HCVS’s Yampa Valley Horse Insect Survival Guide

Insect Management for NW Colorado Horses

You know the drill: spring melt, green grass, flowing rivers, and every flying menace within a 50 mile radius shows up for a buffet on your herd. Here’s the who’s who, why they suck, and how to fight back.

Horses in northwestern Colorado meet a lot of bugs, from backyard “skeeters” and bloodthirsty horseflies to the ever present barn flies and ticks lurking in the brush. Insect control is more than a nuisance fix, it’s key to horse health, comfort and safety. Bites and constant swatting can reduce appetite and energy, and some insects can transmit disease (think equine encephalitis, anaplasmosis, even Equine Infectious Anemia EIA). Colorado’s climate, chilly high altitude nights, hot dry days, irrigated pastures and rushing rivers, means bug season can hit hard when the snow melts. A good integrated pest management (IPM) plan now (sanitation, traps, turnout schedules) pays off later, and rotating chemicals (mixing pyrethroids with other active ingredients) helps avoid the frustrating problem of fly resistance.

Below is a breakdown by pest type, with where/when they bite your horses, what trouble they cause, and proven control tips (sprays with active ingredients, masks/sheets, traps, etc.). We’ll also let you know the real health risks versus the mere nuisances and remind you that these bugs can be sneaky, tough, or downright obnoxious. (A good attitude and fly swatter help, but let’s get strategic.)

House Flies

• Where/when: All summer long in and around barns, stalls and paddocks, especially warm, sunny days. House flies breed in manure, spoiled hay or grain, so any festering pile will attract them.
• Horse impacts: These non biting filth flies annoy horses by landing on them, and are notorious vectors of pathogens (think salmonella, E. coli, pinkeye bacteria). In general they’re mostly a nuisance, contamination of feed and water is the main issue.
• Management: Go clean and contain, scoop manure and old hay daily
  • Fly sprays: Use a horse safe fly spray containing pyrethroids (e.g. permethrin or cypermethrin) or natural pyrethrins. These knock down flies on contact. Remember to rotate active ingredients now and then to avoid resistance. Spray legs and lower body, and use repellent wipes on the face and chest.
  • Fly masks/sheets: House flies don’t bite, but they may irritate eyes and nose. A light fly mask on your horse can keep flies off the face (and block UV!). A fly sheet can deter flies on the body.
  • Traps: Hang glue strips or electric fly traps in the barn corners; consider a baited bait station for localized control. Granular sugar baits (like QuickBayt) can lure and kill congregating flies in feed rooms. Fly predators (tiny parasitic wasps) released weekly can also reduce house fly populations over summer.
  • Environmental: Seal garbage cans, turn compost piles, and minimize spills. In Colorado’s dry climate flies may concentrate around any moist spots, check near water troughs, leaking hoses or wet spots. (Cant dry the river banks or irrigation ditches though)
•  Bottom line: House flies are incredibly annoying but only indirectly harmful. Keep things tidy and the ones left shouldn’t be awful.

Stable Flies (“Biting House Flies”)

• Where/when: Peak in warm summer (midday through afternoon) when horses are out. Stable flies look like house flies but have a piercing mouth, you’ll see them on legs and bellies. They breed in moist, fermenting organic matter: old hay bales, damp bedding, or even decaying clumps of grass and weeds.
• Horse impacts: Ouch! Stable flies bite ankles, bellies and ears with a painful, cutting “needle”. Horses often stomp and twitch tails. They’re more than a nuisance, a swarm can make horses colicky or lame from irritation.
• Management: Attack their breeding sites, since adult flies roam far from birth sites.
  • Fly sprays: Permethrin based sprays are standard; apply to the horse’s belly and legs before turnout. Reapply every few days in summer heat. Consider a permethrin “pour on” or an equine “insecticide ear tag”.
  • Fly sheets/boots: A belly guard or fly sheet that covers the midsection can block bites on the underside. Some owners also use fly boots during turnout to protect fetlocks and lower legs.
  • Traps: Sticky black decoys lure stable flies with sun reflections. Place traps close to horses, especially near water buckets or feeders, since stable flies sit only briefly on animals.
  • Environment: This is key, dry out their nurseries. Remove old damp bedding and rotting hay. Rake out any grass clumps or wet manure where maggots could develop. If using compost piles, cover them tightly or turn frequently to dry. Fans in the stalls provide pesticide free relief, flies simply avoid the breeze.
  • Active ingredients: Pyrethroids (permethrin, cyfluthrin, etc.) are effective; cyclic rotation is advised. Avermectins (ivermectin in dewormer) do not kill biting flies, but note that they will pass through manure and can help control some manure-inhabiting flies if you choose feed-through products (see House Flies).
•  Bottom line: Stable flies are tough customers. Out-hustle them by drying up their larval habitats and using physical barriers (sheets, fans) plus sprays. Even small steps like changing buckets often can pay off.

Horse Flies & Deer Flies

• Where/when: Mid-summer afternoons in sunny weather. These flies buzz in wooded pastures, river edges, or shady creek banks. They breed in mud and wet marshland (not in your barn), so count on them near rivers, wetlands or boggy spots.
• Horse impacts: These are the locusts of the fly world, vicious biters. A horse fly attack feels like a hot needle; deer flies are smaller but also persistent. They typically hit the withers, shoulders or hindquarters. A single bite can bleed or swell; a swarm can cause panic, runs or injuries. Importantly, tabanids mechanically transmit serious diseases, notably equine infectious anemia (EIA) EIA-infected horses can be bit by the fly, then that fly bites the next horse. So keep your Coggins test current, at least annually.
• Management: Brutal honesty: there’s no silver bullet. Adult horse/deer flies are notoriously hard to stop with sprays. They just bite and leave. You can manage exposure:
  • Turnout timing: These flies prefer mid-day sun. Turn horses out early morning or evening when possible. Even moving them to an open field away from woods/marsh helps, sometimes open windy places have fewer flies.
  • Fly masks and sheets: Full-coverage fly sheets (including ear hood) can protect horses during peak fly hours. Lightweight, breathable fabrics work well. Fly masks with nose and ear covers help guard the face and eyes. (Test your horse, some don’t mind sheets; others need training.)
  • Sprays: No repellent is 100% foolproof, but heavy-duty fly sprays: permethrin, or even products with citronella can deter a few seconds of attraction. Reapply often. Note: DEET-based repellents are not FDA approved for horses and may not be very effective on deer flies. Pyrethroid + piperonyl butoxide fly sprays (e.g. Ultrashield) are standard on horses, though tabanids easily ignore light sprays.
  • Traps: The classic Manitoba trap (a black or blue sphere under a funnel) can catch some horse flies. It lures them in and they drown or stick. If you’re swamped, it’s worth a try, place a dark sphere about 3–4 ft off the ground.
  • Environment: Nothing you can do here unfortunately, they don’t breed in your manure or hay. If the flies are thick near streams and irrigation, consider keeping horses a quarter-mile or more away during the worst hours. Not easy to do…
  • Active ingredients: Remember that tabanids do not develop resistance to your sprays, rather, our sprays just aren’t that good. Focus on reducing contact and aim for rotation with other tools.
• Bottom line: Horse and deer flies are sneaky predators. Arm yourself with fly sheets and good manners (and maybe a fly-swatter for one-on-one duels), because these guys are almost impossible to eradicate.

(Non‑biting) Dung & Filth Flies

• Where/when: All summer, especially in and around stalls and manure piles. This category covers house/face flies, cluster flies, muscid flies and the tiny “dung” (sphaerocerid) flies that hang around manure. None of these bite, but they can pester horses and spread disease. Face flies (around cattle farms) bother eyes and carry pinkeye; cluster flies swarm in fall; and little dung flies (yellow dung flies) just swarm manure.
• Horse impacts: Mainly nuisance and hygiene. They can annoy horses by crawling on legs and feeding on secretions. They also accelerate flylife by competing or occasionally carrying pathogens. But because they don’t bite, they’re generally less dangerous than horse flies or mosquitoes.
• Management: Sanitation is your best weapon.
  • Manure management: Pick up droppings often, use well-drained paddocks, and compost or remove manure off-site. Remember: face flies and other non-breeding flies do not rely on horse manure to breed, but house flies do, so remove that cattle barn dung too.
  • Predators & parasitoids: Consider releasing Fly Predators to help with population control https://info.spalding-labs.com/
  • Traps and screens: Sticky ribbons in barns, window/screens, and baffles on water buckets (to prevent flies drinking in) all help. UV light traps inside the barn can catch night-active cluster flies.
  • Sprays: Target spots where flies rest, eaves, windows, manure bins, with residual sprays (pyrethroids). Use outside only on surfaces, not on horses. Inside, space sprays or foggers can knock down adult flies if the barn is closed up tightly for a short while.
  • Active ingredients: Pyrethroid residuals or insect-growth regulators (methoprene, (S)-methoprene) in fly baits. Feed-through larvicides (like cyromazine or diflubenzuron in feed) can break the life cycle if horses are eating the additive and breeding on-site (just be sure they actually do breed there!). Rotate modes of action to slow resistance.
•  Bottom line: Dung and filth flies are obnoxious scavengers. Keep everything clean, use traps, and let natural enemies help. Over time you’ll notice fewer buzzers around the barn.

Black Flies

• Where/when: Early summer near rivers, creeks and mountain streams with fast-moving water. Colorado has ~40 species of Simulium flies, especially active in spring runoff and early summer. They swarm in daylight (often late morning) around streamside pastures and will even follow horses to bite ears and necks.
• Horse impacts: Black flies cause painful welts. They slash with tiny serrated jaws, leaving bumps and bleeding points. Horses can be driven mad scratching at manes and necks. In extreme cases (heavy swarms in confined areas) horses can go down or even die from blood loss or allergic shock. Fortunately that’s rare; usually it’s a miserable few weeks of itch. They are also potential vectors for vesicular stomatitis virus (VSV), which
  Colorado sporadically reports.
• Management: This is tough, since black flies breed far from the horses.
  • Turnout management: On high-risk days (warm, damp mornings near streams), stall horses if possible until late afternoon. If horses are in the field, turn them out near open sunny pastures rather than in the brush by the river. Remember: black flies fly several miles, so even fields aren’t always safe.
  • Fly sprays/repellents: A heavy application of permethrin spray on the neck, chest and belly can offer an hour or two of relief. Oil-based fly repellents (containing IR3535 or citronella) may also help briefly. Reapply often.
  • Fly masks: Use a fine-mesh fly hood that covers the ears. Some come with under-nose cover too. Even draping a light sheet over a horse’s front half can stop many bites.
  • Fans: Running a sturdy fan on a shaded paddock or in the stall area keeps flies at bay, black flies are weak fliers and will not battle the breeze. A fan aimed at the horse’s neck/head can give instant relief inside the barn.
  • Traps: No easy trap exists (they lay eggs in water, not manure).Towns sometimes fog or treat streams for control, but that’s a job for wildlife agencies (and not practical on most ranches).
  • Region tips: Be especially vigilant in May–June after a big snowmelt. In 1995 Colorado saw a huge black fly outbreak after heavy runoff. Also, northwest Colorado’s mountain rivers (Yampa, Colorado River headwaters, etc.) are breeding hotspots, expect some annoyance if your barn is nearby.
•  Bottom line: Black flies are tiny ninjas from the river. Use stall time, fans, masks and
  heavy-duty repellent. Once the runoff subsides, their numbers usually drop, but until
  then, stay one step ahead.

Mosquitoes

• Where/when: Mosquitoes buzz around any standing water in summer, irrigation ditches, ponds, buckets, hoof prints. They bite mainly at dawn and dusk (and some species at night). In Colorado’s altitudes, mosquitoes do exist (especially in green valleys); a wet summer can mean big hatches even at 7,000+ feet. Turns out a dry year can be bad too…
• Horse impacts: Their itchy bites can drive horses crazy and lead to secondary skin infections. More importantly, mosquitoes are disease vectors. Colorado horses face West Nile Virus (WNV) and Western Equine Encephalitis (WEE). Both are carried by mosquitoes feeding on infected birds. An untreated horse with WNV can suffer neurological disease and even death. (Tip: vaccinate horses for WNV/EEE well before fly season!)
• Management: Target both bugs and breeding sites.
  • Stable sheets: A fine-weave turnout sheet (or light stable blanket) can prevent night bites.
  • Mesh tarps draped over run-in sheds like doors reduce access. Some horses may not tolerate this. (Harbor Freight has great mesh tarps that are very affordable!) https://www.harborfreight.com/collections/meshtarps.html
  • Fly sprays: Use a spray labeled for mosquitoes on horses; many common fly sprays (permethrin + piperonyl butoxide) repel mosquitoes too. DEET-based human repellents should not be applied to horses. Treat the animal’s body (especially belly, legs, and back) often.
  • Fans: A fan over the stall or the horse’s run will deter mosquitoes, they can’t fly well in moving air.
  • Larval control (Environmental): Drain or change water daily wherever possible. Add mosquito dunks (Bacillus thuringiensis israelensis) to standing water troughs or ponds, these kill larvae and are safe for livestock. Consider stocking guppy or goldfish in water sources to eat larvae (Just not in water that would make it back to the Yampa).
  • Repellents on property: Burn mosquito coils away from horses, and use flicker LED lanterns (some emit repellant light) at night. Permethrin spread on non grazing areas may help too
  • Mow down tall grasses when possible.
  • Vaccination: Have your vet discuss WNV/EEE/WEE vaccines. A vaccinated horse resists the deadliest effects even if bitten.
  • Region tips: Areas near irrigated fields or standing irrigation ponds get heavy mosquito pressure. Dry summers mean fewer, wet years (or heavy dew) mean more. Altitude generally reduces mosquito breeding, but warm valleys can still harbor them into August.
•  Bottom line: Mosquitoes are persistent parasites. Battle them with netting, fans and
  larval control. Don’t skip the vaccine; it’s far cheaper than treating encephalitis!

Ticks

• Where/when: Spring through summer (and a smaller fall peak). Colorado’s woods, tall grasses and brush patches harbor ticks. They crawl up fence lines, rock piles and trails, waiting on blades of grass. Horses picking their way along wooded pastures or trails are likely to collect ticks. High-use beds (where deer or rodents spend time) can have dense tick populations. The Rocky Mountain wood tick is the usual culprit around here.
• Horse impacts: Ticks attach and feed for days. A few ticks on a horse generally just cause irritation (itching, rash). But disease is the worry. In Colorado, ticks can carry Colorado Tick Fever, Rocky Mountain spotted fever, and rarely Tularemia. (Note: Lyme disease is not considered endemic in CO.) Dermacentor ticks feed on large animals including horses. Heavy infestations of winter ticks (Dermacentor albipictus), usually a moose pest, can also bother horses kept near elk herds. The diseases above are rare here but not unheard of.
  ● Management: It’s all about habitat and protection.
  • Pasture management: Mow grass short, clear brush and leaf litter around fences and paddocks, sunny dry areas limit tick survival. Keep horses out of brushy fence-lines and wood edges (a temporary electric fence can help).
  • Permethrin treatment: Treat the horse directly. Permethrin (a pyrethroid) can be sprayed onto the mane, tail and belly; it kills or repels ticks on contact. Several products and “tick collars” or spot-ons (like flumethrin) made for livestock can help reduce tick load. Reapply in spring and as needed through summer.
  • Check and remove: After turnout or trail rides, groom and inspect horses (especially around ears, under bellies, in chestnuts). Removing a tick within 24 hours usually prevents disease. Use tweezers or a tick hook for clean removal. Personally I use the ZenPet Tick Tornado:
    https://www.amazon.com/ZenPet-Tick-Tornado-Removal-Remover/dp/B010NU1V1I?th=1
  • Chemical barriers: Consider permethrin/pesticide treatments on pastures along wood edges. Professional spraying on perimeter weeds/leaf litter can knock down ticks on property. (Just don’t contaminate feed)
  • Fun fact: Chickens, peafowl or guinea fowl eat ticks
  • Veterinary advice: Talk to your vet if you see any oozing, hair loss, or swelling. They have topical and injectable medications that can help.
  • Region tips: In NW Colorado’s mountains, ticks drop when it’s cool, but can survive through mild winters. Snow cover helps them survive, so heavy-mulched pastures can hide them. Even high-country patches of brush can hold a few. Always think ticks when near wooded fence-rows or creek banks.
• Bottom line: Ticks are stealthy and nasty hitchhikers. Keep pastures tidy, use permethrin, and stay vigilant. A quick pull is better than a costly vet bill.

Integrated Pest Management (IPM): No single thing will completely handle all these bugs. The key is using multiple tactics together: sanitation (clean stalls, manure management) plus physical barriers (fly masks/sheets, fans),insecticides on horses, and habitat reduction. Rotate active ingredients yearly to slow down resistance. Inspect and test new horses (especially for EIA), keep vaccines updated (WNV/EEE), and remember that covering your horse’s eyes can save their and your sanity on a buggy day.

Conclusion: With patience and persistence, you can drastically cut those critters down to size. Remember: A well-planned insect control program now will make your herd happier, and have you breathing easier, all summer long.