Cluster flies in the home: how to identify and control these pests
Many of you are struggling with a common pest problem—flies in the home. Flies living in and around the home transmit various human diseases, such as shigellosis, traveler’s diarrhea, cholera, typhoid fever, and eye infection. Flies do not sting, but some blood-feeding flies can give a painful bite.
All flies have four developmental stages: egg, larva, pupa, and adult. The female fly lays eggs in breeding areas where the larvae (maggots) complete their development. Full-grown maggots stop feeding and usually wander from the breeding site in search of a place to pupate. They convert their last larval skin into a hardened tubular shell (puparium) within which the pupa develops. The pupa transforms into an adult fly, which pops off the end of the puparium and emerges.
Many fly species invade our homes. Knowing the correct species is crucial for an effective control programme, as flies differ in their behaviour and breeding habitat.
Cluster flies are, by far, the most common fly species found in homes during the cool season. They can cause serious nuisance, particularly in taller buildings where they tend to congregate on sunny walls during warm periods and later enter the home to seek winter shelter. During the cool season some flies may inadvertently become active and fly lazily within the living spaces.
Cluster flies are dull-gray with black markings and have golden-yellow hairs on the thorax. Unlike house flies, their wings overlap when they are at rest. Cluster flies are not interested in food, do not develop in garbage, and are not a sign of unsanitary conditions.
Cluster flies do not feed nor reproduce within buildings. They develop as parasites of earthworms and are particularly active in the spring just after earthworms and robins appear. In spring and summer the adult flies lay eggs in soil and the maggots move to and develop within earthworms.
How to control cluster flies
- Seal cracks and crevices around homes, particularly upper stories of south and west sides prior to periods when flies enter in late August and September. Exterior treatment of house walls with a registered insecticide can further limit entrance.
- Locate the area where the flies are hibernating in the home and then treat them directly. Begin searching for resting sites on the southern and western sides of the building, and vacuum the flies off the surfaces on which they are hibernating. In areas inaccessible to vacuuming, a residual insecticide can be applied.
- Devices to trap the flies are also offered on the market. These traps, attached to windowpane where flies are a problem, contain a non-toxic powder that works like quicksand in trapping the insects that fall in.
- Swatting cluster flies is not recommended as it may attract more flies. Also, it is not advisable to control cluster flies by attempting to control the earthworm populations.
- For current chemical control options, consult your local pest control representative. Follow closely the directions and cautions on the label.
Rana Sarfraz is an entomologist and ecologist currently working at the University of British Columbia.