Bot Flies Identification Guide
Oestridae (Family)
Bot flies are parasitic flies whose larvae develop inside the bodies of mammals. Unlike most household flies, bot flies do not breed in garbage or filth. Instead, their larvae burrow under the skin of hosts such as rodents, rabbits, livestock, and occasionally pets.
Taxonomy
Bot Flies Coloration
Common color patterns to help identify bot flies
Bot Flies
Seasonal Activity
When bot flies are most active throughout the year
Where Bot Flies Are Found
Hover over states to see their names. Green regions indicate where bot flies have been reported.
Bot Fly Identification Guide
Physical Characteristics
Adult bot flies are thick, heavy-bodied flies that look more like bumblebees than common house flies. They measure 12 to 20 millimeters long. That makes them much bigger than most flies you see around your home. Dense hair covers their bodies, giving them a fuzzy look. They can be dark brown, black, or gray. Many have bands of yellow or light brown hair on their midsection.
One key feature tells bot flies apart from the bees they look like. Bot flies have only one pair of wings. Bees have two pairs. Their antennae are also short and stubby. Many adult bot flies have weak or useless mouthparts. They do not bite, sting, or feed during their short adult lives. Their only job as adults is to mate and lay eggs.
Bot fly larvae look nothing like the adults. They are cream-colored, barrel-shaped maggots with rows of small spines on their bodies. As they grow inside a host, they turn reddish-brown or dark brown and can reach about 25 millimeters long.
Common Species in North America
Several bot fly genera are found across North America. Each has a different preferred host and life strategy:
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Cuterebra spp. (Rodent and Rabbit Bot Flies): The most common group in North America. These flies lay eggs near the burrows of mice, chipmunks, squirrels, and rabbits. Pets that explore these areas can also become hosts. Adults are large, dark flies with banded abdomens.
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Gasterophilus spp. (Horse Bot Flies): Found wherever horses are kept. Adult females glue tiny yellow eggs to horse hair on the legs, shoulders, and muzzle. When the horse licks or rubs the area, the larvae enter the mouth and latch onto the stomach lining.
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Hypoderma spp. (Cattle Grubs or Warble Flies): Once common in cattle-raising areas. Females lay eggs on cattle hair. Larvae burrow through the skin and travel inside the body before forming a raised lump called a warble under the hide on the back. Control programs have cut their numbers in many regions.
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Dermatobia hominis (Human Bot Fly): Found in Mexico, Central America, and South America. This species does not live in the United States or Canada, but travelers to tropical areas may come home with one. It uses a unique method: it attaches its eggs to mosquitoes or other biting insects that carry them to human hosts.
Bot Fly Behavior and Biology
Bot fly larvae are parasites. They must grow inside a living host to survive. Adults live on their own but do not last long. Most live only a few days to two weeks.
Life Cycle
All bot flies go through four life stages, but the details vary by species:
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Egg Stage: Females place eggs where hosts are likely to find them. Cuterebra lays eggs near burrow openings. Gasterophilus glues eggs to horse hair. Dermatobia sticks eggs to biting insects. The eggs hatch when they sense warmth or touch from a host.
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Larval Stage: After hatching, tiny larvae enter the host through the skin, mouth, or other openings. The larva digs a small pocket under the skin called a warble. It keeps a tiny breathing hole at the surface. The larva feeds on tissue fluids and grows through three stages over several weeks.
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Pupal Stage: When fully grown, the larva crawls out through the breathing hole and drops to the ground. It burrows into the soil to pupate. This stage lasts several weeks to months depending on temperature. Some species spend the winter as pupae.
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Adult Stage: Adults come out of the soil, mate, and start looking for places to lay eggs. Most adults do not feed at all.
Host Interactions
Different bot fly species go after different hosts. Cuterebra species mainly target wild rodents and rabbits. But cats and dogs can pick up larvae when they sniff around burrows or brush against plants where eggs sit. In pets, the larva usually ends up under the skin of the head, neck, or chest.
Horse bot flies follow a cycle tied to horse behavior. The tiny eggs on horse hair hatch when the horse licks or rubs the spot. This lets larvae enter the mouth and attach to the stomach lining. A large number of larvae can upset the stomach and weaken the horse over time.
Seasonal Activity
In most of North America, bot fly activity follows a clear pattern. Adults show up in late spring and early summer. Activity peaks from June through August. Egg laying takes place during these warm months. You are most likely to notice larval lumps on pets and wildlife in late summer and early fall after the larvae have had time to grow. By November, adult activity stops. The cycle pauses until the next spring.
In tropical areas where Dermatobia hominis lives, bot flies can be active all year. Numbers tend to rise during rainy seasons when mosquito populations grow.
Treatment Methods for Bot Fly Infestations
Bot flies are not a typical household pest. They do not infest homes, contaminate food, or breed in garbage. Treatment focuses on addressing larval infestations in animals or, rarely, humans.
Larval Removal in Pets
If you notice a swelling or lump with a small hole on your pet, especially in late summer or fall, it may be a bot fly larva. Here is the recommended approach:
- Do not squeeze the lump. Crushing the larva while it is still inside the skin can cause a serious allergic reaction or infection.
- Cover the breathing hole with petroleum jelly or a similar substance to cut off the larva’s air supply. This may cause it to move toward the surface.
- See a veterinarian. A vet can extract the larva under local anesthesia and treat any infection that may follow. This is the recommended approach. The Companion Animal Parasite Council (CAPC) provides clinical guidelines for veterinary treatment of Cuterebra infestations.
Larval Removal in Humans
Human cases in North America are uncommon. They almost always happen after travel to Central or South America. Doctors typically use one of these methods:
- Covering the breathing hole with petroleum jelly, nail polish, or tape to cut off the larva’s air. Once it moves toward the surface, a doctor can pull it out with forceps.
- Minor surgery when the larva sits too deep or does not respond to the covering method.
- Follow-up care with antibiotics only if a bacterial infection develops at the site.
The CDC’s DPDx resource on myiasis provides guidance for healthcare providers.
Note: This information is for general awareness only and is not medical advice. Always consult a doctor or veterinarian for diagnosis and treatment.
Prevention
Since bot flies are not drawn to homes or food sources, prevention focuses on reducing contact with larvae:
- Supervise pets outdoors, especially near woodlands, fields, and areas with rodent burrows during summer months.
- Check pets regularly for lumps or swellings after time spent outdoors, particularly around the head, neck, and chest.
- For horse owners, scrape bot fly eggs off horse hair often using a bot knife or grooming tool. Talk with your vet about a deworming plan that targets Gasterophilus larvae at the right time of year.
- For travelers heading to Central and South America, use insect repellent, wear long sleeves and pants, and sleep under bed nets. This cuts down on mosquito bites that can carry Dermatobia eggs.
Health Concerns
Bot flies do not spread diseases like mosquitoes or ticks do. The main health concern is the tissue damage and pain caused by larvae growing under the skin. In pets and livestock, problems from bot fly infestation can include:
- Secondary bacterial infection at the larval site
- Allergic reactions if a larva is crushed during removal
- Weight loss in horses carrying many stomach bot larvae
- In rare cases, larvae moving to sensitive body areas in accidental hosts
For most homeowners, bot flies are a curiosity rather than a serious concern. They do not enter homes, do not bite as adults, and do not breed in household environments. If you encounter one, it is likely an adult that wandered indoors by accident.
References
- CDC DPDx - Myiasis (Bot Fly and Related Parasitic Flies)
- Companion Animal Parasite Council (CAPC) - Cuterebriasis Guidelines
- UF/IFAS Extension - Human Bot Fly (Dermatobia hominis)
- Merck Veterinary Manual - Gasterophilus in Horses
- Michigan DNR - Warbles in Wildlife
- University of Florida IFAS - Cattle Grubs (Hypoderma)
Other Flies
Explore other species in the flies family
Commonly Confused With
Bot Flies are often mistaken for these similar pests
Where Bot Flies Are Found
Hover over states to see their names. Green regions indicate where bot flies have been reported.
Common Questions about Bot Flies
What does a bot fly look like?
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Adult bot flies are stout, hairy flies that look similar to bumblebees. They are typically 12 to 20 millimeters long with dense hair covering their bodies. Colors range from dark brown and black to gray with yellowish banding. Unlike bumblebees, bot flies have only one pair of wings and short antennae typical of true flies.
Can bot flies infest humans?
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In the United States and Canada, human bot fly infestations are rare and almost always linked to travel in Central or South America, where the human bot fly (Dermatobia hominis) is native. North American Cuterebra species occasionally infest humans but this is very uncommon. If you develop a boil-like lesion with a central opening after outdoor activity, see a doctor.
Are bot flies dangerous to pets?
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Bot flies can affect dogs, cats, and rabbits. Cuterebra larvae sometimes infest pets that explore rodent burrows or spend time outdoors in wooded areas. Signs include a swelling or lump under the skin with a small breathing hole. Do not try to remove the larva yourself. A veterinarian should extract it to prevent complications like infection or allergic reaction.
How do bot flies reproduce?
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Bot flies have unusual reproductive strategies. The human bot fly attaches its eggs to mosquitoes or other biting insects, which deliver the larvae to a host. North American Cuterebra species lay eggs near rodent burrows. When a warm-blooded animal passes by, the eggs hatch and the larvae attach to the host and burrow under the skin.
Where are bot flies found in North America?
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Cuterebra bot flies are found across much of the continental United States and southern Canada. They are most commonly reported in rural and suburban areas near woodlands where rodents and rabbits live. Horse bot flies (Gasterophilus) are found wherever horses are kept. The human bot fly (Dermatobia hominis) is not native to the U.S. but is common from Mexico through Central and South America.
What time of year are bot flies most active?
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In temperate North America, adult bot flies are most active from late spring through late summer, roughly June through August. This is when they lay eggs and when larval infestations in pets and wildlife are most common. By fall, activity drops sharply, and bot flies are not active during winter months.
How are bot fly larvae removed?
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Bot fly larvae should be removed by a medical professional or veterinarian. Common techniques include applying petroleum jelly or a similar occlusive substance over the breathing hole to force the larva to the surface, then gently extracting it with forceps. In some cases, minor surgical removal is needed. Squeezing or crushing the larva while it is still embedded can cause infection or an allergic reaction.
Do bot flies come inside houses?
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Bot flies do not breed indoors and are not attracted to food, garbage, or other household attractants. Adult bot flies live only a few days to two weeks and spend their time outdoors looking for hosts or egg-laying sites. Finding a bot fly inside a home would be unusual and accidental.
With five years of hands-on experience in the pest control industry, George Schulz is a registered technician with the Virginia Pest Management Association and a proud third-generation professional in a family business that's been protecting homes for over 57 years. He manages and trains a team of service pros while also leading internal research efforts—recently spearheading a deep-dive review of thousands of documents on pest control materials to hand-pick the most kid and pet friendly, most effective solutions tailored specifically for homes in the DC metro area.