Do Gnats Die in Winter? Cold Survival Calculator

Do gnats die in winter?

Some do, some don't, and the ones in your house usually don't notice it's winter at all. "Gnat" covers several kinds of small fly, and each reacts to cold differently. The houseplant fungus gnat slows down sharply as the temperature falls towards 11 °C (52 °F); the phorid fly stops developing below about 12.7 °C (55 °F); and the drain fly keeps breeding indoors all year, more slowly in cool rooms (University of Kentucky). So do gnats die in the winter? Outdoors, their numbers crash; indoors, warm soil and drains keep them going.

Will gnats die in the winter in your home? Only if their breeding site dries up or gets cold. A heated room at 20 °C is perfect for fungus gnats: at 21 °C a new generation takes about 18–25 days, and adults live 7–10 days (University of Kentucky). Extension agents in North Carolina note that indoor fungus gnats have "continuous reproduction and overlapping generations with all life stages likely to be found throughout the year".

• Outdoors in a cold winter: breeding stops for every type; survival depends on the species.
• Unheated garage, porch or greenhouse: fungus gnats slow to a crawl near 11 °C; phorid flies stop below about 13 °C.
• Heated home: all three keep breeding wherever there is wet soil, slime or rotting organic matter.

Can gnats survive the cold? How cold is too cold

Can gnats survive the cold? Many can survive more of it than you might expect, but they can't breed in it. So do gnats survive in the cold for long? It depends on the type — and for some, nobody has measured it. Here is what the studies show for each type — and where the evidence runs out:

Notice the gaps. Nobody we found has measured houseplant fungus gnats below 11 °C, or drain flies at any cold temperature, so we don't claim to know what temperature kills gnats of those kinds. The calculator says "no sourced data" in those cases rather than guessing.

What temperature kills fungus gnats?

The coldest temperature studied for the common greenhouse and houseplant species, Bradysia impatiens and a related Bradysia, is 11 °C. At that temperature females took 93.3–112.8 days to grow from egg to adult — against about 21–27 days at 25 °C — and the populations' growth rate was almost exactly zero: one species grew very slightly (0.004) and the other shrank (-0.006) (Ishihara & Tagami 2019). By 16 °C both were growing again. So does cold kill fungus gnats? A cool room near 11 °C stops the population growing; whether colder temperatures kill them hasn't been tested.

Can gnats survive in cold weather outside?

Some can, very well. Can gnats survive in cold weather far below freezing? One Alaskan fungus gnat does — see the next section. So can gnats survive in the cold indoors, in an unheated room? For the gnats that bother you indoors, the evidence is about breeding, not death: cold stops them multiplying long before it is known to kill them.

Can fungus gnats survive cold? The half-frozen gnats of Alaska

The most remarkable answer to can fungus gnats survive cold comes from Fairbanks, Alaska. While sorting insects under the loose bark of dead balsam poplar trees, Todd Sformo, then a doctoral student at the University of Alaska Fairbanks' Institute of Arctic Biology, found thousands of adult fungus gnats spending the winter there (Rozell 2009). The species, Exechia nugatoria, belongs to the family Mycetophilidae — a fungus gnat, but not the same family as the houseplant gnats (Sciaridae).

In the lab, the gnats froze in two separate steps as they were cooled (Sformo et al. 2009):

• The abdomen froze first, at about -30.1 °C (around -25 °F). After this first freezing event, at -31.5 °C on average, 70% of the gnats survived.
• The head and thorax stayed unfrozen — supercooled — down to about -48.7 °C (around -60 °F). When they froze too, at -50.7 °C on average, none survived.

The researchers think the head and thorax stay liquid because they are much drier (about 47% water against 71% in the abdomen), which stops ice spreading forward. Most insects either tolerate freezing or avoid it; this gnat does both at once, in different halves of its body. So how cold can gnats survive? This one survives temperatures below −30 °C — but it is a wild forest species, and that tells us nothing certain about the gnats in a flowerpot.

Where do gnats go in the winter?

Where do gnats go in the winter? It depends on the gnat:

• Forest fungus gnats such as Exechia spend it as adults under bark, partly frozen (Sformo et al. 2009).
• Drain flies breed outdoors in summer, in low wet spots such as where air-conditioning units drain and in clogged gutters, and come indoors through open doors and windows (University of Kentucky). Indoors, they can carry on all year.
• Phorid flies come indoors in autumn, their peak breeding season (University of Kentucky), and are mainly found in warm regions (LSU AgCenter).
• Houseplant fungus gnats: we found no study of how they get through winter outdoors. Indoors they never leave — they live in the pots.

Do gnats hibernate?

Not in the sense of a deep winter sleep. Do gnats hibernate? The Alaskan species survives by partly freezing, which is a different strategy altogether. For the indoor types, cold simply slows development until it stops — phorid flies below about 12.7 °C. So how do gnats survive winter outdoors? Sheltered, inactive and not breeding, rather than asleep.

Do gnats come back in spring?

Outdoor species become active again as the weather warms — drain flies, for example, breed outdoors through summer (University of Kentucky). We found no study that follows outdoor fungus gnats from winter into spring. For the indoor problem, do gnats come back in spring is the wrong question: if you had them in winter, they were breeding the whole time. North Carolina extension agents describe fungus gnats as "a particular pest in greenhouses in winter and spring".

Why you have gnats in your house in winter

If you're asking why do I have gnats in winter, look first at your houseplants, then your drains. If you're searching for gnats in house in winter, the culprits are nearly always one of two things:

Fungus gnats in winter houseplants

Winter is peak season for fungus gnats in winter houseplants, and watering is usually why. "Houseplants in the winter normally do not require as much water as at other times of the year" (NC State Extension), because they grow more slowly — yet many of us keep watering on the summer schedule. Overwatering feeds the fungi the larvae eat (University of Maryland Extension), and fungus gnats "like high humidity and cooler temperatures" such as a plant on a cool windowsill (NC State Extension).

Drains in a warm house

Drain flies breed in the slime layer in sink, shower and floor drains, and indoors their development "can continue through the year" (University of Kentucky). Phorid flies use drains too, along with any moist organic matter they can find — including some you can't see, under a floor or behind a wall.

For how long each adult lives and when an infestation should end once you fix the source, see the gnat lifespan calculator.

Do phorid flies die in winter?

Phorid flies are the small, humpbacked flies that scuttle across a counter or wall instead of flying away; the common household species, Megaselia scalaris, breeds in moist organic buildup in drains and plumbing (Colorado State University). For how to tell them from fungus gnats and fruit flies, see gnats vs flies, midges and phorid flies.

Phorid flies in winter

Do phorid flies die in winter? Outdoors they stop breeding: a 2024 study found Megaselia scalaris took 62 days from egg to adult at 16 °C, 18.7 days at 22 °C and 17.4 at 25 °C, with development stopping below about 12.7 °C (or 14.6 °C using a different model) (Zhang et al. 2024). We found no study that timed how long they survive below that. Indoors, phorid flies in winter are a sign of a hidden breeding site: the University of Maryland notes that very large numbers on a building's first floor "may indicate a broken drain".

Drain flies in winter

Drain flies in winter are common for a simple reason: the drain is the same temperature in January as in July. The University of Kentucky gives a life cycle of about 10–15 days at about 70 °F (21 °C) and says development "is slower at lower temperatures but can continue through the year indoors". Adults live only 3–4 days (University of Kentucky), so a steady stream of them means the slime is still there.

Do drain flies die in winter? Outdoors, their summer breeding sites go quiet, but we found no study or extension source with a temperature at which drain flies stop developing or die — so the calculator reports "no sourced cold data" for drain flies anywhere but a heated home.

How to get rid of gnats in winter

Cold weather won't do it for you indoors, but winter makes some fixes easier. These non-toxic steps come from extension services:

• Water houseplants less. Let the soil surface dry completely between waterings (NC State Extension) — "when the soil dries, the larvae will die" (University of Maryland Extension).
• Empty saucers so pots don't sit in water, and improve drainage (UC IPM).
• Use yellow sticky cards in the pots to catch adults and find the infested plant (UC IPM; University of Maryland Extension).
• Scrub drains and traps to remove the slime drain flies breed in; hot water alone gives only short-term control (University of Kentucky).
• For phorid flies, find the moist organic matter and remove it or let it dry out completely, and fix leaks (Colorado State University; University of Maryland).
• Tape a sticky trap or clear plastic over a drain overnight to see whether that drain is the source (Colorado State; University of Kentucky).

Fruit flies are a separate case with their own winter story — see do fruit flies die in winter.

How this gnat cold calculator works

The calculator uses a different set of sourced thresholds for each gnat type and never fills a gap with a guess:

• Fungus gnats: 16–30 °C counts as active and breeding, 11–16 °C as slowed down, and below 11 °C as "no data" (Ishihara & Tagami 2019). Egg-to-adult times and adult lifespans come from the same data as our gnat lifespan calculator, including its rule of showing the 25 °C figures for warmer rooms.
• Drain flies: in a heated home at 11–30 °C, active and breeding (University of Kentucky), with stage times measured at about 21 °C (UF/IFAS). Anywhere else, "no sourced cold data".
• Phorid flies: 16–34 °C active, with egg-to-adult times drawn between Zhang and colleagues' measured temperatures; 12.7–16 °C slowed; below 12.7 °C development stops.
• Not sure: plants point to fungus gnats, drains to drain flies, and bins or a damp floor to phorid flies.

Picking a place fills in an example temperature for you to change. Temperatures from -30 to 45 °C are accepted. The Alaskan freezing results are shown as background for outdoor fungus gnats, not used to predict houseplant gnats.

Frequently asked questions

These calculators give estimates for education. Real flies vary with strain, food, humidity and temperature. For a stubborn infestation, your local cooperative extension service can help identify the insect. We give no pesticide advice.