What are cluster flies?

Often confused with the common house fly, cluster flies are roughly the same size. Some characteristics that differentiate the cluster fly: they fly somewhat more slowly than the house fly, they almost always fly toward windows on the warm side of a structure and their wings overlap almost completely, when at rest. Cluster flies are most common along the northern part of the US and in other countries around the world. They will appear inside homes to overwinter only during the cool fall, winter or spring months.

Cluster flies should not be confused with other medium to large size flies which may appear suddenly. Those flies may appear when a small animal such as a mouse, rat, squirrel or bird dies within a wall, ceiling or floor void. You may or may not detect an odor. Such flies will find the hidden carcass and lay eggs on it. The eggs will hatch into larvae (maggots) which feed on the carcass. Soon, the larvae enter the pupae (cocoon) stage and then eventually hatch out as adults (flies), appearing around windows (just like cluster flies). This process may take from a few days to up to two weeks to complete and for the flies to stop appearing. Until then, open windows or doors to let the flies out or vacuum them up as needed.

Cluster flies breed in the ground outside of buildings during the warm weather (late Spring into early Summer) using earthworms as a food source for the immature larva (maggots). The flies later pupate (go into the cocoon stage), then hatch as adult flies. In temperate areas, often in late August or early September, these flies begin to migrate indoors finding any small cracks or crevices that permit entry into a structure. These include areas around window frames, door frames or eaves. Entry tends to be on the same, warm, sunny side (often the southern or western exposure) of the structure that the flies later emerge from.

Unlike other flies species, cluster flies only overwinter inside of a structure; they do not breed there. During the fall, winter or spring months, these flies may emerge, particularly on warm, sunny days thinking that spring has arrived. The flies appear at windows buzzing and “clustering” around those areas to the dismay of the occupants. This fly can become a problem in virtually any structure and they have been a problem in sensitive areas such as hospitals, where they are especially unwelcome.

Why do I have them?

As soon as the fall approaches, the cluster flies begin to enter homes and buildings in large numbers. At this time of the year, the days become shorter and temperature begins to fall, hence they enter human houses in search for overwintering sites.

And as during this time the west and south facing buildings are exposed to more sunlight, so the cluster flies are attracted more to such buildings due to the warmth they can get there. They enter these buildings through small openings or cracks and crevices near window or door frames, open or unscreened windows/vents.

Once inside they gather together in an isolated, safe place such as attics or false ceilings and begin to hibernate. Generally these sites are the upper south or west sides of the buildings. Once the temperature again crosses 12 degree Celsius they become active.

If the temperature inside the building is manually controlled above 12 degree Celsius, the cluster flies may come out thinking it is spring. Cluster flies are strongly attracted to light, so you can also find them near windows and near lamps at night.

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