Cluster flies are harmless, but can be a nuisance, especially in the autumn. There’s no direct association between bats and cluster flies, but they tend to live in the same areas of buildings.
Cluster flies usually arrive in late autumn, gathering on walls, and then move into lofts for hibernation. They either die in large numbers over the winter period or move out in the early spring when they search for light to get outside. This is often how cluster flies end up in the living areas. The flies are likely to return the following autumn and begin the cycle all over again (which means any treatment may need to be repeated yearly). Whenever possible, we recommend tackling the fly problem using non-chemical treatment. These methods can create long-term solutions for you so that the cluster flies do not bother you if they do come back the following year.
Additionally, chemical insecticide treatment of cluster flies within the roof space is not always necessary, effective, or safe to be carried out where bats are present.
This advice is designed for non-chemical cluster fly control in dwellings. If you need to control cluster flies in any other scenario, require the use of chemical control, or you cannot follow the guidance, you should seek personalised advice.
Please do not use fly paper where bats roost
Please note that sticky traps of any kind (e.g. fly paper), must never be used where bats are known or suspected, though they can be effective in the living areas of dwellings. The volunteers on our UK Bat Care Network have dealt with many sad cases of bats getting stuck to these traps.