Like flatlanders who misjudge the potency of alcohol at our altitude, cluster flies sluggishly reel and spin around the room, bumping into the ceiling, windows and lampshades. They're a general nuisance this time of the year as our air warms up.
Cluster flies (Pollenia rudis) are often mistaken for houseflies. If you look closely--and it's possible to do so since they're so sluggish--cluster flies are slightly larger than houseflies that only invade Steamboat homes in the summertime. At rest, their wings overlap at the tip unlike the housefly, and cluster flies do not have stripes on the chest but instead some short yellowish hairs.
Don't blame your indoor plants for harboring these pests. They invade our homes in the fall through tiny openings in the siding, window sashes, foundation cracks, attics, basements, etc. Throughout the cold winter months, they remain in hibernation, and then begin waking up when the temperatures reach the mid-50s. They often are found in sunny rooms, near the windows, seeking a way outside.
Cluster flies do not breed indoors. They seek soil containing earthworms in which to lay eggs. The larvae develop as parasites in the bodies of earthworms. While populations of cluster flies vary from year to year, they seem to become most prolific when the summer is wet.
To control an infestation of cluster flies, you'll need to seal any openings that they may use to enter your home in the fall. Once they're indoors, you can vacuum up these slow moving pests or swat them with a newspaper or flyswatter. Cluster flies often leave a greasy stain on upholstery, wood, curtains, walls and other surfaces when smashed.
There are some insecticides you might want to try that have tube injectors allowing you to spray inside your walls. The following chemicals are registered for controlling cluster flies and are available to homeowners: pyrethrin and permethrin. Foggers and dust applications are also known to be effective over a long period of time. Flypaper, bug zappers and other methods that work well on houseflies are not much use with cluster flies since they don't fly around much.
If you have a large infestation of these flies, the dead bodies can attract larder beetles who will feed on them and then move from the walls to other parts of your home.
Another indoor pest common in Steamboat homes located near livestock are face flies (Musca autumnalis). Face flies are similar to houseflies also, but are slightly larger and darker. They breed in fresh manure of farm animals and overwinter in homes and structures located near the animals. You've probably especially noticed them in the summertime, bothering cattle and horses where they buzz around their nose, mouth and eyes.
Control for face flies is similar to that for cluster flies.
So if you're being driven crazy by punch-drunk flies in the house this spring, take solace in knowing their desire for an escape route is as great as yours is to be rid of them.
Deb Babcock is a Colorado State University Master Gardener through the Routt County Cooperative Extension office. Products mentioned in this article are presented for informational purposes only and are not endorsed by the Master Gardener program. Questions? Call the CSU Cooperative Extension office at 879-0825 or email to: firstname.lastname@example.org