These Collections Care documents have been produced using information from our Conservation Development Officer, Helena, to provide suggested information on Cluster Flies including how to identify and treat them.
Larvae are parasites in earthworms. Adults eat mainly plant fragments and some meat.
Larvae – Long thin maggot, only found in earthworms
Adult – Black fly with large eyes, clear wings, golden hairs on the back
What’s that slowly buzzing around the windows? If they are quite large, black flies with
clear wings, and large brown eyes and there are lots of them, they are probably cluster
The bodies may look paler because they have pale, golden hairs on the back, but they
are different from the metallic sheen of bluebottles or the smaller, faster houseflies with
Eggs and larvae
Cluster fly eggs are laid on the ground and the newly-hatched larvae burrow
into earthworms where they mature for up to a year. In late summer the adult
flies emerge from the soil where they laze in the sun, feeding on nectar and plant
detritus. As the weather turns cold, though, they look for shelter and your cosy
museum may be very attractive.
Cluster flies are also called “attic flies” because the large clusters of them are often
found in spaces at the top of buildings.
The flies will gather in warm places – especially south-facing walls – while they
search for a way into the building. If you spot a large gathering of flies on the
outside, check that windows and vents nearby are well sealed on the inside to
prevent them getting in.
You might find them gathered in visible areas such as windows, where they can
be seen, but sometimes inside panelling or cracks in the building, which makes
them much harder to spot.
In spring they will emerge and, again, you may find large groups of them near
windows, looking for mates and a way out to feed and lay eggs in the soil.
Why are they a problem?
The larvae are only found inside earthworms, and the adults are usually
herbivores, so they don’t attack the collection. However, they still cause three
problems in museums:
1. Fly droppings (and vomit) can leave small unsightly marks on painted
surfaces and objects on open display. These can be difficult to remove safely.
Ask a conservator for advice before trying to clean them off an object. In the
South West, the Conservation Development Officer can help.
2. The nuisance of having large numbers of insects slowly buzzing around and
settling on windows or walls. They can even be so numerous that they set off
3. The dead flies are an attractive food source for pests which are more
dangerous to the collection, such as carpet beetles. Once these pests have
finished eating the cluster flies they will look for other protein nearby, and may
attack objects such as fur, feather, leather or insect collections
What can we do?
- The best defence is not letting them in. Check that windows and vents are closed
or covered with insect-proof mesh when open.
- Seal small cracks and gaps. Use bristle strips under doors or small strips of
polythene foam to block gaps around window frames.
- Remove dead flies promptly. A vacuum with a long nozzle can be very helpful.
- You can use UV fly-killers in suitable areas (make sure the objects are not exposed
to the UV radiation).
- Constrain and similar permethrin-based insecticides may help to reduce the
population if sprayed on window areas where they gather. Never spray pesticides on
or near objects and always follow the manufacturers’ safety instructions.
- If you have potted plants in the museum, cover the soil with a breathable
membrane to prevent flies emerging (or laying eggs on the soil).
Download the below the information sheet on how to treat and prevent Cluster Flies.