Cluster Flies

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Cluster flies are named for the way they tend to hibernate or “cluster” together in groups while overwintering in wall voids or attics during the colder months.

HOW DO I KNOW IF I HAVE CLUSTER FLIES?

  • From late August to early September (depending on the temperature and region), adult cluster flies search for overwintering sites; usually gathering on sunlit surfaces on roofs of buildings, walls, weather stripping, and windows.

  • When it gets cool, cluster flies hide in cracks, crevices, and voids.

  • Once indoors, cluster flies can be found under clothing in closets, under curtains, behind pictures and furniture, etc.

  • Cluster flies also may leave their hiding places and appear on sunny warm days during winter buzzing and "clustering" at windows.

WHAT DO CLUSTER FLIES LOOK LIKE?

  • Cluster flies (Pollenia rudis) are slightly larger than the common house fly, about ¼” to 3/8” in length.

  • Cluster flies are generally dark gray with golden yellow hairs on the thorax and irregular light and dark gray areas on the abdomen.

  • The wings of the adult cluster fly overlap when the fly rests.

  • Cluster fly larvae (or maggots) have long, wedge-shaped bodies that are cream colored.

  • Cluster fly eggs are rarely seen as they are deposited in the soil.

ARE CLUSTER FLIES DANGEROUS?

  • No. Cluster flies do not bite humans or animals and are not known to carry any diseases.

  • However, 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.

  • Dead cluster flies inside wall voids, cracks, and crevices may trigger allergic reactions in sensitive individuals.

  • In addition, dead cluster flies can attract unwelcome pests that feed on them, such as hide beetles, larder beetles, carpet beetles, and carpenter ants.

CAN MY HOUSE BE DAMAGED BY CLUSTER FLIES?

  • Generally, cluster flies will not damage your home.

  • Cluster flies do not breed or feed inside your home.

  • However, cluster flies can leave excrement on windows and walls.

WHY ARE CLUSTER FLIES IN MY HOUSE AND YARD?

  • In the fall, cluster flies begin looking for protected places to survive the winter.

  • They congregate, often in great numbers, on the sunny sides of homes.

  • Cluster flies usually gain access to interiors through cracks or holes in the exterior walls, or under the eaves of the buildings.

  • Cluster flies may be found throughout the winter flying about, often near windows and lights.

  • The cluster flies you see during the winter have become active during warm days and have crawled out of their over-wintering nests in the walls or attic.

WHAT DO CLUSTER FLIES EAT?

  • Cluster fly larva (maggots) burrow into the soil and feed on earthworms.

  • Adult cluster flies feed on flower nectar and plant sap, fruit, flowers, and other types of organic matter.

WHAT IS THE LIFECYCLE OF CLUSTER FLIES?

  • Female cluster flies lay eggs in cracks, crevices, and openings in the soil.

  • Eggs hatch in three days.

  • Newly hatched larvae penetrate the body cavities of earthworms to feed.

  • Cluster flies 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.

  • After that, the flies emerge and repeat their life cycles.

  • The Cluster flies’ development time from egg to adult is about 45 to 66 days.

  • In our area, there are three to four generations of cluster flies per year.

  • In August and September, the last generation of flies seeks shelter in homes and buildings to over-winter.

ITS WINTER AND I CAN’T WAIT UNTIL SUMMER… WHAT CAN BE DONE RIGHT NOW TO GET RID OF CLUSTER FLIES?

  • Temporary -- but immediate -- relief from cluster flies can be obtained by using a vacuum cleaner and space sprays in individual rooms.

  • To reduce the number of overwintering flies, install insect light traps (ILTs) in attics or other potential overwintering sites indoors. Remember to check traps periodically and remove dead flies; otherwise they will attract unwanted bugs that feed on them.

  • Vacuum up any dead cluster flies to prevent attracting other insects that will feed on the dead cluster flies.

3 INTERESTING FACTS ABOUT CLUSTER FLIES:

  1. Cluster flies are related to blow flies, such as the blue bottle flies, green bottle flies, etc.

  2. Unlike their relatives, the larvae of cluster flies do not feed on carcasses of dead animals, garbage or manure, and therefore do not contaminate food and are not a health risk

  3. Cluster flies emit an odor like buckwheat honey when crushed.