Common Name: Cluster fly
Scientific Name: Pollenia rudis
The common name reflects this species habit of forming compact clusters of hibernating individuals, typically in wall voids or attics. It is widely distributed throughout the United States and common in Missouri. Specifically, cluster flies occur wherever their host earthworm, Allolbophora rosea occurs, which is usually in a well-drained, silt-loam soil with grass cover.
Adult cluster flies are 3/8 inch long, robust and colored dark gray with numerous short golden hairs on the thorax and with irregular lighter areas on the abdomen. The wing tips overlap at rest. Overwintering cluster flies are typified by sluggish movements indoors.
(1) The housefly (Musca domestica) and closely related flies have 4 distinct stripes on top of the thorax and lack the golden hairs. (2) Blow flies/bottle flies (family Calliphoridae) have the abdomen or entire body metallic colored. (3) Other flies either have stripes on top of the thorax or lack the golden hairs on the thorax.
Adult cluster flies overwinter in sheltered places, emerging in the spring to mate. Eggs are laid in soil cracks and hatch in about 3 days. The larvae are parasites of their earthworm hosts, entering at almost any point along the body wall. Development time (egg to adult) varies from 27 to 39 days. There are usually 4 generations per year.
As days shorten and the weather cools (in September and early October), cluster flies often enter structures to overwinter, sometimes traveling more than a mile to do so. They usually occupy attics and wall voids of the sun-warmed south, east and west exposures of houses and buildings. Typically they use the same structure year after year. They do not breed within structures.
They can be a problem or nuisance in the autumn, winter and spring: They enter during autumn to hibernate, temporarily break hibernation on warm, sunny winter days and finally in the spring when they attempt to leave the structure. They can be stimulated by warmth to resume activity almost anytime. Sometimes it takes no more than the furnace to be turned on and thoroughly warm the inside of the structure to start activity. However, winter activity usually requires a bright sunny day to warm the walls from the outside. Once stimulated, cluster flies seek light. This is why they usually come out around loose fitting wall switches and outlets, ceiling fixtures, skylight casings, window and doorframes, window pulleys and ventilation duct penetrations in floors and walls.
Cluster flies can usually be found at windows crawling on the panes or frames, or around lamps or lights. They are sluggish in their movements in comparison to houseflies. They give off a buckwheat honey-like odor and leave a greasy spot when crushed.
Rottler technicians prefer to apply treatments for cluster flies on strategic exterior surfaces of buildings in early fall. Preventative chemical barriers involve applying a highly repellent, long-lasting residual liquid insecticide to structural junctures on all outside vertical walls and the adjacent overhang. This application is made just before the last seasonal generation of adults emerges. If adults have already begun to congregate and attempt entry into buildings, it may be too late for preventative action. Once buildings are entered, the best solution is physical removal with a good shop-type vacuum cleaner.
Rottler technicians prefer not to inject insecticides into the overwintering sites of cluster flies in buildings because the bodies of dead flies in wall voids attract dermestid beetles (e.g., larder beetles, carpet beetles and cabinet beetles). Dermestid larvae wander and will readily enter the living space, causing numerous complaints. However, temporary relief is possible by using a vacuum cleaner and sealing interior entrances, where practical and accessible.
Cultural Control And Preventative Measures
Control begins outside. Reducing the outside population is impractical for cluster flies since their larvae breed in earthworms. Therefore, the use of preventative physical barriers involves exclusion. Although total exclusion is probably not possible, all vents (roof, gable, soffit/eave) should be screened with at least 16-mesh screening. Weep holes should be stuffed with copper gauze or steel wool. Caulk should be applied around cable entrances, windows, doors and overhangs. These steps should be taken before late August.
Temporary but immediate indoor relief from cluster flies can be achieved by removal with a vacuum cleaner fitted with a hose attachment. To prevent entry of more flies, it is important to seal the possible routes of entry using silicone caulk and similar materials. Entry points include around window pulleys, window frames, doorframes and baseboards. For electrical outlets, switch boxes, heating duct and air return vents, it is recommended to remove the cover plates, seal and replace. For light fixtures and ceiling fans, one should remove the fixture to its base plate, seal and replace.
If cluster flies are in a false ceiling area, the population can be reduced by installing an insect light trap (ILT); if used, it is important to periodically empty the catch tray or replace the sticky panels. An alternative is to install a continuously burning 60-watt fluorescent bulb, which attracts the flies, causes them to exhaust their food (fat) and die right around the light. The dead flies ought to be removed with a vacuum cleaner. To speed the process, adhesive fly strips, glue boards, or other sticky traps can be suspended from the uppermost surface of the false ceiling void near the light. Sticky, fly-capturing devices should be replaced when filled or covered with flies and other insects. To reduce the number of flies coming into a room from a false ceiling, all cracks through which light enters should be sealed using duct tape or caulk. In elevator shafts, a continuously burning 60-watt bulb can be installed just above the pit floor. Again dead flies should be removed periodically using a vacuum cleaner.