Types Of Flies In Montana

Montana’s wide-open rangelands and mountain streams each attract flies suited to those specific environments. Understanding the types of flies in Montana helps you prepare for everything from a backyard barbecue to a backcountry fishing trip. You will find species that bite, species that swarm, and species that just annoy—each with its own season and habitat.

Flies are more than a nuisance here. Some carry diseases, others ruin outdoor meals, and a few are actually beneficial for pollination. Knowing what you are dealing with makes a big difference in how you handle them. This guide covers the most common flies you will encounter across the state, from the high plains to the trout streams.

When people ask about the types of flies in Montana, they usually want two things: identification and control. The state hosts over a hundred fly species, but only about a dozen regularly bother humans or livestock. Below you will find the main groups, broken down by where they live and what they do.

House Flies

House flies are everywhere in Montana. They breed in manure, garbage, and rotting vegetation. You see them most in summer around homes, barns, and picnic areas.

Color: Gray with four dark stripes on the thorax
Size: About 6-7 millimeters long
Risk: Spread bacteria like Salmonella and E. coli
Control: Keep trash sealed and clean up pet waste

These flies do not bite, but they land on food and surfaces constantly. They can transmit over 100 pathogens. In Montana, house fly populations peak in July and August when temperatures stay above 70°F.

Horse Flies And Deer Flies

Horse flies and deer flies are the biters you remember. They slice the skin with scissor-like mouthparts and feed on blood. Females are the ones that bite; males feed on nectar.

Horse flies: Large, dark eyes, often with patterned wings
Deer flies: Smaller, with dark bands on the wings
Habitat: Near water sources, livestock pastures, and forests
Season: Late spring through early fall

These flies are aggressive. They chase you, land silently, and bite hard. Some people have allergic reactions to their saliva. In Montana, horse flies are worst along the Yellowstone River and in the Bitterroot Valley.

Stable Flies

Stable flies look like house flies but have a distinct, pointed proboscis for biting. They attack livestock and people, usually on the legs and ankles.

Appearance: Gray with checkerboard pattern on the abdomen
Bite: Painful, leaves a red welt
Breeding: In wet hay, straw, or manure mixed with bedding
Control: Remove rotting organic matter from barns and corrals

Stable flies are a major problem for cattle operations in eastern Montana. They reduce weight gain in calves and cause stress in dairy herds. For humans, they make outdoor work miserable in late summer.

Blow Flies And Bottle Flies

Blow flies and bottle flies are metallic green, blue, or bronze. They arrive quickly after an animal dies or if meat is left out.

Metallic green: Common bottle fly
Metallic blue: Bluebottle blow fly
Behavior: Attracted to carrion, garbage, and open wounds
Risk: Can cause myiasis in livestock and pets

These flies are important for decomposition, but they are a sign of poor sanitation near homes. In Montana, they are common around hunting camps and livestock operations. Keep meat covered and dispose of carcasses properly.

Mosquitoes

Mosquitoes are technically flies in the order Diptera. Montana has over 40 species, and they are most active after snowmelt and summer rains.

West Nile virus: Carried by Culex species
Peak season: June through August
Habitat: Standing water, marshes, irrigation ditches
Prevention: Use DEET repellent and remove standing water

Mosquitoes are worst in the Flathead Valley and along the Missouri River breaks. They breed in any water that sits for more than a week. Some species bite during the day, others at dusk.

Black Flies

Black flies are small, humpbacked flies that swarm in huge numbers near fast-moving water. They bite relentlessly and can cause severe itching and swelling.

Size: 2-5 millimeters
Color: Black or dark gray
Habitat: Clear, flowing streams and rivers
Season: Late spring to early summer

Black flies are a rite of passage for Montana anglers and hikers. They do not carry disease in the US, but their bites can become infected from scratching. They are thickest along the Madison, Gallatin, and Big Hole rivers.

Fruit Flies

Fruit flies are tiny, red-eyed flies that appear around overripe produce and fermenting liquids. They are more of a kitchen pest than an outdoor problem.

Size: About 3 millimeters
Color: Tan with bright red eyes
Breeding: In moist, sugary residues
Control: Throw out rotten fruit and clean drains

In Montana, fruit flies are common in homes and restaurants during late summer. They breed fast—a single female can lay 500 eggs. They are harmless but annoying.

Flesh Flies

Flesh flies are larger than house flies, with a checkerboard pattern on the abdomen. They are gray and have three dark stripes on the thorax.

Size: 8-14 millimeters
Behavior: Attracted to carrion, feces, and garbage
Live birth: Females deposit live larvae, not eggs
Risk: Can carry bacteria from decaying matter

Flesh flies are common in rural Montana near livestock and wildlife. They are often mistaken for stable flies but do not bite. They are scavengers that help break down dead animals.

Where Flies Live In Montana

Different regions of Montana have different fly problems. Knowing the local species helps you plan your outdoor activities.

Western Montana Mountains

In the Rockies, black flies and mosquitoes dominate the summer. The dense forests and cold streams create ideal breeding grounds. Horse flies are common in open meadows and along trails.

Black flies: Worst in May and June
Mosquitoes: Peak in July after snowmelt
Horse flies: Active in July and August

If you hike in the Bob Marshall Wilderness or Glacier National Park, bring head nets and long sleeves. The flies are thickest near water.

Eastern Montana Plains

The plains are drier, so house flies and stable flies are more common. Livestock operations attract large populations. Blow flies appear quickly around dead animals.

Stable flies: Major problem in cattle feedlots
House flies: Abundant near towns and farms
Blow flies: Common in late summer

Wind helps keep some fly numbers down, but sheltered areas around buildings can be terrible. Fly control on ranches is a constant battle.

River Valleys And Lakes

Montana’s major rivers—Yellowstone, Missouri, Flathead—are fly magnets. The combination of water, vegetation, and warm weather creates perfect conditions.

Black flies: Along fast-moving sections
Mosquitoes: Near slow water and marshes
Deer flies: In wooded areas along banks

Anglers deal with flies constantly. Many wear bug repellent and use head nets. The worst times are early morning and evening.

How To Identify Flies In Montana

Identifying flies helps you choose the right control method. Here is a simple step-by-step process.

Look at the size: Tiny (under 5mm) is likely a black fly or fruit fly. Medium (5-10mm) could be house fly or stable fly. Large (over 10mm) is probably a horse fly or flesh fly.
Check the color: Metallic green or blue means blow fly. Gray with stripes means house fly. Dark with no stripes could be stable fly.
Observe behavior: Biting flies are aggressive and persistent. Non-biters land on food or surfaces. Swarming flies like black flies stay in the air.
Note the location: Near water? Likely black flies or mosquitoes. Near livestock? Stable flies or house flies. Near garbage? Blow flies or flesh flies.
Look at the wings: Deer flies have dark bands. Horse flies have clear or patterned wings. House flies have clear wings held slightly apart.

If you are still unsure, take a photo and compare it to online resources. The Montana State University Extension has good identification guides.

Fly Control Methods For Montana Homes

Controlling flies starts with sanitation. Without removing breeding sites, sprays and traps only provide temporary relief.

Indoor Control

Keep screens on windows and doors in good repair
Clean up food spills immediately
Take out trash daily in summer
Use fly swatters or electric zappers for occasional flies
Set up sticky traps near windows

For persistent problems, check for dead animals in walls or attics. A dead mouse can attract blow flies for weeks.

Outdoor Control

Remove standing water for mosquitoes
Clean up pet waste and manure regularly
Compost properly—turn piles to reduce fly breeding
Use fly traps baited with attractants
Keep garbage cans tightly sealed

For livestock areas, consider biological control. Parasitic wasps target fly larvae and are safe for animals. They work well in barns and feedlots.

Personal Protection

Wear light-colored clothing—dark colors attract some flies
Use EPA-approved repellents with DEET, picaridin, or oil of lemon eucalyptus
Wear head nets in heavy black fly areas
Avoid perfumes and scented lotions

For anglers, consider treating clothing with permethrin. It repels and kills flies on contact. Reapply after washing.

Fly-Borne Diseases In Montana

Flies can carry diseases that affect humans and animals. While the risk is low, it is worth knowing what to watch for.

West Nile Virus

Mosquitoes transmit West Nile virus. In Montana, cases occur every year, mostly in late summer. Symptoms include fever, headache, and body aches. Severe cases can cause neurological problems.

Prevention: Use repellent and avoid dusk/dawn activity
Reporting: Dead birds may indicate local virus activity

Bacterial Infections

House flies and blow flies can carry Salmonella, E. coli, and Campylobacter. They pick up bacteria from manure and transfer them to food.

Symptoms: Diarrhea, fever, stomach cramps
Prevention: Keep flies away from food and cooking areas

Myiasis

Blow flies can lay eggs in open wounds or on animals with poor hygiene. The larvae hatch and feed on living tissue. This is rare in humans but common in livestock.

Treatment: Clean wounds and remove larvae
Prevention: Keep wounds covered and animals clean

Beneficial Flies In Montana

Not all flies are bad. Some help with pollination, decomposition, and pest control.

Hover Flies

Hover flies look like small bees or wasps but are harmless. They pollinate flowers and their larvae eat aphids.

Color: Yellow and black bands
Behavior: Hover in place, then dart quickly
Benefit: Natural aphid control in gardens

Robber Flies

Robber flies are predators that catch other insects in mid-air. They are large, with a long abdomen and a beard of bristles.

Size: 10-30 millimeters
Behavior: Perch on branches and ambush prey
Benefit: Control grasshoppers, beetles, and other flies

Bee Flies

Bee flies are fuzzy and look like small bumblebees. They hover near flowers and feed on nectar. Their larvae parasitize other insects.

Color: Brown or gray with patterned wings
Benefit: Pollination and natural pest control

Flies And Fishing In Montana

For anglers, “flies” means something different. Fly fishing is huge in Montana, and matching the hatch is key to success.

Common Fly Patterns

Adams: Imitates many mayflies
Woolly Bugger: Imitates leeches, baitfish, and nymphs
Hopper patterns: Imitate grasshoppers in late summer
Stonefly nymphs: Imitate the large nymphs found under rocks
Midges: Tiny flies that work in winter

Knowing the insect hatches on your river helps you choose the right artificial fly. Local fly shops are the best source of current information.

Matching The Hatch

Look at the water for rising fish
Check what insects are flying or floating
Match the size, color, and shape of the natural insect
Present the fly naturally with the current

Montana’s rivers have different hatches at different times. The salmonfly hatch on the Madison River in June is legendary. The trico hatch on the Missouri River in August is another classic.

Seasonal Fly Activity In Montana

Flies follow predictable patterns based on temperature and moisture. Here is what to expect each season.

Spring

Black flies emerge first, often in May. Mosquitoes appear as snow melts and standing water warms. House flies start breeding in manure and compost.

Black flies: Peak in late May
Mosquitoes: Start in May, increase through June

Summer

Summer is the worst season for flies. Horse flies, deer flies, and stable flies are at their peak. House flies multiply rapidly in hot weather.

Horse flies: July and August
Stable flies: Late July through September
Mosquitoes: Peak in July

Fall

Fly activity drops as temperatures cool. Mosquitoes remain active until the first hard frost. House flies move indoors seeking warmth.

House flies: Enter homes in September and October
Mosquitoes: Decline after frost

Winter

Most flies die off in winter. Some house flies survive indoors. Cluster flies may appear in attics and walls on warm days.

Cluster flies: Large, slow-moving, seek shelter
Indoor flies: Usually house flies or fungus gnats

Frequently Asked Questions

What are the most common types of flies in Montana?

The most common are house flies, horse flies, deer flies, stable flies, black flies, and mosquitoes. Each has a different habitat and season.

How can I keep flies away from my Montana home?

Sanitation is the first step. Remove manure, garbage, and standing water. Use screens on windows and doors. Fly traps and repellents help in heavy infestations.

Do flies in Montana carry diseases?

Yes. Mosquitoes can carry West Nile virus. House flies and blow flies can spread bacteria like Salmonella and E. coli. The risk is low but real.

What is the best repellent for Montana flies?

DEET-based repellents work best for mosquitoes and biting flies. Picaridin and oil of lemon eucalyptus are good alternatives. For black flies, head nets are more effective than repellent alone.

When is fly season in Montana?

Fly season runs from May through September. Black flies peak in late spring. Horse flies and deer flies peak in mid-summer. Mosquitoes are active from June through the