Flies are a common nuisance for horses, particularly during warmer months. Their attraction is rooted in specific sensory cues that horses emit. Flies use their olfactory system, thermal detection, and visual perception to locate and approach their equine hosts.
Chemical Signatures: The Scent Trail
Flies detect a range of chemical compounds released by horses. Horse sweat, for instance, contains proteins and salts which break down into ammonia and other odors that flies find appealing. A 2020 study indicated that ammonia compounds in horse sweat can increase fly attraction by 40% compared to dry skin, with different horses having varying sweat compositions that alter their appeal to flies. Lactic acid, also present in sweat, is another strong attractant for flies.
Horses also emit distinct blends of organic compounds through their skin oils and coat bacteria, creating a unique scent profile that acts as a signal for flies. Some research suggests that equine pheromones might also play a role in this attraction. The decomposition of horse manure releases gases and compounds that attract certain fly species. Biting flies, like horse flies, are drawn to compounds found in blood, which they detect to locate a host for a blood meal.
Thermal and Respiratory Cues
Flies detect horses through thermal and respiratory signals. Horses exhale amounts of carbon dioxide (CO2), which acts as a powerful attractant for many fly species. Flies possess specialized receptors that allow them to sense CO2 plumes, guiding them towards a living host. This cue is so effective that CO2 is often used as bait in artificial fly traps.
Body heat is another draw for flies. Flies sense the infrared radiation emitted by a horse’s body, helping them locate a potential host from a distance, even in low light conditions. Darker-colored horses may absorb and emit more heat, potentially making them more attractive to heat-seeking flies. The combination of CO2 and body heat guides flies to their target.
Visual Indicators and Moisture Sources
A horse’s physical presence attracts flies through visual cues. Their large size makes them an easily identifiable target. Movement, such as tail swishing or head tossing, draws attention, as flies are attracted to moving objects. Certain colors and contrasts can influence fly attraction; darker coats, for example, tend to attract more horse flies than lighter ones.
Flies are drawn to moisture sources on a horse’s face. Face flies, for instance, often congregate around a horse’s eyes and nostrils, seeking moisture, proteins, and nutrients found in ocular and nasal secretions. These secretions, including tears and mucus, provide a direct food source for non-biting flies and can become more prevalent with eye irritation, further attracting insects.