Big flies in your Marshalltown house this fall, and what a smell in one room means

It is November, the screens are long since down, and there are big flies in the house. Maybe they are buzzing at the back bedroom window. Maybe they are crawling on the glass of an upstairs room nobody has opened in a week. Before anyone reaches for a swatter, smell the room. Flies indoors this late in an Iowa year come from one of two places, and your nose is the quickest way to tell them apart.

What does a dead-animal smell in one room mean in late fall?

A sweet, heavy, rotten smell that stays in one room and gets stronger near one wall usually means something died inside the wall, the ceiling or the floor. In a Marshalltown house in late fall that is most often a mouse that came in during the October push and never came back out. It needs finding and the next one needs stopping, which is ordinary work for pest control in Marshalltown.

A mouse is the usual answer, not the only one. A bird that came down a chimney with no cap, a bat that got stuck behind a gable vent, or a squirrel that made it into the attic and could not find its way back out will all do the same thing on a bigger scale. Size matters for the smell. A mouse is a small, sharp smell you notice walking past one spot. A squirrel fills a room.

Old houses have plenty of smells, and most of them spread. A damp basement smells all over the basement. A dead animal smells in one spot, and opening a window does not clear that corner.

Why are shiny green or blue flies showing up in that one room?

Those are blow flies, and they are hatching from the animal. A blow fly finds a carcass by smell, even one sealed in a wall cavity, lays eggs on it through whatever gap it can reach, and the next generation comes out into the house near the same spot. That is why they show up in one room, and often some days after the smell first got noticed.

They are easy to tell from a house fly. A blow fly is bigger and loud in flight, and its body has a hard metallic shine, bright green on some, steely blue on others. Indoors they head for the light, so they collect at that room’s window or circle the ceiling fixture. Sometimes a few small pale larvae turn up on the floor or a windowsill below the spot as they leave the carcass to finish growing.

Swatting them does no harm, and vacuuming them off the glass is fine. They have been walking on a dead animal, so keep them away from the kitchen and wipe down any counter they land on. The flies will keep coming for a while whatever you do at the window, since the source is still in the wall.

How do you narrow down where it is without opening every wall?

Follow your nose low and slow, then let the flies vote. Smell along the baseboards and at the outlets and switch plates on each wall of the room, since an outlet box is an open window into the cavity. In a kitchen, check the toe-kick under the lower cabinets and the gaps behind the range and the refrigerator, two warm spots where mice like to spend time.

A few other places give a source away:

• A damp, yellow or brown stain coming through the ceiling drywall, often with a fly or two settled on it.
• The attic insulation directly over the room, if the smell is stronger up there than below. A mouse that died on top of the ceiling is in the attic, not the wall.
• The heat register in the room, when the smell blows out with the furnace. A duct run or the space beside it can hold a carcass.
• The one wall the flies keep landing on or crawling out from. Where they emerge is usually close to where they grew.

When something has already died in a void, finding the odor source is part of a rodent job here. If a wall does end up opened, it is the last step and a small one, made where the smell and the flies point to the same spot.

Why do mice end up dying inside the walls?

A mouse goes home to die. The nest is in the cavity, so a mouse that is sick or poisoned heads back to the warm, dark place it lives, which is behind the drywall, under a floor or in the insulation over a ceiling. That is how a store-bought bait packet set out in a kitchen so often turns into a smell in the next room a little later.

Bait does what it says. It kills mice. What it cannot do is choose where the mouse is when that happens, and in a lived-in house the mouse usually picks the cavity. A trap is different. A trap leaves the mouse where somebody can pick it up.

That is the reasoning behind how rodent control that keeps mice from dying in the walls is laid out for a house in town. Traps go along the walls mice actually run, the gaps they came in through get sealed so no new mice arrive, and any bait goes in secured, tamper-resistant stations placed with the traps, never a loose packet behind the stove. A follow-up visit pulls the traps and checks that the sealing is holding. For a house that already has the smell, the cost page lists what a fall rodent visit costs.

Does the smell go away on its own?

Yes, eventually. A dead mouse dries out inside the wall, and once it does, the smell fades and the flies stop. How long that takes depends on the size of the animal and on how warm and humid the cavity is. A mouse in a cold outside wall goes faster than one in the warm space beside a chimney or over a kitchen ceiling, and a squirrel takes longer than either.

Moving air helps more than anything you can spray. Run a fan pointed out an open window, keep the door to that room shut, and close the heat register if it is the one carrying the smell. Covering the smell with something stronger mostly gives you two smells. If the smell comes back later in a different room, it is likely a second mouse, which means the gap that let the first one in is still open.

What are the slow flies at the upstairs windows if nothing smells?

Cluster flies. They are a bit bigger than a house fly, dull gray rather than shiny, with fine golden hairs on the back, and they move slowly, crawling on the glass more than flying. They show up at the sunny upstairs windows of an older house, sometimes a lot of them, and the room they are in smells like nothing at all. They do not bite, and they do not breed inside.

The life story explains the windows. Cluster fly larvae grow in lawn soil, living off earthworms. In late summer the adults gather on the sunny south and west sides of tall houses and work in under the upper siding, around the window trim and through the attic vents to spend the winter in the attic and the wall voids. In Marshalltown that is mostly the older two-story houses in the near-north neighborhoods and off Center Street, plus any tall house that faces a big lawn or the grass along the Iowa River bottoms. On bright days the warmth draws some of them out toward the light, and that is when they turn up on the glass.

That is a different visitor from the fall invaders that crowd the warm walls. Boxelder bugs and lady beetles mass on the outside of the house in daylight where everybody can see them. Cluster flies go in quietly and only show up once they are already inside.

Spraying the window does little, since the flies on the glass are a small share of what is in the attic. Vacuum them up as they appear. The real fix is sealing the upper siding, window trim and attic vents before the next late summer.