Unlike the more familiar houseflies and bluebottles, cluster flies don’t usually spend much time around people. They are unlikely to spread diseases or be a major irritation. However, they do enter buildings during the winter to hibernate, often in large numbers. At this point they still aren’t exactly a health hazard, but they might constitute a nuisance.
What are cluster flies?
Hibernating cluster flies may be the common cluster fly (Pollenia rudis) or any of a number of related species from the genus Pollenia. Often, individuals from more than one species hibernate together. They look very much like houseflies, albeit a little larger and slower. Cluster flies do not bite, sting or carry infections dangerous to humans, but they may produce a somewhat unpleasant smell when they congregate in large numbers.
Because flies are small, complete exclusion may not always be practical. However, sealing up gaps and cracks around windows and doors at the end of the summer helps stop cluster flies entering a building. If the building isn’t going to be used over the winter anyway, for example in the case of a garden shed, consider leaving the cluster flies alone. They won’t harm equipment and will leave come the spring of their own accord.
Natural fly repellents, in particular citronella essential oil, might make local cluster flies choose another hibernation site. Start burning a citronella candle or a few drops of the essential oil in an oil burner in the autumn before the weather gets really cold. Burn the candle for about five minutes a day a couple of times a week. Do not leave the candle or burner unattended or you might have a far more serious problem than cluster flies. Alternatively, use a spray with a few drops of the oil in water and mist the site occasionally.
Extermination or removal
Because cluster flies form an important part of the ecosystem and are not really a pest, extermination should be the last resort. The most natural killing method is simply crushing them, but even this is not necessary, with removal being the better option. Catch as many of the flies as you can when they move in using an aquarium net and a plastic tub and release them at least few hundred metres away from your home. Seal gaps afterwards to stop them returning.
The places that cluster flies hibernate may also be used for the same purpose by other animals, notably bats. Before disturbing the site, using repellents or commencing repair work, check carefully for evidence of bat occupation, remembering that it is illegal to disturb bats in the UK without a permit.