Elis Pest Control

Cluster Flies

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Get Rid Of Cluster Flies

Cluster flies also known as grass flies or attic flies are of the genus Pollenia in the family Polleniidae. Unlike the more familiar blowflies such as the bluebottles they are completely harmless to human health as they do not lay eggs in human food.

Where can you find them?

These flies have the habit of coming indoors in late autumn to hibernate during the cold winter months, they are often found in groups or clusters of many hundreds if not thousands. During the summer cluster flies live outdoors and can often be seen sunning themselves on fences, walls and tree trunks. Indoors, they congregate in roof spaces and dark corners of attics and little-used rooms. Flies can also enter via sash window boxes, cracks round window frames and unused Venetian blinds or curtains.

Occasional warm days in the cooler months bring cluster flies out of their hiding places and they crawl or fall into rooms. You may have experienced windowsills covered with buzzing flies spinning round and round in uncoordinated attempts to fly. The following spring, the flies start moving again, and the problem recurs. The appearance of these lethargic flies causes considerable annoyance, although they are quite harmless. Although we understand why clustering flies seek shelter in this way, we still do not know why they only enter certain buildings in preference to other adjacent or nearby properties. Please be aware that once a building has been invaded, it is almost certain that more flies will return in subsequent years.

How do you treat the problem?

Complete control of cluster flies is almost impossible since the over wintering adults are concealed in walls, attics, etc. and often it is impossible to prevent them getting in, and success with the killing of one species may be negated by the later appearance of more flies, often of another species. Treatment of roof spaces can be carried out using an insecticide smoke generator or a ultra-low volume fogging treatment which can be carried out by a trained technician from the Elis Pest Control Team. Please be aware that bats may be living in areas of buildings such as roof spaces, which are used by cluster flies. Bats a protected species in Ireland and so care must be taken to survey areas before treatment for signs of bats.

Additional Information


The larvae of a Cluster fly live as a parasite inside earthworms. Eggs are laid in the soil and hatch in about a week. The young larvae bore into worms where they stay throughout the winter. In spring the larvae develop quickly, usually killing the worms, then pupate in the surrounding soil. The adult flies emerge in summer. These are large flies (10 mm long), with a shifting pattern of silver and grey-brown markings on the abdomen, and crinkly yellow hairs on the thorax.


The Yellow Swarming fly a predacious species, the larvae feeding on aphids living on the roots of grasses. There are two generations each year, the later one usually over wintering as pupae in the soil. However, a warm spell in late autumn may cause the adults to emerge prematurely, and they then invade buildings for shelter when the weather becomes colder. The adult flies are very small (2.5 – 3 mm) and are bright yellow with black markings.


The Autumn fly breeds in animal dung in fields. Adult females are almost identical in appearance to common house flies, but males are distinctive in having an orange abdomen with a black mark down the center. Contact Elis Pest Control for more cluster flies problems. 


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