A very common, and frequently disturbing, household insect encounter of late winter is the emerging cluster flies. Almost invariably any large dark-colored flies found in a Montana home at this time of year are cluster flies (Pollenia pseudorudis) with incidence most common at higher elevations. The flies tend to collect around windows and during periods of activity may rebound zombie-like around the room during flights. Infestations are concentrated in upper stories, on south and west sides, with some of the largest numbers being found in church steeples, attics - and pricey penthouse real estate.
Although disconcerting to find flies in one's dwelling, the habits of the cluster fly are quite innocuous. They develop as parasites of earthworms during spring and summer and bring with them none of the associated health problems of filth-breeding species such as houseflies and flesh flies. Furthermore, the cluster flies found in homes are just transient visitors, neither breeding within the dwelling nor directly feeding and damaging household objects.
Problems with cluster flies begin in late August as they move to winter quarters. Because they seek a warmed site with protective cracks for shelter, buildings hold a particular attraction. When ready to go into winter dormancy they perch on the warmed south facing surfaces and many ultimately find their way to the upper stories. Once inside they remain quiet and may cluster together within wall voids. As winter progresses, and particularly following a warm day, increasing numbers awake from their somnolence and resume activity. Those that make a wrong turn end up in the living space of some non-appreciative human. Cluster fly problems can be partially prevented, but at this time of year little can be done. All of those currently found within the home had infiltrated walls months ago. A "mini-vac" is the best control of the individual flies that are currently present. Late August and early September are the times to limit visits by this insect. Thoroughly caulking all exterior cracks on the sun-exposed sides (south and west) is fundamental. Sprays of the insecticide permethrin (e.g., Astro, BugStop, Intercept) during late summer have also been shown effective, when combined with sealing of cracks. Pest control companies may also dust insecticides (typically silica aerogel and pyrethrins) into wall voids that can help control some of those insects that do make it into the building. (Whitney Cranshaw, Colorado State University)
Categories: Cluster Flies
Date: 11/26/2001