Cluster flies are a common household nuisance, often appearing indoors during cooler months. These insects are larger than typical house flies, measuring 8 to 10 millimeters. They have dark gray bodies with golden or yellowish hairs on their thorax, and their abdomen often displays a black and silver checkered pattern. When at rest, their wings overlap across their back, distinguishing them from other fly species.
Where Cluster Flies Lay Their Eggs
Cluster flies do not lay their eggs inside homes or other buildings. Their reproductive cycle is distinct, relying on a parasitic relationship with earthworms. Female cluster flies deposit their eggs individually in the soil, near the entrance to earthworm burrows, in late summer or early autumn.
Once the eggs hatch, the tiny larvae, known as maggots, seek earthworm hosts. They then burrow into the earthworm’s body and feed internally, developing within the host for 13-22 days. After larval development, they emerge from the earthworm and pupate in the soil. The entire life cycle, from egg to adult, takes about one month, with several generations occurring each year.
Why Cluster Flies Enter Homes
Cluster flies enter homes to find sheltered locations for overwintering. As outdoor temperatures drop in late summer and early fall, these flies seek warm, protected spaces to hibernate. Buildings, especially those with sun-exposed, south or west-facing walls, attract them.
They gain entry through small openings like cracks and crevices around windows and doors. Loose-fitting screens, attic vents, and gaps in siding are common entry points. Once inside, they congregate in undisturbed areas like attics, wall voids, or unused rooms, remaining inactive until warmer temperatures stimulate them in spring.
Managing Cluster Flies in Your Home
Managing cluster flies involves both preventing their entry and removing those already inside. Sealing entry points is a primary preventive measure. This involves caulking cracks and gaps around window and door frames, ensuring screens are intact and properly fitted, and sealing openings around utility penetrations and attic vents.
For flies already indoors, vacuuming is an effective and safe method of removal. Light traps or sticky traps can supplement these efforts by attracting and capturing flies. While some chemical sprays are available, using them indoors can pose health risks, so they should be applied cautiously.