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Cluster Flies

A pervasive nuisance in homes and other manmade structures during the fall and spring, cluster flies overwinter to survive the drop in temperature. Cluster fly larvae are parasites of earthworms but tend to cause little damage outside of their parasitic behavior. Nevertheless, given their tendency to congregate in large numbers, cluster flies may require the attention of pest control professionals.

Appearance / Identification

cluster fly
cluster fly
Photo credit: MSU Diagnostic Services

What Do They Look Like?
Size: With an average size of 5/16 of an inch, adult cluster flies grow slightly larger than the common house fly.
Color: The body of the cluster fly is largely nonmetallic gray with black markings.
Characteristics: Some distinguishing characteristics of the species include a pair of wings, longitudinal black lines on the thorax, and yellowish hairs that give off the appearance of a golden sheen.

Behavior

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.

Food

What Do They Eat?

Larvae feed on earthworms as they develop into adults. Once fully developed, cluster flies feed on organic matter, plant sap, fruit, flowers, feces, and proteins in animal meat.

Life Cycle / Reproduction

Female cluster flies lay eggs in cracks in the soil during the late spring and summer months. Within three days, the eggs hatch and the emerging maggots eat their way into an earthworm where they will go through the larval and pupal stages of the cluster fly life cycle. The entire developmental process lasts an average of 27 to 39 days. As such, cluster flies are able to produce three or four generations each year. Adult flies live for several months.

Detection

How to Find Them

  • Look for clusters of adult flies in the home around late autumn and early spring.
  • Check for small, dark spots of excrement on windows and walls.
  • May notice the emergence of adults during brief warm spells in the winter months.

Problems Caused by Cluster Flies

Surprisingly, the cluster fly causes very few problems. Though they overwinter in large numbers, the insects do not feed on cloth, nibble holes in furniture, or cause any sort of structural damage. Furthermore, the species waits until summer to lay eggs and does not feed on human food. Though previous research suggests that large numbers of cluster flies can cause problems in hospital settings, the insects are not known to carry diseases of medical importance to humans. Instead, the cluster fly is merely classified as a nuisance rather than a pest insect.

Signs of Infestation

Cluster flies get their common name from their tendency to congregate in clusters on the sunny exteriors of buildings in the fall and on windows inside the home on warm winter days. As such, the sight of large congregations of flies around windows during unseasonably warm weather is a fairly certain indicator that a cluster fly infestation exists somewhere in the home. Residents should check nooks, dark corners, in wall cavities, false ceilings, and attics for the insects.

In addition to congregating in large groups, cluster flies leave behind small, dark spots of excrement on walls and windows, which serve as further evidence of an infestation problem. Furthermore, once the weather becomes consistently warm in the spring, cluster flies will emerge from their overwintering sites and begin to appear around the house. Typically, emerging flies move sluggishly and will clumsily fly around homes looking for a way outside.

Prevention Tips

Prevention remains the best method of dealing with cluster flies, as it is challenging to remove them from the home once they find a place to overwinter. Exclusion works best, so homeowners should take care to seal cracks around windows, doors, siding, utility pipes, behind chimneys, and similar locations. Replacing damaged window screens also helps to prevent a cluster fly infestation.

Get Rid of Them

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.

Attics
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.

Outside the Home
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.

Insecticide
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.