Horses are fly magnets for a simple reason: they produce exactly what flies are looking for. Their large bodies generate plumes of carbon dioxide, body heat, and chemical compounds in sweat that flies can detect from surprisingly far away. Their dark coats, moist eyes, and open wounds offer both visual targets and nutrient-rich feeding sites. For biting flies in particular, horse blood provides the protein female flies need to produce eggs.
What Draws Flies to Horses
Flies locate horses using a combination of smell, heat, and vision. The primary long-range attractant is carbon dioxide, which horses exhale in large volumes due to their size. Trapping studies have shown that CO2 alone increases horse fly catches three- to four-fold compared to unbaited traps. Horses also release a compound called octenol through their skin, a chemical found in mammalian sweat and breath. When octenol is combined with CO2, the attractant effect is even stronger, pulling in more fly species and greater numbers.
Once closer, flies use visual cues. Horse flies in particular have a remarkable ability to detect polarized light, the kind of light that bounces off smooth, dark surfaces. A dark-coated horse standing in sunlight reflects highly polarized light, making it stand out sharply against the surrounding vegetation. Research from the Royal Society found that the darker the coat color and the higher the degree of polarization of the reflected light, the more attractive the horse is to female horse flies. They’re most effective at spotting hosts using blue light wavelengths, and movement also plays a major role in triggering their approach. Field observations on free-ranging horses confirm this pattern: darker colored horses attract significantly more flies per capita than lighter ones, especially on sunny days.
The Different Flies That Target Horses
Not all flies bother horses for the same reason, and the species involved have very different feeding strategies.
- Horse flies and deer flies are the most painful biters. Female horse flies have blade-like mouthparts lined with curved, pointed teeth that slice through skin to reach blood vessels underneath. Anchoring teeth on the inner mouthparts grip the tissue while the mandibles cut deeper. The bite is crude and effective, creating a wound that bleeds freely.
- Stable flies look like ordinary house flies but have a forward-pointing piercing mouthpart. They typically bite the lower legs, and they’re persistent enough to keep returning even after being swatted away.
- Black flies number over 1,000 species worldwide. They tend to swarm around the head and ears.
- Face flies don’t bite at all. They cluster around the eyes and muzzle to lap up protein-rich secretions, along with nasal discharge, saliva, and blood from existing wounds.
- House flies and blow flies breed in manure and decaying organic matter, both of which are abundant around horses. They feed on moisture and nutrients from wounds, body openings, and feces.
Why Female Flies Need Blood
For biting species, the attraction to horses is driven by reproduction. Female horse flies and stable flies require a blood meal to develop their eggs. Vertebrate blood is rich in protein, and after feeding, a female fly’s body converts those amino acids into the lipids, carbohydrates, and proteins that get packed into developing eggs through a process called vitellogenesis. A single blood meal can trigger the production of an entire batch of eggs. Males of most biting species don’t bite at all. They visit horses mainly to find females who are already attracted to the host.
This reproductive need makes female flies aggressive and persistent feeders. A horse fly interrupted mid-meal will often circle back or move to a nearby horse to finish feeding, which is part of what makes them effective disease carriers.
Diseases Flies Carry Between Horses
The feeding habits of horse flies create a direct route for pathogen transmission. Equine infectious anemia (EIA), a viral disease with no cure, can be mechanically transmitted when a horse fly feeds on an infected horse and then moves to a healthy one. Research on one species of horse fly found that transmission was successful when flies were interrupted and allowed to refeed within 30 minutes, but not after four hours or longer, suggesting the virus survives only briefly on mouthparts. Notably, horse flies can travel more than four miles, meaning the standard practice of separating infected horses by 200 yards may not be enough in areas with heavy fly populations.
Flies also play a role in a parasitic condition called summer sores. House flies and stable flies serve as intermediate hosts for stomach worms. Adult worms in the horse’s gut produce larvae that pass out in manure, where fly maggots ingest them. When those flies later land on a horse’s lips, the larvae are deposited and swallowed, completing the worm’s life cycle. But when larvae end up in wounds, the nostrils, or the genitalia instead, they can’t complete their development and trigger an intense inflammatory skin reaction.
How Fly Pressure Affects Horses
Fly harassment isn’t just an annoyance. It changes how horses eat, where they go, and how much energy they spend defending themselves. When fly burdens are high during warm months, free-ranging horses will abandon prime grazing areas entirely, retreating to bare ground, elevated ridges, or windy spots where flies are less active. Field data from barrier island horse populations show that per capita fly loads drop as wind speed increases and rise with temperature, so horses actively seek conditions that reduce exposure.
This matters because the same warm months when flies peak are the months horses need to build body condition for winter or recover from foaling. Chronic reductions in grazing time, especially when high-quality forage has to be abandoned for long stretches, carry real nutritional costs. Horses may only return to good pasture when fly numbers dip, creating a cycle of interrupted feeding that can affect weight and condition over the course of a season.
How Horses Defend Themselves
Horses have a surprising toolkit for fighting flies. Tail swishing is the most visible defense, and it works better than it looks. Engineering research found that a swishing tail deters about 50% of approaching insects, not by making contact, but by generating a small breeze of roughly 1 meter per second at the tip. That’s enough to blow mosquitoes and small flies off course before they can land. Approaching insects appeared unaware of the tail until they were close enough to feel the airflow, at which point they veered away.
Beyond tail swishing, horses use skin twitching (a rapid contraction of the muscles just beneath the skin), head shaking, and stomping to dislodge or discourage flies. They also engage in mutual grooming and will stand head-to-tail with a companion so each horse’s tail covers the other’s face. Grouping behavior increases in fly-heavy conditions, and horses with docked or short tails are at a measurable disadvantage, relying more heavily on stamping and head tossing to compensate.
Coat color ties back into defense as well. Since darker horses attract more flies, lighter-coated individuals in mixed herds tend to carry lower fly loads. Some researchers have speculated that the striped patterns of zebras may have evolved partly as a fly deterrent, since the alternating black and white disrupts the polarized light signals that horse flies use to locate hosts.