Cluster flies do not reproduce inside homes. While you might see large numbers of them in your house, especially in attics or around windows during fall and winter, their full life cycle—specifically their reproduction—happens outside, not indoors.

Cluster flies are a little different from common house flies. Their life starts in outdoor soil. Adult female cluster flies lay eggs in the soil during spring or summer. When those eggs hatch, the larvae (which look like worms) seek out and enter earthworms in the ground. These larvae feed on the earthworm for a few days and then turn into pupae (a resting stage, like a cocoon) before emerging again as adult flies.

What’s most important is this: cluster fly eggs, larvae, and pupae all require the soil and earthworms, which you just don’t find inside homes. The only reason you see cluster flies in your house is that they’re looking for warm, sheltered places to spend the cold months. Your home is a perfect spot for them to “overwinter” (basically, to wait out the winter until it’s warm again). They come inside through cracks, gaps in siding, around windows, or through other small openings. Once inside, they hide in walls, attics, or unused rooms. When warm sunshine or indoor heating wakes them up in late winter or early spring, they try to get back outside—which is why you’ll often see them collecting around windows.

Here are a few important points:

• Cluster flies do not lay eggs inside your home.
• Their larvae need earthworms and soil to survive.
• Any flies you see indoors came in as adults, not as larvae or pupae, and they’re not breeding in your house.
• Seeing lots of cluster flies does not mean there’s a hidden infestation or a breeding site inside.
• If you see cluster flies in your home, it’s simply because they chose your house as a good place to shelter for winter, not because they’re reproducing indoors.

So, while cluster flies can be annoying and alarming when they appear in large numbers, you do not need to worry about them multiplying or breeding inside your home. Blocking potential entry points and vacuuming any flies you see are the most effective ways to deal with them. Their presence is seasonal and temporary, and they will leave when outdoor temperatures rise.

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