Blow flies inside your house almost always mean something is decaying nearby, usually a dead animal hidden in a wall, attic, or crawlspace. These shiny, metallic green or blue flies are nature’s first responders to decomposition, and they can show up on a carcass in less than ten minutes. If you’re suddenly seeing a swarm of them indoors, the priority is finding and removing whatever attracted them.
How to Identify Blow Flies
Blow flies are easy to distinguish from common house flies. They have a distinctive metallic sheen, typically bright green or blue, sometimes mixed with copper. They’re also slightly larger and louder in flight. House flies, by comparison, are smaller (under 3/8 inch), dull gray, and have four dark stripes running down their upper body. Both types have sponging mouthparts, meaning they don’t bite or feed on blood. If the flies buzzing around your home look iridescent under light, you’re dealing with blow flies.
Why They’re in Your House
The most common cause of a sudden blow fly appearance indoors is a dead animal you can’t see. Mice, rats, squirrels, and birds that die inside wall voids, ceilings, attics, or crawlspaces create the exact conditions blow flies need to breed. A single female can lay up to 2,000 eggs during her short life, and she deposits them in clusters of about 250 at a time in natural openings like the mouth, eyes, and ears of a carcass.
Beyond dead animals, blow flies also breed in rotting food and feces. An overflowing outdoor garbage can near a door, a forgotten bag of meat scraps, or pet waste in a yard can all draw them in. But when dozens appear inside at once, a hidden carcass is the most likely explanation. These mass emergences happen when a whole generation of flies matures at the same time and suddenly appears in living spaces, often startling homeowners who had no idea anything was decomposing nearby.
How Fast They Multiply
Blow flies develop quickly. After eggs are laid, they hatch into first-stage maggots within 24 hours. The maggots feed, molt through two more stages, and eventually move away from the food source to transform into pupae. The full cycle from egg to adult takes two to three weeks. Large masses of feeding maggots generate their own heat, raising the surrounding temperature by more than 10°C, which can actually speed up development further.
Adult blow flies live anywhere from two weeks to about a month. That means even after the original food source is gone, you may continue seeing adults for several weeks as the last generation finishes developing and emerges.
How to Find the Source
Start with your nose. The carcass will generally be in the smelliest part of the house. Beyond that, several other clues can narrow the search.
- Follow the flies. Blow flies cluster near the carcass. They won’t lead you to the exact spot every time, but they’ll get you in the general area.
- Look for stains. A decomposing animal in a ceiling or wall may leave a dark, wet spot that seeps through drywall or paint.
- Check for maggots. If the carcass is overhead, maggots sometimes drop through ceiling gaps or light fixtures. Where they fall gives you a good indication of where the body is.
- Feel for temperature changes. Decomposition generates heat. Running your hand along a wall or ceiling may reveal a warm spot near the carcass.
- Think about the animal. Squirrels tend to end up in the upper half of the house, often falling into wall cavities from an opening near the roofline. Rats that die from rodenticide tend to stay in crawlspaces, sometimes in tight spots that are hard to reach.
If you heard scratching inside a wall before the flies appeared, that’s a strong clue. Mark the location of any scratching sounds while you can still hear them, because once the animal dies, pinpointing its position becomes much harder. Even a few inches in the wrong direction matters when you’re cutting into drywall.
Getting Rid of Them
Killing the adult flies helps reduce numbers in the short term, but the real solution is removing whatever they’re breeding in. Without eliminating the food source, new adults will keep emerging for weeks. Here’s the practical sequence:
First, locate and remove the carcass or decaying material. This sometimes means cutting into a wall or ceiling, which isn’t pleasant but is the fastest way to end the cycle. If you can’t access the carcass yourself, a wildlife removal service can help. Once the source is out, clean the area thoroughly. Decomposition fluids soak into building materials and can continue to attract insects if left untreated.
For the adults already flying around, a standard fly swatter or a vacuum cleaner works. Sticky fly traps placed near windows where blow flies tend to congregate can catch large numbers passively. Opening windows on the opposite side of the house from the source can help draw them out. If maggots are visible, remove them physically and clean the area with hot, soapy water.
After removing the source, expect to see stragglers for up to two or three weeks as the last pupae finish developing into adults. This is normal. If new flies are still appearing a month later, there may be a second carcass or another breeding site you haven’t found.
Health Risks
Blow flies feed on dead animals, feces, and rotting food before landing on your kitchen counter or your plate. They can carry bacteria like Salmonella and E. coli on their bodies and transfer them to food and surfaces. The risk of actual illness from a few blow flies is low, but you should avoid leaving food uncovered while they’re present, and wipe down any surfaces where they’ve been landing.
Preventing Blow Flies From Getting Inside
Once you’ve dealt with the immediate problem, a few structural fixes can keep blow flies (and most other flying insects) from entering your home in the future.
Check for gaps under exterior doors. Even 1/16 of an inch is enough for insects to slip through. Install door sweeps or thresholds, and apply caulk along the bottom edges and sides of door thresholds. Garage doors should have a rubber bottom seal (vinyl doesn’t hold up well in cold weather). For sliding glass doors, line the bottom track with foam weatherstripping.
Repair any torn or loose window and door screens. Seal utility openings where pipes, wires, and vents enter the foundation or siding, using caulk, cement, expandable foam, or copper mesh. Install quarter-inch wire mesh over attic vents, roof vents, and crawlspace vents to block both insects and the animals whose carcasses attract them. A chimney cap is worth the investment for the same reason: it keeps birds and squirrels from entering and getting trapped.
On the sanitation side, keep outdoor garbage cans tightly sealed and away from entry doors. Clean up pet waste regularly. If you use rodenticide, be aware that poisoned animals often die in hidden spots inside your home’s structure, which creates exactly the conditions that bring blow flies indoors. Snap traps let you remove dead rodents immediately, avoiding that problem entirely.