If large, metallic or gray-striped flies suddenly appear inside your home, a dead animal is almost certainly the cause. These are blow flies or flesh flies, species that breed on decaying carcasses, and their presence indoors points to a rodent, bird, or other small animal that died in your walls, attic, crawlspace, or chimney. The good news: this is a temporary problem with a clear solution.
How to Identify Corpse Flies
Two fly families show up on dead animals indoors. Blow flies (Calliphoridae) are the shiny, metallic ones, ranging from copper and green to blue or black. They’re noticeably larger than common houseflies and often buzz loudly near windows. Flesh flies (Sarcophagidae) are gray with distinct black stripes running along the top of their bodies just behind the head. Flesh flies are typically the first to arrive at a carcass, but both species behave similarly once they’re inside.
If you’re seeing clusters of these flies in one room, especially near a window, that’s a strong clue. They emerge from the carcass and fly toward light, so the room where they congregate most heavily is usually closest to the source.
Why They Appeared
The most common scenario is a mouse or rat that died inside a wall void, attic, or crawlspace. This happens frequently after rodent baiting, because poisoned rodents often retreat into hidden spaces before dying. Birds that enter through vents or chimneys and become trapped are another common source. Squirrels, raccoons, and opossums can also die in attics or under porches.
Female blow flies can detect a carcass within minutes of death and lay around 250 eggs at a time. Those eggs hatch into maggots within 24 hours. The maggots feed for roughly a week, then pupate, and after about 8 more days emerge as adult flies. So if you’re suddenly seeing dozens of adult flies, the animal likely died two to three weeks earlier. The flies you’re seeing now represent a generation that developed entirely on the carcass before emerging into your living space.
Finding the Source
A dead animal behind a wall produces a smell that’s hard to miss. The odor from a decaying carcass is strong enough to make your eyes water, and it’s usually strongest in the room closest to the animal. Follow your nose: press it close to walls, baseboards, and vents in the room where flies cluster. The smell will intensify as you get closer. Check along the base of walls for staining or moisture, which can seep through drywall from a decomposing animal on the other side.
Also inspect accessible spaces directly. Look in the attic with a flashlight, check crawlspaces, peer into the fireplace and chimney cleanout, and examine any gaps around plumbing or ductwork where a rodent could have traveled. If the animal is in an accessible spot, removal is straightforward. If it’s sealed inside a wall, locating the exact position often requires cutting into drywall, guided by the strongest point of odor.
How to Get Rid of the Flies
Removing the carcass is the only permanent fix. Spraying adult flies with a pyrethrin-based aerosol (sold at any hardware store) will knock them down quickly in a closed room, but it won’t stop more from emerging as long as the source remains. These sprays have no lasting residual effect. You’ll keep killing adults while new ones hatch.
If you find the carcass yourself, handle it safely:
- Wear double nitrile gloves. Don’t use leather or cotton gloves, which absorb fluids.
- Use a tool, not your hands. Tongs or a shovel work well for small animals.
- Double-bag the remains in heavy-duty dark garbage bags and dispose of them in an outdoor trash bin.
- Disinfect the area. Mix one part household bleach to nine parts water, apply it to the surface, and let it sit wet for at least 20 minutes before wiping.
- Bag your gloves and cleaning materials separately and throw them out.
Sweep or vacuum up any maggots you find. They can’t survive without the food source, so once the carcass is gone, the infestation ends.
When the Carcass Is Inaccessible
Sometimes you can smell it but can’t reach it without tearing into walls or ceilings. In this case, you have two options. You can call a pest control or wildlife removal professional who has experience locating and extracting animals from wall voids. This often involves cutting a small access hole in the drywall, removing the remains, disinfecting, and patching. Or, if you’d rather wait it out, the flies will stop emerging once the carcass dries out completely. Depending on the size of the animal and humidity levels, this takes anywhere from two weeks for a mouse to several weeks for something larger. The smell follows a similar timeline, peaking around one to two weeks after death and gradually fading.
During that waiting period, keep affected rooms ventilated, use aerosol sprays to manage adult flies as they appear, and place activated charcoal or baking soda near the strongest odor point to help absorb the smell.
Preventing It From Happening Again
The flies are a symptom. The real problem is that an animal got inside your home’s structure in the first place. Seal cracks and openings where rodents or wildlife can enter, paying close attention to gaps around utility lines, dryer vents, roof soffits, and foundation joints. Install tight-fitting screens on windows and repair any broken ones. Cap your chimney if it’s open.
If you’ve been using rodent bait stations indoors, reconsider. Poisoned rodents frequently die inside walls, creating exactly this problem. Snap traps placed along walls allow you to find and dispose of rodents immediately rather than waiting for them to decompose somewhere you can’t reach.