Flies at the window are a fairly common occurrence, but circumstances surrounding their visits are sometimes quite curious. A few years ago, I received a delightful collection of metallically colored flies that accumulated on a windowsill in the laundry room. I recognized these instantly as bottle flies which are often found breeding in a variety decaying things including the remains of dead animals. After assuring me that the body of a long lost relative or neighbor was not interred in a basement wall, we investigated other possibilities.
It happened that an exterminator was summoned to deal with small rodents in the home a few weeks prior to the discovery. Poisoned baits used in the exorcism likely resulted in small mousy bodies in a wall void thereby providing sumptuous meals for larvae of the carrion flies. As adults emerged and attempted to escape, they buzzed noisily around the basement window until expiring on the sill. When a friend recently called about similar haunting of flies, we discussed the possibility of exterminators, bodies in walls, and dismissed this as possibility. Another common winged visitor to homes in winter is the cluster fly. Cluster flies move indoors sometimes in great numbers in autumn through unscreened vents in soffits or other openings to the home. On warm winter days, cluster flies appear in living quarters as they attempt to escape to the wild. In spring cluster flies find their way outside and lay eggs in soil where their larvae develop inside the bodies of earthworms. After examining a neatly bagged specimen of my friend’s winged intruder, I was surprised to discover the unwanted fly was not a cluster fly, but was instead a member of the house fly family known as Muscina pascuorum.
My attempt to learn a bit more about this fly led to an interesting account written by Charles B. Johnson in 1923 who reported the appearance of said flies on windowsills during the autumn and winter in several homes in New England. One such collection from a home in Hamilton Massachusetts yielded “125 Muscina pascuorum, 93 Pollenia rudis, 7 Phormia regina, and 6 Muscina assimilis.” Apparently, larvae of Muscina pascuorum consume fungi commonly found in forests and following bumper-crop years for fungi, flies abound. Autumn rains and mild temperatures created wonderful conditions for fungi this fall and likely supported a goodly crop of fungus-loving flies. My friend also shared that a construction project at her home may have provided easy access through unprotected windows and eves just about the time that flies might seek shelter – the perfect storm for a home invasion. As with many other unwelcome guests like brown marmorated stink bug, boxelder bugs, and lady beetles, homes well-sealed, screened, and caulked, enjoy fewer winter visitors than ones with easy access to the outdoors. While renewed activity of home invaders is a nuisance during this season, take heart for these are the harbingers of good days to come. Spring is just around the corner.
We thank Liz for sharing her curious fungus-loving flies. The marvelous article by Charles B. Johnson served as a reference for this episode. To learn more about home-invading flies, please visit the following web site.