Cluster flies gather on houses in the fall in large numbers. Many of them manage to get inside the walls. Whenever the walls get warm from the sun, the flies become active. They squeeze through cracks in search of light and they often end up inside the living space.
Cluster flies are pests in the fall when they try to get into the house to get warm. They are also pests when they come out of the wall or the attic inside the house.
If cluster flies are already inside the house, the simplest remedy is to vacuum them up. It may be necessary to do this again over several days because they will come out any time the temperature goes up.
Make a note of every spot where the flies are coming out. If they are coming from holes in the wall, cracks around electric boxes, or around ceiling fixtures, caulk, spackling, or even tape may help keep them out of the living space. During the summer, after all of the flies have gone, it will be possible to seal the openings permanently.
Do not try to spray anything into the wall void to kill the ones that are hiding. It will be impossible to get the dead flies out of the wall and they will attract beetles and other scavengers.
If cluster flies are in a large, open attic, several experts suggest installing a light with an un-shaded large-wattage bulb. The flies will fly around the light until they wear out and drop. Use fly strips or sticky traps to catch the flies around the light. Use a vacuum to remove the dead flies.
If there is room to get into the attic to remove the dead flies, a fogger or flying insect aerosol can also be useful to kill many of the flies. Do not spray flies in a confined attic if there is not room to remove them after the treatment.
If you have to let this year's cluster flies emerge as they have in the past, take comfort in knowing that this may be their last visit inside your home. When summer comes, start planning ways to prevent a cluster fly problem in the future.
Inspect the outside of the house to find all of the cracks the flies have used to get into the house. Caulk any cracks around doors and windows. Repair damaged screens on windows and vents – check the attic vents too.
Make sure the exterior doors close tightly. Use weather-stripping to eliminate gaps under the doors. Caulk around openings for cables and pipes. Secure the access to the crawl space.
It will not be possible to kill all of the cluster flies outdoors because the larvae feed on earthworms in the soil. When it is time for them to gather on the house, a liquid insecticide can help discourage them from staying.
The insecticide label will have directions for mixing and applying. The extension service can suggest the best time to make the applications for your particular area. Many people call on a pest control professional for this part of the process.