Cluster Fly Control for Attics and Wall Voids

Cluster Fly Control

Why are there flies at my windows in January? If they are slow, a little larger than a house fly, dark gray with short golden hairs on the back, and they overlap their wings at rest, they are cluster flies. Cluster flies spend the winter inside buildings. On a warm, sunny winter day the wall they are resting in heats up, they wake, and they crawl toward the light. That is why they appear at windows, often on the south and west sides of the house, and often in an upstairs room.

Cluster flies have nothing to do with garbage or sanitation. Their larvae develop inside earthworms in the soil. The adults emerge from lawns, pastures and fields through the summer, and in late summer and early fall they look for a sheltered place to spend the cold months. A house with sun on its siding, gaps at the soffit, unsealed vents, or cracks around window frames is exactly that. Once inside they gather in attics, wall voids and behind trim, sometimes by the hundreds.

Wooded and rural lots in southern Virginia and northern North Carolina have plenty of the earthworm rich soil cluster flies come from. Houses that back onto pasture or woods tend to see them year after year. The number that shows up indoors is a reading of how many got into the voids the previous fall.

Our approach starts with an inspection to confirm the species and find the entry points. Cluster flies are treated differently from house flies, because the source is outside and the population is inside the walls. Treatment aims at the exterior surfaces and openings they use to enter, and at the attic and voids where they gather. Sealing the gaps we find reduces the number that get in next season.

Timing matters. The best time to treat is late summer into early fall, before they move in. A treatment in January, when they are already inside the walls, reduces the flies you see but cannot reach every void. Indoors, vacuuming up the adults that appear at windows is the cleanest way to deal with them.

We will not tell you we can stop cluster flies entirely on a rural lot. What we can do is cut the number that get into the structure, hold what is left at the lowest practical level, and show you which openings to close.