Cluster flies (Pollenia), also known as "grass flies" and "attic flies" belong to the same family as Blow flies, and are often confused with House flies because of their similar appearance.
They are known for heading indoors when outdoor temperatures drop, and for heading into the highest parts of your house as they can (ideally the attic). They come out of hiding on winter days when the sun shines and it warms up, and "cluster" around windows, attempting to exit.
Cluster flies are also well-known for moving very slowly and lethargically, compared to other home-invading nuisance flies. You can take steps to help prevent them from overwintering in your home.
What Do Cluster Flies Look Like?
Cluster Flies vs. House Flies
Adult Cluster flies are often mistaken for House flies, are dark grey in color, and about 1/3" - 2/5" in length. They also feature golden-colored hairs on their bodies, with a prominent light and dark checkered pattern along their abdomens, and distinct striped patterns behind their heads.
Like Blow flies, their larvae (maggots) look like tiny, whitish worms.
Where do Cluster Flies Live?
Cluster flies are found mostly in Europe, North America, as well as in Australia and New Zealand. They tend to primarily exist in proximity to humans.
These flies hide in cracks, crevices, and voids, especially indoors when it cools off outside. In your home, Cluster flies are often found behind curtains, underneath clothing in closets, behind picture frames and furniture, and as vertically high up in the house as they can.
Cluster Fly Habits
Adult Cluster flies outdoors live close to the ground level, and feed on flower nectar, plant sap, fruits, and other organic materials. Their maggot larvae are parasites of earthworms, and adult females lay their eggs in the soil near earthworm burrows so that the larvae can burrow into the ground and feed on the worms.
What Causes Cluster Flies in the House?
Cluster flies are attracted to light, which explains their fixation with windows on sunny days. They are also attracted to artificial lights at night. These flies are not attracted to garbage and other refuse like so many other types of filth flies.
Are Cluster Flies Dangerous?
Though Cluster flies are considered a nuisance as overwintering pests, they do not bite humans or animals (other than earthworms), and they do not spread bacteria or lay eggs in food. They pose no health threat whatsoever.
The Cluster Fly Life Cycle
Generally speaking, each season brings with it around three to four generations of these flies.
Once adult Cluster flies emerge from overwintering in the spring, they lay eggs in cracks in the soil --these eggs hatch within about 3-4 days and the larvae proceed to feed on earthworms for two to three weeks.
The larvae will then pupate in the soil for 11-14 days, after which a new generation of adult Cluster flies emerge. From egg to death, the Cluster fly lifespan lasts from one to three months.
Will Cluster Flies Eventually Go Away on Their Own?
If Cluster flies have made it into your home in the fall to overwinter, they will stay out of the way, and emerge occasionally on warmer, sunny winter days, attempting to get outside through your windows. They lethargically float around in a zombie-like state, and sometimes just fall to the floor like they're dead. At any point, Blow flies can appear to be half-dead or dead, but, like zombies, they never quite go away if left alone.
Cluster Flies in the House: How to Prevent and Control Cluster Flies in Your Home
Make sure all door and window screens are in good repair. Also, sealing cracks, crevices, and holes around the outside of your house, as well as screening plumbing/cable penetration points will help you keep invading Cluster flies outside.
Spot- and area-treating for Cluster flies outdoors during the summer (along with treating for other outdoor pests) will help reduce their populations and will help keep more of them from coming indoors in the fall to overwinter.
How to Get Rid of Cluster Flies Naturally
Cluster flies are quite harmless outdoors, and you will mostly have to deal with their nuisance existence indoors. But outdoor prevention steps always help mitigate indoor pest problems. Pest control products would ideally not be used until all other methods have been implemented and you're still seeing a Cluster fly problem.
Try an effective plant oil-based indoor fly killer for spot-treatment, like Maggie's Farm Home Bug Spray, or Maggie's Farm Flying Insect Killer. Plants don't like flies and other bugs any more than you do, and the natural oils they produce to protect themselves are amazingly effective at killing and repelling bugs of all sorts. For effective personal protection against flies (and mosquitoes), try plant oil-based Maggie's Farm Natural Insect Repellent.
For longer term, more thorough coverage, you can treat wider areas of your lawn with effective hose-end plant oil-based pest control products like Maggie's Farm Yard Bug Spray.
Always carefully follow any directions on pest control product labels, including for storage and disposal.
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How do you control Cluster flies in your home and yard? Leave us a comment! We want to hear your tips and tricks!
For scientifically-tested, effective fly control in your home that is friendly to the environment, try Maggie’s Farm pest control products. Our promise is that our plant and mineral-based products are developed by scientists and seasoned pest control professionals to be the most effective.