6 Biting Flies in Colorado You Need to Identify and Avoid

Colorado’s diverse ecosystems harbor several species of biting flies that can turn your outdoor adventures into painful ordeals.

While you’re enjoying the state’s 300+ days of sunshine, these persistent insects are actively seeking their next blood meal.

Understanding which biting flies in Colorado pose the greatest threat—and how to identify them—can save you from weeks of itchy, swollen bite marks and potential disease transmission.

Whether you’re hiking the Rocky Mountain trails, fishing along the South Platte River, or simply enjoying your backyard barbecue, you’ll encounter at least one of these six problematic species.

Each has distinct behaviors, preferred habitats, and peak activity times that directly impact when and where you’re most vulnerable.

The key to effective protection lies in species-specific identification and targeted prevention strategies rather than generic bug spray applications.

This guide covers Colorado’s most aggressive biting flies, from the lightning-fast deer flies that target moving objects to the nearly invisible no-see-ums that slip through standard window screens.

You’ll learn to distinguish between species that cause temporary irritation and those that deliver painful, long-lasting wounds requiring medical attention.

Deer Flies

Deer flies represent Colorado’s most aggressive daytime biters, with females capable of pursuing you for hundreds of yards once they’ve locked onto your scent and movement patterns. These medium-sized flies, measuring 1/4 to 1/2 inch in length, feature distinctive dark bands or spots across their clear wings and large, iridescent eyes that can appear golden or green depending on the light angle.

Key Insight: Deer flies use visual and thermal detection to track moving targets, which explains why they’re particularly attracted to joggers, cyclists, and horseback riders rather than stationary individuals.

You’ll most commonly encounter deer flies from May through September in Colorado’s wetland areas, including the Colorado River corridors and mountain meadows with nearby water sources. Unlike mosquitoes that probe gently for blood vessels, deer flies use their scissor-like mouthparts to slash through skin, creating wounds that continue bleeding after the fly departs.

The pain from deer fly bites is immediate and intense, often described as feeling like a small knife cut. Bites typically swell within hours and can remain itchy for days, with some individuals developing allergic reactions that require antihistamine treatment. Colorado State University Extension research shows that deer fly saliva contains anticoagulants that prevent blood clotting, explaining why these bites bleed more than other insect wounds.

Protection Strategies for Deer Flies:

• Wear light-colored, tightly woven clothing that covers arms and legs
• Apply DEET-based repellents to exposed skin and clothing edges
• Avoid wetland areas during peak feeding times (10 AM to 4 PM)
• Use physical barriers like head nets when hiking through marsh areas
• Consider permethrin-treated clothing for extended outdoor activities

Pro Tip: Deer flies typically attack the head, neck, and shoulders, so focus protection efforts on these areas. A wide-brimmed hat can provide significant defense against aerial attacks.

Stable Flies

Stable flies earn their reputation as the “dog flies” of Colorado due to their painful bites that feel similar to bee stings. These gray-brown flies, approximately 1/4 inch long, can be distinguished from house flies by their forward-pointing proboscis and the distinctive checkerboard pattern on their abdomens. Unlike other biting flies that target specific body areas, stable flies will bite any exposed skin they can access.

Common Mistake: Many people confuse stable flies with house flies and assume they’re harmless until experiencing their surprisingly painful bite.

What makes stable flies particularly problematic in Colorado is their association with livestock operations and rural properties. They breed in decomposing organic matter, including manure piles, wet hay, and compost areas common throughout the state’s agricultural regions. Research from Colorado’s agricultural extension services indicates that a single stable fly can take multiple blood meals daily, making them persistent attackers once they locate a suitable host.

The bite pattern of stable flies differs significantly from other species—they prefer to feed on lower extremities, particularly ankles and legs, where they can quickly escape if disturbed. Their bites create small, red welts that often develop into hard, itchy bumps lasting several days. Unlike deer flies that slash the skin, stable flies probe with their needle-like mouthparts, often requiring several attempts to find a suitable feeding site.

Stable flies are most active during cooler parts of the day, typically early morning and late afternoon, when temperatures range between 70-85°F. They’re also attracted to movement and carbon dioxide, making them particularly bothersome to people working outdoors or enjoying patio activities.

Effective Stable Fly Management:

1. Source Reduction: Remove or properly manage organic waste that serves as breeding sites
2. Barrier Protection: Wear long pants and closed shoes when in areas with livestock or compost
3. Repellent Application: Use products containing picaridin or DEET on lower legs and ankles
4. Timing Avoidance: Plan outdoor activities during midday heat when stable flies are less active
5. Property Management: Keep grass short and remove wet organic debris near living areas

Horse Flies

Horse flies command attention as Colorado’s largest biting flies, with some species reaching nearly an inch in length and delivering bites painful enough to draw blood immediately. These robust insects feature large, prominent eyes that often appear metallic green or copper-colored, and their substantial size makes them impossible to miss when they’re actively hunting. Female horse flies require blood meals for egg production, making them aggressive pursuers during their active season.

Colorado’s horse fly populations peak during July and August, coinciding with the state’s warmest weather when many residents and visitors spend extended time outdoors. You’ll encounter these flies most frequently near lakes, ponds, and slow-moving streams, where they complete their aquatic larval development. Popular recreational areas like Chatfield Reservoir and the plains reservoirs often experience heavy horse fly activity during peak summer months.

The bite mechanics of horse flies make them particularly problematic. They use powerful mandibles to tear through skin rather than piercing like mosquitoes, creating wounds that can take weeks to fully heal. Horse fly saliva contains potent anticoagulants and enzymes that can trigger significant allergic reactions in sensitive individuals. Some people experience delayed reactions, with bites developing into large, painful welts 24-48 hours after the initial attack.

Important Note: Horse fly bites can become infected if not properly cleaned and treated, as their large mouthparts can introduce bacteria deep into tissue.

Horse flies demonstrate remarkable persistence when targeting victims, often following people for considerable distances. They’re particularly attracted to dark-colored clothing, rapid movement, and the carbon dioxide produced during physical exertion. This combination makes them especially troublesome for hikers, swimmers, and anyone engaged in outdoor sports near water sources.

Horse Fly Defense Strategies:

Pro Tip: Horse flies often circle their target before attacking. If you notice large flies repeatedly buzzing around you, seek shelter or dense vegetation where their flight patterns are disrupted.

Biting Midges (No-See-Ums)

Biting midges, commonly called no-see-ums, present Colorado’s most underestimated biting fly threat due to their microscopic size and disproportionately painful bites. These tiny insects, measuring only 1-3 millimeters in length, can easily pass through standard window screens and most protective clothing weaves. Despite their diminutive stature, female biting midges deliver bites that can cause intense burning sensations and long-lasting skin reactions.

Colorado’s semi-arid climate might seem unsuitable for these moisture-loving insects, but they thrive in specific microenvironments throughout the state. You’ll find concentrated populations around irrigation ditches, livestock watering areas, and the muddy edges of mountain lakes and reservoirs. Colorado State University entomologists have documented significant midge populations in the South Park region and along the Eastern Plains wherever consistent moisture creates suitable breeding conditions.

The biting pattern of no-see-ums sets them apart from larger flies. They typically attack in groups during dawn and dusk hours, when their small size makes them nearly impossible to detect until multiple bites have occurred. Their salivary proteins cause intense itching that can persist for days or even weeks, with some individuals developing papular urticaria—a condition characterized by persistent, hard bumps that resemble small blisters.

Key Insight: No-see-ums can bite through thin clothing and target areas where fabric touches skin, such as waistbands, collar lines, and sock tops.

What makes biting midges particularly problematic in Colorado is their seasonal timing. Peak activity occurs during late spring and early summer when many residents emerge from winter hibernation to enjoy outdoor activities. Unlike larger flies that are easily swatted or avoided, no-see-ums attack unnoticed, often resulting in multiple bites before you realize you’re under assault.

The medical significance of no-see-um bites extends beyond simple irritation. Some species can transmit bluetongue virus to livestock, though human disease transmission is rare in Colorado. However, the intense allergic reactions some people experience can require medical treatment, including prescription antihistamines or topical corticosteroids for severe cases.

No-See-Um Protection Protocol:

• Use fine-mesh screens (at least 20 mesh per inch) on windows and doors
• Apply repellents containing DEET or picaridin to all exposed skin
• Wear tightly woven fabrics that prevent penetration through clothing
• Avoid outdoor activities during dawn and dusk peak feeding times
• Use fans on patios and decks to create air movement that disrupts their flight
• Treat severe bite reactions with cool compresses and antihistamines

Common Mistake: Many people don’t realize they’re being bitten by no-see-ums until hours later when the intense itching begins, making prevention more challenging than treatment.

Black Flies

Black flies, also known as buffalo gnats due to their distinctive hunched appearance, represent Colorado’s most persistent swarm-forming biting flies. These small, dark insects typically measure 2-5 millimeters in length and are characterized by their broad, short bodies and clear wings with distinctive vein patterns. Unlike solitary hunters like deer flies, black flies attack in overwhelming numbers that can make outdoor activities unbearable without proper protection.

Colorado’s black fly populations are closely tied to flowing water sources, as their larvae require well-oxygenated streams for development. The state’s extensive mountain creek systems and irrigation networks provide ideal breeding habitat, with peak emergence occurring during snowmelt periods from April through July. Areas near Rocky Mountain National Park and the state’s many trout streams experience some of Colorado’s heaviest black fly activity.

The feeding behavior of black flies creates unique challenges for outdoor enthusiasts. These insects use pool-feeding techniques, creating small wounds that continue bleeding while they consume blood and tissue fluids. Black fly saliva contains powerful anticoagulants and proteins that can trigger significant allergic reactions, including a condition called “black fly fever” characterized by swelling, nausea, and fever in sensitive individuals.

Important Note: Black flies are attracted to dark colors, carbon dioxide, and movement, making them particularly problematic for hikers and anglers wearing dark clothing in their preferred riparian habitats.

Black flies demonstrate remarkable host-finding abilities, using visual cues, carbon dioxide gradients, and body heat to locate victims from considerable distances. They typically attack exposed areas of the head, neck, and arms, with their bites often going unnoticed initially due to their anesthetic saliva. This delayed pain response allows multiple flies to feed simultaneously, potentially resulting in dozens of bites in a short period.

The medical implications of black fly encounters extend beyond immediate discomfort. In areas with heavy infestations, the sheer number of bites can lead to systemic reactions requiring medical intervention. Additionally, scratching bite sites can introduce secondary bacterial infections that may require antibiotic treatment.

Black Fly Management Strategies:

1. Habitat Avoidance: Stay away from flowing water sources during peak emergence periods
2. Protective Clothing: Wear light-colored, long-sleeved shirts and pants with tight-fitting cuffs
3. Repellent Selection: Use products containing 20-30% DEET for maximum effectiveness
4. Head Protection: Employ head nets or wide-brimmed hats when hiking near streams
5. Activity Timing: Plan water-based activities during midday heat when black flies are less active

Pro Tip: Black flies are poor fliers in windy conditions. Choose exposed, breezy locations for rest stops when hiking through black fly territory.

Sand Flies

Sand flies, despite their name, thrive in Colorado’s diverse environments far beyond sandy areas, establishing populations wherever organic matter accumulates in moist, protected locations. These tiny flies, measuring only 1.5-3.5 millimeters, are distinguished by their pale coloration, hairy wings held vertically when at rest, and distinctive hopping flight pattern that sets them apart from other small biting flies.

Colorado’s sand fly activity centers around areas with decaying vegetation, animal burrows, and rocky crevices that provide the humid microenvironments these insects require for survival. You’ll encounter them most frequently in wooded areas with deep leaf litter, around compost piles, and in canyon areas where moisture collects. The Colorado foothills and canyon systems provide ideal habitat, particularly during the warmer months when organic decomposition accelerates.

The medical significance of sand flies in Colorado extends beyond their painful bites. While local transmission of leishmaniasis remains rare, these flies can carry various bacterial infections and cause severe allergic reactions in sensitive individuals. Sand fly bites typically develop into small, hard papules that can persist for weeks, often accompanied by intense itching that worsens at night.

Key Insight: Sand flies are weak fliers that rarely venture more than a few hundred yards from their breeding sites, making source identification and elimination highly effective control strategies.

Sand flies exhibit crepuscular activity patterns, becoming most active during twilight hours when many people enjoy evening outdoor activities. Their small size allows them to penetrate most protective clothing and standard window screens, making them particularly challenging to exclude from indoor spaces. Unlike day-active flies that are easily spotted and avoided, sand flies often attack unnoticed during social gatherings on patios and decks.

The bite pattern of sand flies reflects their opportunistic feeding behavior. They typically target any available exposed skin, with bites often occurring in clusters where clothing fits loosely or gaps exist in protective coverage. Their saliva contains potent allergens that can cause delayed hypersensitivity reactions, with some individuals developing chronic dermatitis at bite sites.

Sand Fly Control Approaches:

• Remove organic debris and leaf litter from around living areas
• Seal cracks and crevices where sand flies might breed or shelter
• Use fine-mesh screens on windows and doors to prevent indoor entry
• Apply repellents containing permethrin to clothing and gear
• Install outdoor fans to create air movement that disrupts their weak flight
• Time outdoor activities to avoid peak feeding periods around sunset

Common Mistake: Many people assume sand flies are harmless due to their small size and don’t take adequate protective measures until after experiencing their surprisingly painful and persistent bites.

Conclusion

Effective protection against Colorado’s biting flies requires a multi-layered approach that combines species-specific knowledge with proven prevention techniques.

The most successful strategies integrate physical barriers, chemical repellents, environmental modifications, and behavioral adaptations tailored to your specific outdoor activities and locations.

Personal Protection Essentials:

The timing of your outdoor activities significantly impacts your exposure to different biting fly species. Early morning hours (6-10 AM) generally offer the best conditions for hiking and fishing, as most species are less active during cool temperatures. Midday heat (11 AM-3 PM) provides relief from black flies and stable flies but increases deer fly activity near water sources.

Seasonal Protection Planning:

• Spring (April-May): Focus on black fly protection near water sources
• Early Summer (June-July): Peak activity for most species—maximum protection required
• Late Summer (August-September): Continued deer fly and horse fly activity
• Fall (October+): Minimal activity except during warm weather spells

Environmental modifications around your property can significantly reduce biting fly populations. Eliminate standing water sources, maintain short grass, and remove organic debris that serves as breeding habitat. For properties near livestock or agricultural areas, coordinate with neighbors to address stable fly breeding sources that affect the entire community.

Understanding the behavioral patterns of Colorado’s biting flies allows you to make informed decisions about outdoor activities while maintaining your quality of life. With proper identification skills, targeted protection strategies, and awareness of peak activity periods, you can enjoy Colorado’s outdoor recreational opportunities without surrendering to the state’s most persistent flying pests.

Remember that biting fly activity varies significantly based on weather conditions, recent precipitation, and local habitat changes. Stay informed about current conditions in your area and adjust protection strategies accordingly to ensure comfortable and safe outdoor experiences throughout Colorado’s extended outdoor season.