Flies at the Window: A Clue to a Hidden Animal Carcass

Fly clues worth recording

A few minutes of observation can save hours of searching later.

Which window or light fixture the flies collect at, and at what time of day
Whether they are large and shiny green or blue, or small and dull gray
Whether any are crawling out from an outlet, a recessed light, a baseboard gap or a vent grille
Whether small pale larvae have appeared on the floor or sill, which happens when they leave to pupate
How many you see each day, so you can tell if the activity is rising or fading
Whether the smell is stronger in the same room as the flies

Quick reference

Large shiny flies plus rotten smell: Strong sign of a carcass nearby
Larvae on the floor or sill: The source is usually close, often above or beside that spot
Flies stop appearing after a few weeks: The carcass may be drying out, but residue can still smell
Swatting and spraying: Controls adults only; the source still has to be found
Best next step: Use the fly location to narrow the room and wall, then plan access

How to use the flies to find the source

Blow flies home in on decay quickly and lay eggs on the carcass. A week or two later, a new generation emerges and heads for light, which is why they collect at windows and light fixtures. The window where they gather is usually in the same room as the source, or the room directly below an attic or ceiling source.

Larvae crawling out onto a floor or windowsill are a sign the carcass is very close, often directly above or beside that spot. Note where they appear before cleaning them up.

Common questions

Should I spray insecticide into the wall?

It is rarely worth it. It does nothing about the odor, may contaminate the cavity, and the flies stop once the carcass is removed or dried out.

Is a fly problem a health risk?

Blow flies move between decaying material and surfaces, so cover food, clean counters and deal with the source promptly.

How long do the flies keep coming?

Usually a week or two, until the carcass dries out. A steady stream for longer may mean a second source.

General information for Sloan's Lake, not a quote or professional advice. In an emergency, call 911.