Cluster flies are the most common and annoying flies found within homes during the cool seasons. They can be particularly serious pests of office buildings, often concentrating in upper stories. Piles of dead cluster flies also are commonly encountered when returning to mountain vacation homes that are unoccupied during the months when the flies emerge. 

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 parasitoid upon the earthworm host, wintering at almost any point along the body wall. Developmental time (egg to adult) varies from 27-29 days. There are usually 4 generations per year.

Blow/Bottle flies are more than just a nuisance, they are of medical importance because of their mechanical transmission of disease organisms and ability to cause myiasis (infestation of tissues/cavities) in humans and animals. About 80 species occur throughout the United States and Canada. 

Blow/Bottle fly females lay up to 2,373 eggs on suitable larval food material. Upon hatching, the larvae may feed on the surface and the burrow into the food material which is less decayed. Most species develop in meat or animal carcasses but if these are not available they will use animal excrement, decaying vegetation, and/or garbage. 

Cluster fly (2)
Cluster Fly
Blow fly
Blow Fly


Cluster fly adults are about 3/8″long, robust. Color dark gray, non-metallic; thorax lacking distinct stripes but with numerous short golden hairs (may be lost in older specimens); abdomen with irregular lighter areas. Wings with 4th (3rd long) vein (M) sharply bent forward near tip towards and almost meeting 3rd vein at wing margin; wing tips overlap at rest. Sluggish movements. With buckwheat honey odor when crushed. 

Blow/Bottle fly adult about 1/8-5/8″ long. Color partly or wholly metallic blue, green, or dull brassy, sometimes black. Mouthparts sponging. Antenna with arista plumose (feathery) at least on basal 2/3’s. Thorax with postscutum (area below scutum) not developed. Wing with 4th (3rd long) vein (M) strongly angled forward, cell R5 narrowed but rarely closed distally (at wing margin).


Cluster fly control starts outside, reducing the outside populations in impractical for cluster flies since their larvae breed in earthworms. Although total exclusion is probably not possible, all vents (roof, overhang, weepholes, ect) should be screened with at least 16-mesh screening. Caulk around cable entrances, windows, doors, and overhangs. Temporary but immediate indoor relief can be achieved by removal with a vacuum.

Blow/Bottle fly starts with identification. Sanitation involves removal or elimination of the larval developmental sites. This may involve the timely emptying and cleaning of garbage receptacles to render breeding materials unsuitable by drying them out. Sanitation should eliminate the bulk of the fly problem so that mechanical measures will be more effective. Mechanical controls consists of garbage receptacles with tight-fitting closures, tight-fitting windows and doors, windows securely screened if they can be opened, doors with self-closures, all holes through exterior walls for utilities, ect., sealed, all vents securely screened, ect. and the use of air curtains, insect light traps, sticky-surfaced traps, ect. Insect light traps (ILTs) are particularly effective in reducing the number of flies indoors. 



Identification of the pest is crucial for the correct prescription treatment and will reduce the unnecessary use of pesticides.

Set Action Thresholds

An Action Threshold is the pest population level at which the pest's presences is a :Nuisance, Health Hazard; or Economic Threat

Pest Prevention

As a first line of pest control, habitat modification is one very effective and cost-efficient strategy that presents little of no risk to people or the environment.

Pest Control

Control methods will be determined on the pest management evaluation and prevention.
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