Cluster Flies

In the spring, cluster flies lay their eggs in soil near earthworms. After they hatch into larvae, or maggots, the flies find earthworms to crawl into, feed on, and complete the larva part of their lifecycle. After this, they enter the soil to pupate, after which they have completed the transition to adulthood. During the summer months, adults feed on fruit juices and flower nectar. As summer wears on and daylight hours shorten, cluster flies seek protection from the impending cold weather. Colder temperatures cause the flies to crawl upwards, and in this way they enter cracks and openings in buildings. Gaps under eaves and siding are common places for cluster flies to enter. They will remain in buildings until early spring, but they will return to roof spaces previously occupied the following winter so re-infestation can be expected.

When do cluster flies appear in Ireland? In Ireland, cluster fly activity begins in late August and September as daylight hours shorten and temperatures start to drop. The peak period for cluster fly entry into buildings is September and October. Large numbers can appear very suddenly during this period — homeowners who had no fly problem all summer can find dozens or hundreds of flies in attic spaces, behind window frames, or on south-facing walls within the space of a few days. This seasonal pattern is the most reliable way to identify cluster flies: a sudden appearance of large dark flies in autumn, clustered near light or warmth, with no obvious breeding source inside the building.