Besides lady beetles, another unwelcome winter guest is the cluster fly. These are the large, black, pesky flies that show up in bedrooms and on window sills from late fall through early spring. These flies are known as cluster flies, a name that describes their habit of clustering in large numbers inside attics, walls, and similar spaces in our structures.
Cluster flies do not reproduce indoors, and home owners 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 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. Casual observation of client reports suggests houses located on an exposed hill top or high ground are most attractive to these migrating flies.
As the flies warm throughout the winter, and especially in the early spring, they come out of their cold temperature dormancy and begin sluggishly moving around. Their random crawling brings them into the house by way of electrical outlets, window pulley holes, and small openings around windows, moldings and base boards.
Cluster flies are difficult to control in homes because they hibernate within inaccessible places. Because they hide inside walls or under insulation, they are usually not vulnerable to treatment until they appear within the living space of the house. Preventing attic flies is a job for the summer and fall. As much as possible, seal cracks and openings around the outside of the house, especially under the eaves, as you would for energy conservation.
Unfortunately, there is little that can be done for flies already inside the attic and walls. Space sprays and fogs into the attic have little if any affect, as the flies are usually under insulation or deep in cracks and crevices. They do not fly around much in attics. Therefore, fly paper, fly strips and bug zappers are of no value. Flies buzzing within a room can be dispatched with a fly swatter, hand vacuum, or shop-vac.
Anne Lenox Barlow has had experience in the agricultural field as a horticulture educator with Cornell Cooperative Extension in Clinton County. She can be reached by e-mail at firstname.lastname@example.org.