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How to identify it
- Shiny metallic green or blue bodies, often with a copper tint, set them apart from house flies; they are also called blue or green bottle flies.[1][2][3]
- Adults are about 1/4 to 1/2 inch long and robust, and Kentucky calls their buzzing flight very annoying.[2][3]
- Larvae are maggots that look and behave much like house fly maggots.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| House fly | Under 3/8 inch with four dark stripes down the back of the thorax, and not metallic green or blue.[1] |
| Cluster fly | Cluster flies gather in large groups in attics and wall voids to overwinter and have short golden hairs on the thorax; blow flies have no distinct clustering behavior, so only a few turn up indoors in winter.[2][3] |
Where it lives
Blow flies are common around homes, garbage and pet waste; in cities the most common breeding sites are human food waste and pet feces, and Minnesota says they are sometimes more abundant than house flies in urban areas.[1][2]
Biology and life cycle
Females lay eggs on garbage with meat scraps, dead animals, animal wounds and feces-caked hair or wool on pets and livestock; in cities, UC IPM says human food waste is the most common breeding site.[2][1]
In warm weather maggots can reach full size by the second or third day after hatching. Mature maggots leave the food to find a drier spot to pupate, which is why they crawl out of trash cans.[2][1]
A dead animal in a wall or ceiling can produce adult flies several days later, and maggots may drop through gaps to the floor. It ends when the food is gone, which may take up to a week for a small animal like a squirrel.[2]
Some adults enter homes in fall to overwinter and become active on warm winter days without reproducing indoors; Kentucky notes they are drawn to lamps and lights.[2][3]
Damage and signs
- They're strongly attracted to food and garbage, which makes cooking outdoors difficult, and they can carry disease agents from garbage or animal feces onto food.[1]
- Females also lay eggs in animal wounds and on feces-caked hair or wool of pets and other domestic animals, so wounded or soiled animals draw them.[2]
Health risk
Blow flies have sponging mouthparts and don't bite, but they may carry disease agents from garbage or feces to food. Their maggots can also infest people (myiasis). In a U.S. study of 42 cases from 20 urban and suburban medical centers, most infestations were in existing wounds, no tissue invasion was recorded, and the most common species was the green blow fly; homelessness, alcoholism and peripheral vascular disease were frequent cofactors, and 5 percent of cases began in the hospital. The CDC says myiasis is uncommon in the United States, that untreated or open wounds raise the risk, and that a health care provider should remove the larvae; keep wounds covered and clean them daily.[1][5][4]
Control
Keep them out
Seal openings that let flies into walls and attics; Kentucky warns that clustering flies are hard to control once they are inside voids.[3][2]
Sanitation
Seal household garbage and pet waste in plastic bags before putting them in outdoor cans, set cans out at least weekly even if not full, and wash cans with soap and water.[1][2]
Remove dead birds and animals, manure, rotting mulch and lawn clippings from the yard.[2]
Traps and physical control
Use odor-baited traps placed well away from the house; vertical sticky fly ribbons don't work because blow flies rarely rest on vertical surfaces.[1]
Swat or vacuum flies that get inside.[2][3]
Pesticides
House fly baits are designed for house flies and won't control blow flies well, though placed on the ground around trash cans they may help some.[1]
Where sources disagree
How fast they develop: UC IPM says egg to adult can take as little as 7 days at ideal temperatures; Minnesota says the full life cycle usually takes 10 to 25 days.[1][2]
Where they breed: Kentucky says manure or carrion; UC IPM says human food waste is the most common source in urban areas, with pet waste also producing them.[3][1]
Indoors in winter: Kentucky says blow flies can often be found in homes during winter and early spring; Minnesota says they overwinter in buildings less often than cluster and face flies, so few are found indoors then.[3][2]
Common questions
- Why are there suddenly lots of shiny flies in one room?
- Often an animal or bird has died hidden in a chimney, wall void or ceiling. The flies stop once the carcass is consumed, which may take about a week for a small animal.[2]
- Can blow fly maggots get into a wound?
- Yes, though it is uncommon in the United States. Most U.S. cases involve neglected existing wounds; keep wounds clean and covered, and see a provider to have any larvae removed.[5][4]
Sources
- University of California Statewide IPM Program, “Flies (Pest Notes, Home and Landscape).” ipm.ucanr.edu/home-and-landscape/flies/ (accessed 2026-11-11)
- University of Minnesota Extension, “Flies (Household Insects).” extension.umn.edu/garden-and-home/home-maintenance/household-insects/flies (accessed 2026-11-11)
- University of Kentucky Entomology, “Cluster Flies, Face Flies, and Blow Flies in Homes (ENTFACT-624).” entomology.mgcafe.uky.edu/ef624 (accessed 2026-11-11)
- Centers for Disease Control and Prevention, “About Myiasis.” www.cdc.gov/myiasis/about/index.html (accessed 2026-11-11)
- Archives of Internal Medicine, via PubMed, “Wound myiasis in urban and suburban United States (Sherman 2000).” pubmed.ncbi.nlm.nih.gov/10888974/ (accessed 2026-11-11)
- iNaturalist, “Lucilia sericata taxon page (order and family).” www.inaturalist.org/taxa/1632739 (accessed 2026-11-11)