Cluster Flies are a seasonal pest in Ontario. They become a pest in the spring and fall when they are found, often in great numbers in homes. Cluster flies enter homes in the fall of the year looking for shelter for the winter. They enter through tiny cracks around windows and doors and especially around the eaves and gable ends of houses.
On sunny days during the fall and early spring, the flies become active and attempt to leave. They often become trapped in living spaces at these times. Cluster Flies are a rural pest because the larvae are a parasite of earthworms. The more earthworms there are, the more cluster flies there will be. Earthworms are particularly populous near manure or compost piles, and in areas with lots of grasses and moist soil. Cluster flies are not attracted to food or waste. Adults live on flowers, and the larvae live on earthworms. However, they can be quite a nuisance, buzzing loudly and bumping into windows, and when in large groups (clusters), they can give off an unpleasant smell.