Cluster Fly

Overview

Cluster flies, Pollenia rudis (Fabricius 1794), seek protected places, including homes in fall, to survive the winter (Robineau-Desvoidy, 1863). They usually gain access to interiors via any cracks or holes in the exterior walls or under eaves of the buildings and become a nuisance (Pimentel and Epstein, 1960). The presence of cluster flies inside a structure can create health and liability concerns, especially in sensitive locations, such as hospitals, food processing and handling facilities, etc. Dead bodies of cluster flies inside wall voids, cracks and crevices may trigger allergic reactions in sensitive individuals; in addition they would attract unwelcome pests that feed on them, such as hide beetles, larder beetles, carpet beetles, carpenter ants, etc.

Identification

Cluster flies are dark gray and about 1/4 to 3/8 inch in length. They have golden yellow hairs on the thorax and irregular light and dark gray areas on the abdomen. The wings overlap when the fly rests.

Life Cycle

Female cluster flies lay eggs in cracks, crevices and openings in the soil near the holes of earthworms. Eggs hatch in three days. Newly hatched larvae penetrate earthworm bodies to feed. They go through three larval instars in two to three weeks after which they leave the earthworm to pupate in the soil for four to six weeks (Keilin, 1911). After that, the flies emerge and feed on pollens and repeat their life cycles. There are three to four generations of cluster flies per year.

Habits

In the late summer and early fall, between late August to early September (depending on the temperature and region), the adults search for over-wintering sites. They usually gather on sunlit surfaces on roofs of buildings, walls, weather stripping on windows, etc. When it gets cool, they hide in cracks, crevices, and voids. Once indoors, they can be found under clothing in closets, under curtains, behind pictures and furniture, etc. They also may leave their hiding places and appear on sunny warm days in winters.

Management

Prevention

• During the summer and before late fall or early winter, people whofrequently encounter cluster files should seal or caulk all cracks, crevices, voids and all possible entry ways of the structure. They also should screen windows and doors, ventilator and louvers.
• Vacuum cluster flies as they appear in building and do not crush, as this will leave greasy marks on light surfaces.

• In sensitive locations, insect light traps can be placed close to potential over-wintering sites indoors to help in monitoring and reducing the cluster fly population.

• A licensed pest management professional (PMP) should apply a preventative exterior treatment to provide quick knockdown for cluster flies before they enter the structure, and to leave behind long-term residual effects up to 30 days or more. The PMP will apply crack, crevice and spot treatments to adequately cover the common cluster fly entry points and avoid dripping or runoff in accordance with the pesticide label and EPA directions. Close attention should be paid to the sunny sides of the structure.
• Due to an extended summer or heavy rains, the initial liquid treatment may become ineffective; accordingly, another treatment may be needed/justified in late fall to maintain protection.

• A treatment to cracks, crevices and voids along with space applications in infested attics or vacant roof spaces is RECOMMENDED in winter and spring when flies become active and are emerging from their hiding areas in a big number.

Frequently Asked Questions:

Q: I can’t wait until summer… What can be done right now?

A: Temporary but immediate relief can be obtained by using a vacuum cleaner and space sprays in individual rooms. To prevent reentry seal all possible entry points, use caulk, foam sealant or even tape around window frames and pulleys, doorframes, baseboards etc. Remove outlet covers, switch plates seal then replace. Light fixtures, ceiling fans etc. can be removed to the base plate, seal the wall and replace the fixtures.

In suspended ceilings you can use No Pest resin strips, sticky traps or fly paper. A 60-watt light can be placed in the suspended ceiling. The flies are attracted to it, which causes them to exhaust their energy and die right around the light (vacuum them up!).