Weather Forecast


Ag Matters - With warmer spring temperatures, cluster flies come calling

DETROIT LAKES - Within days, the hours of sunshine will exceed those of darkness for each 24 hour period. Not only do we have more hours of sunshine, but the sun seems to have more "power" as well. With the advancing hours of daylight, and sunshine that seems to have more energy, spring must be near.

The warmer afternoons and increasing day length also brings about an increase in fly activity in our south-facing windows. Flies active at this time of year are most likely cluster flies, a name that describes their habit of clustering in large numbers.

Cluster flies develop as parasites inside the bodies of earthworms.

There are three generations of flies produced each summer, and the final generation of the season migrates to houses and other buildings during mid to late September. So the flies you're seeing now entered your home last fall.

As the weather warms, these large black, pesky flies begin to show up in bedrooms and on windowsills, becoming a household pest. Their random crawling brings them into the house by way of electrical outlets, attic doors/entryways, window pulley holes, and small openings around windows, moldings and baseboards.

Despite their annoying behavior, these flies are harmless. Cluster flies do not reproduce indoors, and homeowners bothered by these pests do not need to fear the flies are "hatching" from a dead animal or other unpleasant material within the attic or walls.

Cluster flies hibernate in inaccessible places, making them difficult to control. Hidden within walls or under insulation, they are protected from most treatments until they appear within the living spaces of the house.

Unfortunately, there is little that can be done for flies already inside the attic and walls. Insecticide sprays and fog treatments in the attic have little if any affect, as the flies are usually under insulation or deep in cracks and crevices, not to mention adding potentially dangerous chemicals in your home.

Most insects, such as cluster flies, are attracted to light, such as windows and light fixtures. You can take advantage of this attraction to light to focus your control efforts. Low-tech methods such as a fly swatter, sticky traps, or a vacuum cleaner are tedious, but effective.

Recently, my wife discovered another option that has been very successful in our home, a small transparent sticky trap. This simple and relatively inexpensive traps look very much like short strips of clear packing tape, but are deviously stickier. These traps can be placed near the top of the pane, behind window treatments, and will trap the flies as they buzz about the window.

As the flies bounce happily about the window, without a care in the world, they become hopelessly stuck when they land on the sticky surface -- which they assume is the local fly gathering spot filled with the opposite sex. Once this sticky "dance floor" is full, simple discard it and savor the victory of the fly wars.

Preventing attic flies from entering your home is the ultimate victory, but that's a job for the summer and fall. If you have a bumper crop of cluster flies each year, find their entry points and the battle will be nearly won. Start with the most obvious entry points, such as damaged window screens, missing screens on attic vents or fans. Also, be sure to seal cracks and openings around the outside of the house, especially under the eaves, as you would for energy conservation.

These simple changes will hopefully eliminate the need for you to host sticky dance floor parties for cluster flies each spring.

For more information, contact me at the Polk County Extension office in McIntosh at 800-450-2465, or at the Clearwater County office on Wednesdays at 800-866-3125. If e-mail is your thing, contact me at