Flies in Winter: Why You Still See Them and What to Do

Flies in Winter: Why They Are Still Around

Sign or symptom	Likely cause	Risk level	What to do next
Fresh activity related to Flies in Winter	flies are active nearby or recently passed through the area.	High if signs repeat or appear in multiple rooms.	Inspect the surrounding cracks, seams, food sources, and travel paths.
Old or isolated evidence	A past problem, accidental introduction, or inactive nesting site.	Moderate until you confirm whether activity is current.	Clean and mark the area, then recheck in 24 to 48 hours.
Multiple signs together	A developing infestation rather than a one-off sighting.	High because populations can spread before they are obvious.	Start control steps immediately and consider professional inspection.

Most people expect flies to disappear when cold weather arrives. So finding flies buzzing around your windows in December or January can be puzzling and frustrating. Winter flies are usually a different problem than summer flies, and understanding the distinction is key to addressing them effectively.

Why You See Flies in Winter

Overwintering Flies

The most common winter fly is the cluster fly. These flies entered your home in late summer or early fall, seeking a warm place to spend the winter. They settle into wall voids, attics, and other sheltered spaces in a dormant state.

On warm, sunny winter days, the heat penetrating south- and west-facing walls warms the flies enough to become active. They emerge from their hiding spots and fly toward light sources, congregating at windows in slow, sluggish clusters.

Other flies that may overwinter in buildings include:

• Blow flies (occasionally)
• Some species of house flies in warmer regions

Indoor Breeding

Some fly species can breed indoors year-round if conditions are right:

• Fruit flies: Will breed in kitchens throughout winter if fermenting material is available
• Drain flies: Breed in drain biofilm regardless of outdoor temperature
• Phorid flies: Can breed indoors in organic material under floors or around plumbing
• Fungus gnats: Breed in overwatered houseplant soil year-round

Dead Animals

If you see blow flies or green bottle flies indoors during winter, especially metallic-colored flies, a dead animal in a wall void or attic is the most likely explanation. Mice and other rodents that die inside buildings during cold weather attract blow flies that may have been dormant nearby.

Managing Winter Cluster Flies

Immediate Control

• Vacuum them up. This is the most practical approach for cluster flies. Use a vacuum with a disposable bag and discard it immediately.
• Fly paper or window traps. Place adhesive traps on windows where cluster flies congregate.
• UV light traps. Window-mounted traps can intercept flies as they emerge.
• Do not spray into wall voids. Dead flies in walls attract carpet beetles and other secondary pests.

Prevention for Next Year

The time to prevent winter cluster flies is late summer, before they enter:

1. Seal entry points in July and August:

   • Caulk around all windows, especially on south and west walls
   • Seal gaps under eaves and around soffit vents with fine mesh
   • Repair or replace damaged screens
   • Fill gaps around utility penetrations

2. Apply exterior treatments:

   • A residual insecticide applied to building exteriors in August can deter cluster flies from landing and entering
   • Focus on sunny sides, around windows, and under eaves
   • This is often best done by a professional

Managing Indoor-Breeding Winter Flies

Fruit Flies in Winter

If fruit flies appear in winter:

• Check for overripe produce, especially items stored in pantries
• Inspect recycling containers for residue
• Clean drains with enzymatic cleaner
• Set up vinegar traps
• See our guide on how to get rid of fruit flies

Drain Flies in Winter

Drain flies in winter are treated the same as in any season:

• Clean affected drains mechanically
• Apply enzymatic drain cleaner
• Fix any leaks or moisture issues
• See our guide on how to get rid of drain flies

When to Worry

Some winter fly appearances warrant immediate attention:

• Multiple blow flies appearing suddenly: Check for a dead animal in walls or attic
• Large numbers of small flies near a specific area: May indicate a broken drain line or hidden organic material
• Flies emerging from unusual locations (baseboards, light fixtures, air vents): May suggest a pest problem in the building structure

Spring Emergence

As temperatures rise in spring, overwintering cluster flies become more active and eventually leave buildings to breed outdoors. You may see a temporary increase in visible flies during this transition period. This is normal and will resolve on its own as the flies exit.

Help them along by:

• Opening windows on warm spring days
• Using fans to encourage flies toward open windows
• Vacuuming visible flies

For comprehensive information about all fly species and year-round management, visit our complete guide to flies.

Professional Insight

Winter fly calls are a common part of my practice, and they often catch homeowners off guard. In my 15 years of IPM experience, I have found that the most effective winter fly management starts with prevention the previous summer. I always schedule cluster fly exclusion and exterior treatment appointments for August, well before the fall migration begins. For clients who missed that window and are now dealing with cluster flies in January, I focus on vacuuming and monitoring, while planning a comprehensive prevention program for the following season.

Sources and References

• Penn State Extension - Overwintering Pests - Penn State's comprehensive guide to managing pests that overwinter in buildings, including cluster flies.
• University of Florida Entomology - Seasonal Fly Activity - UF resources on seasonal patterns of fly activity and their implications for pest management timing.
• NPMA - Winter Pest Prevention - National Pest Management Association tips for preventing and managing pests during winter months.
• EPA - Seasonal IPM Strategies - EPA guidance on adapting integrated pest management approaches to seasonal pest patterns.

How to Identify

Two species commonly appear indoors during winter, and distinguishing them shapes the response. Cluster flies (Pollenia rudis) are 8--10 mm, dark gray with golden hairs on the thorax, and move sluggishly in warm rooms. Their wings overlap scissor-fashion when at rest, unlike house flies whose wings splay apart. They typically emerge from wall voids, loft spaces, and window frames on sunny winter days.

Blow flies appearing in winter are usually metallic blue or green, 6--12 mm, and more active than cluster flies at the same temperature. Their presence indoors in cold weather almost always indicates a dead animal in the structure, not outdoor migration.

Any small, tan-to-brown flies active near drains or rotting produce in winter are likely fruit flies or drain flies that have found an indoor breeding site, and these require sanitation rather than exclusion treatment.

Prevention

Cluster fly prevention must happen in summer, not winter. Treat exterior walls, roof soffits, and window frames with a residual insecticide in July or August, before adults seek overwintering sites. Seal gaps around window frames, fascia boards, and eaves with caulk or expanding foam.

Once cluster flies are already inside wall voids, exclusion work should wait until spring to avoid trapping dead flies inside the structure. Vacuum emerging adults directly and seal the vacuum bag immediately.

For indoor-breeding flies active in winter, the prevention strategy is identical to warm-weather control: eliminate standing moisture, seal drains when not in use, store food in closed containers, and remove any dead organic material. Year-round sanitation prevents winter populations from establishing regardless of temperature.

Main Causes

Indoor flies activity is driven by accessible breeding material and warmth. House flies and blow flies breed in garbage, pet waste, compost, and dead animals; fruit flies breed in overripe produce, drain biofilm, fermenting liquids, and unrinsed recycling; drain flies breed in the gelatinous film inside infrequently used drains; phorid flies breed in broken sewer lines and decomposing material under slabs. Adults find their way inside through torn screens, gaps around doors, vents, and any opening to the outside. Warm weather accelerates the entire life cycle, and a sustained population always points to an unaddressed source either inside the structure or close enough that adults keep arriving in volume.

Risk and Severity

Flies are mechanical disease vectors, picking up pathogens from feces, decomposing material, and garbage on their bodies and depositing them on food and surfaces. House flies in particular regurgitate digestive fluids when feeding, contaminating any surface they land on. Documented transmissible pathogens include Salmonella, E. coli, Shigella, and Campylobacter. Blow flies in homes signal a dead animal in or near the structure — a secondary health concern from decomposition gases and additional pest activity around the carcass. Biting flies (horse flies, stable flies, black flies) deliver painful bites and can trigger allergic reactions; in some regions they transmit parasites or bacterial infections. Children, elderly, and immunocompromised individuals face elevated risk.

Solutions and Actions

Effective fly control requires locating and eliminating the breeding source — adult-only treatments produce only temporary relief. For house flies: remove and seal garbage, clean pet waste daily, manage compost properly, and check for dead animals in wall voids or attics if blow flies are present. For fruit flies: discard overripe produce, clean drains with enzymatic cleaner weekly, rinse recycling, and empty kitchen compost containers daily. For drain flies: brush drain walls thoroughly and treat with enzymatic drain cleaner weekly for at least three weeks. For phorid flies: investigate for broken sewer lines or moisture intrusion under slabs. Adult control through sticky cards, UV light traps, and targeted residual sprays supplements but never substitutes for source elimination.

Frequently Asked Questions

Why do I have flies in my house in the middle of winter?

Winter flies typically come from one of three sources: overwintering cluster flies that entered your home in fall and emerge on warm days, indoor-breeding species like fruit flies and drain flies that breed year-round when conditions are favorable, or blow flies attracted to a dead animal in a wall void or attic. Identifying the species is the key to determining the source and appropriate response.

How do cluster flies get inside in winter if they entered in fall?

Cluster flies enter building exteriors in late summer and early fall through gaps around windows, under siding, through soffit vents, and around eaves. They settle into wall voids and attics in a dormant state. On warm winter days, heat penetrating sun-facing walls wakes them up, and they fly toward light sources, emerging through gaps around window frames, electrical outlets, and light fixtures into living spaces.

Should I spray insecticide into wall voids to kill winter flies?

No. Spraying insecticide into wall voids will kill some cluster flies, but the dead insects attract carpet beetles and other secondary pests that can cause additional damage. It is better to vacuum visible flies as they emerge and seal entry points to prevent them from reaching your living spaces. Plan exterior exclusion and treatment for the following August to prevent the next generation from entering.

Why do winter flies wake up on warm sunny days?

Use this clue as a prompt to recheck the source, not as a standalone diagnosis. For Flies in Winter, compare where the flies appear, what food or moisture is nearby, and whether activity repeats after cleaning. If the same pattern returns within a few days, focus on the breeding site or entry route before adding more sprays, traps, or repellents.