Blow Flies: What These Metallic Flies Mean in Your Home

Blow Flies: What Their Presence Means

Feature	Blow Flies	Similar problem	Best next step
Main clue	Look for the traits described in this guide, then confirm with direct evidence.	Compare size, behavior, location, and damage before choosing treatment.	Match your control method to the pest you can verify.
Common mistake	Acting on one sign alone.	Assuming the same tools work equally well for both.	Inspect droppings, entry points, and activity areas together.
Control impact	Requires the method, placement, and follow-up timing that fit Blow Flies.	Requires the method, placement, and follow-up timing that fit Similar problem.	Recheck results after several nights and adjust if signs continue.

Blow flies are the metallic-colored flies you have probably noticed buzzing around garbage cans, outdoor grills, and unfortunately, sometimes inside your home. While a stray blow fly coming through an open door is normal, finding multiple blow flies indoors is often a signal that something is decomposing inside your walls, attic, or crawl space.

Identifying Blow Flies

Blow flies belong to the family Calliphoridae and are among the easiest flies to identify thanks to their distinctive appearance:

• Size: 8 to 14 millimeters, slightly larger than house flies
• Color: Metallic blue, green, bronze, or coppery sheen. Green bottle flies are a particularly common and recognizable species.
• Body shape: Rounded, robust body
• Eyes: Large, reddish-brown compound eyes
• Sound: Loud, distinctive buzzing in flight

Several species fall under the blow fly umbrella. The most common include blue bottle flies (Calliphora vomitoria), green bottle flies (Lucilia sericata), and bronze bottle flies. Despite their different colors, they share similar habits and biology.

Blow Fly Biology

Feeding and Breeding

Blow flies are among the first insects to detect and colonize decomposing organic matter. They can locate a food source from over a mile away using their highly sensitive olfactory receptors. Females lay eggs directly on or near decaying animal matter, feces, or open wounds on animals.

A single female can lay 150 to 200 eggs per batch, and these eggs hatch within 24 hours in warm conditions. The resulting maggots feed voraciously on the decaying material, completing their larval development in as little as four to five days.

Life Cycle

The blow fly life cycle follows the standard pattern of complete metamorphosis:

• Eggs: Laid in clusters on decomposing matter, hatching in 12 to 24 hours
• Larvae: Three larval stages lasting 4 to 10 days total
• Pupae: Larvae burrow into soil or migrate away from the food source to pupate, lasting 6 to 14 days
• Adults: Live for 2 to 8 weeks, feeding on nectar, decaying matter, and animal fluids

What Blow Flies in Your Home Mean

Finding a few blow flies near open windows or doors during summer is normal. However, finding multiple blow flies indoors, especially during cooler months when doors and windows are closed, is a strong sign of infestation that typically indicates one of these situations:

A Dead Animal

This is the most common reason for sudden indoor blow fly appearances. Mice, rats, squirrels, birds, or other animals that have died in wall voids, attics, chimneys, or crawl spaces attract blow flies that somehow find their way indoors after emerging from the carcass.

Signs that a dead animal is the source include:

• A sudden appearance of multiple blow flies over a few days
• An unpleasant odor, often faint at first, growing stronger
• Flies concentrated near specific walls, ceilings, or vents
• Staining on walls or ceilings in severe cases

Garbage or Waste Issues

Improperly stored garbage, a forgotten bag of trash, or pet waste brought indoors on shoes can attract blow flies. Check garbage cans, recycling bins, and areas around pet feeding stations.

Entry from Outside

Blow flies can enter through damaged screens, open doors, or gaps around pipes and vents. If you live near farms, stables, or areas with wildlife, outdoor blow fly populations may be high enough that some regularly find their way inside.

Health Concerns

Blow flies present several health risks:

• They carry pathogenic bacteria including Salmonella, E. coli, and Staphylococcus on their bodies and in their gut
• Their feeding behavior (regurgitating on food surfaces) makes them efficient disease vectors
• They can cause myiasis (infestation of living tissue by fly larvae) in pets and livestock, particularly in wounds or soiled fur
• They pose food safety risks in commercial kitchens and food processing facilities

Control and Elimination

Finding the Source

If blow flies appear indoors, prioritize finding the source:

1. Follow the concentration of flies to identify the general area
2. Check for dead animals in wall voids by sniffing along baseboards and around electrical outlets
3. Inspect the attic, crawl space, and chimney
4. Check for forgotten garbage or organic waste

Removing the Source

If you find a dead animal, remove it while wearing gloves and a dust mask. Clean the area with an enzymatic cleaner to remove residual fluids and odor. If the carcass is inaccessible (inside a wall), you may need to wait for decomposition to complete, which typically takes two to four weeks, or cut an access hole for removal.

Supplemental Controls

While addressing the source, reduce adult fly numbers with:

• Fly traps placed near windows
• Fly paper in affected rooms
• Vacuuming visible flies
• UV light traps in enclosed spaces

For persistent problems or when you cannot locate the source, contact a professional pest control service. For comprehensive fly management, see our complete guide to flies.

Professional Insight

In my 15 years of IPM work, I have responded to hundreds of calls about sudden blow fly appearances indoors. In roughly 80 percent of those cases, the source turned out to be a dead animal in the wall void or attic. I always tell clients that finding five or more metallic-colored flies indoors during cooler months is a strong indicator of a carcass somewhere in the building structure. My first step on any blow fly call is always a thorough sniff test along baseboards and around electrical outlets on interior walls.

Sources and References

• University of Florida Entomology - Calliphoridae - Blow fly identification keys and biological information from the UF Department of Entomology.
• CDC - Myiasis - CDC information on myiasis, a condition associated with blow fly larvae infesting living tissue.
• NPMA - Blow Flies - National Pest Management Association resources on identifying and managing blow fly infestations.
• Penn State Extension - Blow Flies - Penn State's guidance on blow fly biology, health risks, and control methods.
• EPA - Integrated Pest Management Principles - EPA framework for IPM approaches to managing filth fly populations.

Prevention

Preventing blow fly infestations centers on denying decaying protein and blocking entry points. Seal all garbage bins with tight-fitting lids and clean them weekly with disinfectant to remove residual odor. Never leave meat scraps, fish waste, or pet waste exposed outdoors. In areas with wildlife pressure, remove carcasses promptly or contact animal control. Seal gaps around utility penetrations, pipe entries, and vents with caulk or steel wool to deny fly entry. Rodent control is critical: mice, rats, or squirrels that die in wall voids become blow fly breeding sites within 48 hours of death. Check snap traps daily and retrieve caught rodents the same day. Inspect outdoor pets and livestock for open wounds daily during warm months, as blow flies deposit eggs in untreated wounds within minutes. For livestock, establish a wound inspection and treatment protocol with your veterinarian before fly season begins. Consistent garbage management and active rodent control together eliminate the conditions behind most residential blow fly problems.

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.

How to Identify

Identify the species before treating, because effective control depends on locating the correct breeding site. House flies are gray with four dark thoracic stripes and feed on garbage and feces. Fruit flies are tiny, tan or yellow with red eyes, and breed in fermenting produce or drain biofilm. Drain flies are fuzzy, moth-like, and emerge in small slow flights from drains. Blow flies are large and metallic blue or green and indicate a dead animal nearby. Phorid flies hover in jerky paths and breed in broken sewer lines under slabs. Cluster flies are slow and dark and overwinter in attics. Sticky cards placed near suspected sources for 24 to 48 hours both confirm the species and pinpoint the breeding zone.

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

How long will blow flies last if there is a dead animal in my wall?

The blow fly activity typically peaks within the first one to two weeks after the animal dies and then gradually diminishes over two to six weeks as the carcass decomposes. The duration depends on the size of the animal and temperature. A mouse may only produce flies for two weeks, while a larger animal like a squirrel or rat can sustain a blow fly population for a month or more.

Are blow flies dangerous to humans?

Blow flies do not bite humans, but they pose health risks through mechanical disease transmission. They carry bacteria including Salmonella, E. coli, and Staphylococcus on their bodies after feeding on decomposing matter. When they land on food or food preparation surfaces, they transfer these pathogens through contact and regurgitation.

How can I tell the difference between blow flies and house flies?

The easiest way to distinguish blow flies from house flies is by color. Blow flies have a distinctive metallic blue, green, or coppery sheen, while house flies are dull gray with four dark stripes on the thorax. Blow flies are also slightly larger, typically 8 to 14 millimeters compared to the house fly's 6 to 7 millimeters.

Should I hire a professional to find a dead animal causing blow flies?

If you cannot locate the source after checking accessible areas like the attic and crawl space, a pest control professional can help. They have experience pinpointing carcass locations using fly concentration patterns and odor detection. In some cases, cutting an access hole in drywall is necessary, which many pest control companies can handle or coordinate.