[Fly Control Issue] Overwintering Flies

Editor’s note: The following was excerpted from the upcoming 10th edition of the Mallis Handbook of Pest Control. Stay tuned to the pages of PCT for details about the date the book will be available, pricing information and more.

Cluster flies (Pollenia spp.) belong to the genus Pollenia which is in the blowfly family Calliphoridae. There are six species of cluster flies, Pollenia spp., in the United States, three of which, P. angustigena, P. pediculata, and P. rudis, are known to have some or significant pest status in invading buildings. The following is dedicated primarily to P. rudis.

Unlike typical blow flies, cluster flies are not believed to be a health hazard because they are not attracted to human foods. However, a study conducted as a result of a mass infestation of a hospital in Germany, drew the following conclusion: “The results obtained led to the assumption that mass infestations of cluster flies occurring in sensitive areas, especially in hospitals, may cause a low, but not neglectable health threat due to mechanical transmission of bacterial pathogens.”

The cluster fly is widely distributed throughout Europe, Canada and the United States, except for the states bordering the Gulf of Mexico. They enter houses in the fall to hibernate and often aggregate in secluded and sometimes inaccessible places, such as wall cavities, attics and false ceilings. In the spring, cluster flies become active and in attempting to leave their hibernation site, they commonly end up inside the living space of structures where they become a nuisance. In early spring, they are sluggish, but do not fly about noisily in buildings like other house-invading Calliphoridae.

P. rudis is slightly larger than the house fly, being about 8 millimeters long and has a nonmetallic dark gray color. The dorsum of the thorax has three diffuse longitudinal black bars that are not conspicuous. On newly emerged specimens, the dorsum of the thorax is covered with many golden hairs. These hairs are often rubbed off in older specimens. The dark gray abdomen has irregular lighter patches thus presenting a checkered appearance. The wings overlap when at rest. When crushed, this fly gives off an odor like buckwheat honey.

These flies normally live outdoors where they frequent flowers and ripe fruits. With the approach of cool weather, they enter buildings to overwinter. Here they hide, often in clusters, in nooks and dark corners, under clothing in closets, beneath curtains, in angles of the walls, behind pictures and furniture, in hats, under the edges of closets, etc. In the spring the flies often become a nuisance in houses. The increasing warmth of the spring days induces activity and if the hibernating quarters have been in the walls of a house or around the window frames, the flies may emerge inside the house instead of on the outside. Just as they become a nuisance in the fall while seeking hibernating quarters, they are also bothersome in the spring as they try to escape. Once inside the house, the flies crawl sluggishly over the walls and often fall into food on the table. They often are rolled up under window blinds and leave unsightly greasy spots where they are crushed.

CLUSTER FLy LIFE HISTORY. The larvae of cluster flies are parasitic in the earthworm, Allolobophora chlorotica and A. rosea. The eggs are laid singly in cracks in the soil. During the summer, the eggs hatch in three days and the emerging maggots can penetrate practically any part of an earthworm. The larval stage may last from 13 to 22 days, and the pupal stage from 11 to 14 days, with a total development period of 27 to 39 days. Four generations occur during the summer. The pupal period is seven days at 80ºF (26.7ºC) and 100 percent relative humidity.

THE FACE FLY. The face fly, Musca autumnalis, was introduced from the Palearctic region and it quickly spread across North America in the 20 years after World War II. Adults feed on cattle and horses and the larvae develop in fresh cattle dung.

The face fly is nearly identical in appearance to the house fly. Like the house fly, it has four distinct dark stripes on the top of its prothorax. The face fly can be distinguished from the house fly by examining the calypteres, which are the flat appendages that cover the halteres found on the side of the thorax just behind the wings. The calypteres on a face fly have a tuft of bristles or “hairs” where they attach to the thorax. The calypteres of a house fly are bare with no tuft of bristles.

The face fly hibernates in buildings in the fall and causes annoyance similar to that of the cluster fly. It is widely distributed throughout southern Canada and in the United States north of the 33rd parallel, where it is especially annoying on the faces of cattle as it laps exudations from the eyes, nostrils and mouth. There is a correlation between face abundance and the occurrence of pinkeye in cattle. Adult face flies feed on animal secretions, nectar and moist manure. Adult face flies normally only lap exudations from bovine eyes, nostrils and mouth.

FACE FLY LIFE HISTORY. In the spring, the face fly deposits its eggs in fresh manure. The three larval instars require three to 10 days for completion depending on temperature. The puparium is dirty white in color. Pupation takes place near the edge of the manure and the egg-to-adult life cycle and may require about two weeks. The life cycle outdoors is 17 to 18 days at 77°F to 86°F (25°C to 30°C).

The eggs of the face fly are easily separated from those of the house fly by the presence of a respiratory stalk on the egg of the face fly. These eggs are deposited just below the surface of manure and may be readily uncovered by lightly abrading the surface of the manure. If disturbed a little later, the yellowish, newly hatched larvae may be seen trying to re-enter the dung. In contrast, the larvae of Musca domestica are creamy white and tend to penetrate throughout most of the droppings. The puparium of the face fly is off-white in color in contrast with the reddish-brown color of house fly puparium.

The habits and behavior of the face fly and the house fly are different. Face flies are generally found on cattle farms because they develop only in fresh cow manure. Structures that become infested with face flies are likely to be those on or near farms or such other areas where cattle might be present in large numbers.

Face flies usually cause few problems in structures until the late summer and fall. Like cluster flies, face flies begin to seek protected overwintering sites in late August and September. As daytime and evening temperatures begin to drop, the adult flies will be attracted to the warm walls of buildings. The walls on the south and west sides of buildings are the ones usually affected because the late summer sun shines most on these sides. Once they are on the building, the flies crawl into cracks in the exterior — crawling as far back as they can go. Large numbers of these flies will huddle inside wall voids, attics and false ceilings. They may even be found harboring in the same voids used by cluster flies. House flies do not engage in this type of overwintering behavior.

CONTROL. Unfortunately, once cluster flies or face flies have entered a building to overwinter, they can be very difficult to locate and control. Any void in which these flies are found can be treated using an appropriate residual insecticide. Insecticide dusts are ideal for wall voids and other enclosed cavities. Because cluster flies breed in earthworms and face flies in manure, no practical means exists for directing control measures against the breeding source. A vacuum device with HEPA filter can be used to capture adult flies as they appear inside the living spaces of buildings or to remove clusters of flies found in attics, crawlspaces or false ceilings. Electric outlet plates may need to be removed and the wall voids behind them treated with a residual dust insecticide. Occasionally, wall voids may need to be drilled in critical areas and dusted. Some suggest using boric acid or Drione (silica gel) dust to treat wall voids.

The best method for cluster and face fly control is to exclude them from entering structures. Caulking, sealing, patching and screening exterior cracks, crevices, holes and other points of entry will greatly reduce the number of flies gaining access into wall cavities.

Unfortunately, some buildings cannot be completely fly-proofed and insecticide applications might be necessary. Exclusion efforts need to be instituted in the summer before flies begin seeking overwintering sites. Sealing entry points during the fall/winter is not recommended as it may force the flies already inside to enter the living spaces.

Some recommend that wettable powder or microencapsulated insecticide formulations be used for treatment. Timing of these applications, however, is important. Treatments should be applied just before adult flies begin seeking overwintering sites.

In some cases, many of the overwintering sites cannot be located. The best approach to deal with flies as they “awaken” in false ceilings or attics is to install insect light traps in these dark locations. The light traps attract and capture most of the flies before they can enter the living spaces of the building and may reduce fly populations to the point where insecticide treatments may not be necessary. ILTs should be checked frequently to see if flies are being captured and how many. The glueboards in ILTs can fill up quickly when large populations of flies are present.

The author is an urban and structural entomologist and consultant based in Southern California. He can be reached at hgulmahamad@giemedia.com.