easier to spot on sills.
The problem can feel endless even after you kill the adults. Adults live only about a week and are the smallest part of the population. Most of it is hidden in the soil as eggs and larvae, so swatting adults doesn’t touch it, and new adults keep emerging as long as the soil stays damp.
What Happens to Outdoor Gnats When It Gets Cold
Fungus gnat populations fall sharply in winter and climb again in spring once temperatures pass about 7°C (45°F).8Acta Horticulturae. Biology, monitoring and controlling techniques of fungus gnats in growing media Like other insects, gnats can’t make their own body heat, so cold slows them down.9NC Museum of Natural Sciences Blogs. What Time is it in Nature: Water in Winter Many enter a dormant period, hiding as eggs or larvae until spring.10NC Museum of Natural Sciences Blogs. What Time is it in Nature: Water in Winter A hard frost therefore rarely wipes out a local population.
Plenty of insects survive below-freezing air by sheltering where temperatures stay milder. Soil and leaf litter stay warmer than the air above them, so cold mostly delays insects rather than eliminating them.
A quiet winter yard doesn’t mean the species is gone. The hidden stages in soil and litter can still hold a large population. Dormant eggs and larvae resume development as the ground warms, and a warm spell in late winter can bring a brief burst of adults.
Some species don’t pause at all. Some fly relatives are adapted to cool seasons and are active in winter and spring.11Ecosphere. Abundance of spring‐ and winter‐active arthropods declines with warming Fungus gnats were the most abundant arthropod family in one forest and were most common in the cooler season.12Ecosphere. Abundance of spring‐ and winter‐active arthropods declines with warming Streams and ponds can also host insects that are active even on the coldest days, since water stays above freezing.13NC Museum of Natural Sciences Blogs. What Time is it in Nature: Water in Winter
How Gnats Go Dormant for Winter
Many insects enter a built-in winter dormancy when days shorten. In one small wasp, days shorter than about 10.5 to 11 hours triggered it, depending on temperature.14Annals of the Entomological Society of America. Diapause in Microplitis croceipes (Hymenoptera: Braconidae) Cold alone, around 15°C (59°F), was enough to trigger dormancy in that wasp even with long days.15Annals of the Entomological Society of America. Diapause in Microplitis croceipes (Hymenoptera: Braconidae)
For the insect, being dormant means development stops at a set life stage, often larva or pupa. It barely moves or eats, and it restarts when warmth and moisture return.
Winter dormancy is different from simply being too cold to move. A cold insect that hasn’t entered it is just slowed down, and it speeds up again as soon as it warms. An insect in winter dormancy has a programmed shutdown, so it stays dormant even if a warm spell arrives. That stops insects from restarting in a January thaw and then being killed when the cold comes back.
Indoor gnats don’t go dormant. Indoor lights and steady warmth never send the shortening-day and cold signals, so indoor gnats keep breeding instead of pausing.
How Some Gnats Survive Freezing
Cold-hardy insects take one of two routes. Some avoid freezing by chilling below freezing without ice forming. Others tolerate freezing.16PubMed Central. Freezing induces a loss of freeze tolerance in an overwintering insect Freeze-tolerant insects often freeze around -10°C (14°F) and can then survive about 40°C colder.17PubMed Central. Freezing induces a loss of freeze tolerance in an overwintering insect Freeze tolerance has evolved separately in several insect groups, including flies.18PubMed. Climatic variability and the evolution of insect freeze tolerance
Ice that forms in the spaces between cells draws water out of them, which dries the cells but doesn’t necessarily kill them. Ice that forms inside a cell can puncture and wreck its structure, which is usually fatal.
One Alaskan fungus gnat lets part of its body freeze, pulling water out of cells so ice crystals don’t puncture them.19Geophysical Institute, University of Alaska Fairbanks. Fungus gnats survive winter half-frozen It also floods its body with its own antifreeze before winter.20Geophysical Institute, University of Alaska Fairbanks. Fungus gnats survive winter half-frozen
The two strategies carry different risks. An insect that chills below freezing without turning to ice stays liquid, but that state is unstable, and a bump, a bit of dust, or a lower temperature can set off ice suddenly. If that insect can’t survive freezing, the moment ice forms is the moment it dies. A freeze-tolerant insect survives that same moment, which gives it a larger safety margin. Even so, snow-dwelling crane-fly relatives in Scandinavia stay active while chilled below freezing near -5°C (23°F) but die around -7.5°C (18°F).21European Journal of Entomology. A review of Fennoscandian arthropods living on and in snow
Where Outdoor Gnats Hide in Winter
Leaf litter acts as a reliable insulating blanket for small soil creatures in winter, more consistently than snow, in a study of ticks.22PubMed Central. Impacts of Deciduous Leaf Litter and Snow Presence on Nymphal Ixodes scapularis (Acari: Ixodidae) Overwintering Survival in Coastal New England, USA Gnat eggs and larvae in damp litter, mulch, and compost are likely similarly shielded. Insects often overwinter hidden as eggs or larvae in protected spots like these.23NC Museum of Natural Sciences Blogs. What Time is it in Nature: Water in Winter
Damp, decaying material also suits the larvae. Fungus gnat larvae feed on fungi and decaying plant matter, so rotting leaves and mulch supply both food and moisture. Decay can give off a little warmth, which may make a pile milder than the air.
Several yard spots tend to produce spring gnats: thick mulch and wet leaf piles near the house, compost bins that stay damp, and overwatered outdoor pots. Checking pots before you move them indoors for winter can prevent an indoor outbreak.
How to Get Rid of Gnats in Winter
The most important step is letting the top 1 to 2 inches of soil dry between waterings.24Colorado State University Office of Engagement and Extension. Fungus Gnats as Houseplant and Indoor Pests Dry soil kills many larvae and makes the pot less attractive to egg-laying females.25Colorado State University Office of Engagement and Extension. Fungus Gnats as Houseplant and Indoor Pests
To judge whether a pot is dry enough, push a finger into the soil to the first or second knuckle and water only if that depth feels dry. A pot can feel dry on top and still be wet underneath, so the finger test beats looking at the surface. Lifting the pot helps too, since a dry one feels noticeably lighter than a freshly watered one.
A winter routine helps. Water less often, since plants use less. Empty saucers so pots don’t sit in water, and consider bottom-watering to keep the surface drier.
Some tools support the main fix. Yellow sticky cards catch adults and show whether numbers are falling. A thin sand or gravel top layer discourages egg-laying. Repotting a badly infested plant into fresh, treated soil can reset the problem.
New soil needs attention too. Because some fungus gnats survive freezing, leaving soil in the cold won’t reliably clean it.26Ask Extension. Fungus Gnats in potting soil Let bagged soil dry out completely, or bake it at about 350°F for roughly half an hour.27Ask Extension. Fungus Gnats in potting soil28lovethatleaf. Do Gnats Like the Cold? How They Survive Winter
To bake soil, spread it thinly on a tray, moisten it slightly, and cover it with foil. Expect an earthy smell, so ventilate the kitchen. Let the soil cool fully before potting.
Do Biting Gnats Spread Disease in Winter
Biting midges can stay active inside livestock barns through winter in Germany.29PubMed Central. Winter activity and virus infection of Culicoides biting midges (Diptera: Ceratopogonidae) in German livestock housing before and during the 2024 BTV-3 outbreak But virus replication inside the midges is largely blocked at winter temperatures there.30PubMed Central. Winter activity and virus infection of Culicoides biting midges (Diptera: Ceratopogonidae) in German livestock housing before and during the 2024 BTV-3 outbreak In southern California, a livestock virus persisted mainly through ongoing transmission in milder weather, not infected midges surviving winter.31Entomology Today. Biting Midges’ Overwintering Activity Offers Clues to Persistence of Bluetongue Virus
These are livestock viruses, not houseplant gnat concerns. Fungus gnats don’t bite people and aren’t known to spread disease to people.
Could Warmer Winters Mean Fewer Gnats
In an experimental warming study in a southeastern US forest, fungus gnat numbers fell about 64% per °C of warming during a record-warm spring.32Ecosphere. Abundance of spring‐ and winter‐active arthropods declines with warming Insects adapted to cool seasons take the biggest hit when those seasons warm.33Ecosphere. Abundance of spring‐ and winter‐active arthropods declines with warming An insect tuned to be active and develop at lower temperatures may be pushed past the range that suits it, even though warmth helps many other insects. Insects in deep dormancy, on the other hand, use so little energy that they may cope with warmer winters.34uwo.scholaris.ca. Linking energetics and overwintering in temperate insects The 64% figure comes from one experiment in one forest during a record-warm spring, so it is not a forecast for your yard.