This time of year is delightful for many reasons, but the number of nuisance flies in and around the home can be annoying.
Several species of flies enter homes in Colorado and most are mere nuisance problems. Cluster flies and some other blow flies or face flies are found during fall and winter and they use homes for shelter from the cold, but they do not reproduce during this time.
Cluster flies are, by far, the most common fly found in homes during the cool months. They can sometimes be serious nuisance problems, particularly in taller buildings where they tend to concentrate on upper stories on the south and west sides.
Cluster flies are moderate-sized, generally dark gray and are distinguishable by the presence of golden hairs on areas of the thorax.
Cluster flies are actually a type of blow fly, but have very different habits. They develop as parasites of earthworms and are not associated with garbage, animal wastes or other materials that provide breeding of the various “filth flies.” In spring and summer, the adult flies lay eggs in soil and the maggots move to and develop within earthworms.
Flies that are present in late summer seek winter shelter and they survive winters as a semi-dormant adult fly. They may be seen sunning themselves on sun-exposed sides of buildings during warm periods and later infiltrate cracks in the building.
In the process of seeking sheltering sites within the building, they tend to migrate upwards and, thus, are found most abundantly in upper floors of buildings. During the cool season some flies may inadvertently become active and fly lazily within the living spaces. However, cluster flies do not feed or reproduce within buildings.