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What Are Cluster Flies & Other Over-Wintering Colorado Pests

Although a majority of residents are adapted to the habit of swatting flies during the summer, many are baffled at the appearance of flies in the autumn and winter months within Colorado homes.

Common house flies are not typically active during this time, which leaves residents wondering where the flies are coming from.  Typically, these flies, named for their habit of clustering are in fact Cluster flies.  Slightly larger than common house flies, Cluster flies have a distinguishing feature of golden-yellow hairs on their thorax which tend to give the appearance of a golden sheen.  The hairs are abundant on the lower side of the thorax, specifically in the area around the legs.

Cluster flies are often seen in autumn, congregating on the sunny exposures on the exterior of homes and buildings.  Soon after, as the weather begins to change, Cluster flies work their way into cracks and crevices from the exterior in order to locate a warmer place in which to over-winter.

While the good news is that Cluster flies do not reproduce inside of homes due to the lack of soil containing earthworms on which the young maggots feed, that does not stop cluster flies from appearing in large numbers sometimes seeming like waves of activity on warmer winter days when the flies attempt to emerge from wall voids and attic spaces where they are over-wintering to get back outside and inadvertently end up indoors.

These flies are commonly found clustering on sunny windows inside the home.  Fortunately, Cluster flies do not damage the home, aside from excrement that they occasionally leave on windows and walls, and they do not carry any known diseases or medical importance to humans.  These flies are typically slow in movement and flight, making them easy to swat.  Much like other over-wintering pests including box elder and conifer seed bugs, Cluster flies are considered a nuisance pest.

In the spring, Cluster flies fortunate enough to find their way out of the areas in which they have over-wintered will lay eggs on the soil, restarting the lifecycle.  The maggots feed on earthworms prior to pupating and emerging as adults.  Multiple generations of Cluster flies tend to occur each summer with the final crop of adults seeking shelter before the winter.

The key to Cluster flies is prevention.  Sealing the exterior gaps cracks and crevices on the interior and exterior of the home prior to autumn can prevent entry into the structure for over-wintering by adult Cluster flies.  High-quality silicone or silicone-latex caulk should be used where possible to seal entry areas.

Larger gaps should be reduced and properly sealed as well.  In areas where Cluster flies were a problem in the past, an exterior application of insecticides applied by a licensed pest control professional may provide additional relief from potential infestations.  This should be completed in autumn just prior to flies beginning to congregate and should only be considered as a portion of your prevention program.

Once Cluster flies have accessed attic or wall void spaces your pest control professional will likely advise against the use of treatment of the areas where the flies over-winter.  Although treatments may kill potentially thousands of Cluster flies within those spaces, secondary pest problems such as carpet stinging insects may occur as pests begin to feed on the dead insects.  Determining the location from which the flies are entering the home and sealing the interior gaps and cracks provides the most effective relief from these nuisance pests once they have entered the structure.