Horse flies (family Tabanidae) are biting insects. Only the female bites, requiring a blood meal to gain the protein necessary for producing eggs. Because these pests are strong fliers and can transmit diseases, understanding how to avoid them is important. This guide provides practical strategies for personal protection, area control, and livestock management.
Understanding What Attracts Horse Flies
Horse flies locate hosts using sensory cues, primarily relying on vision. They are highly attracted to large, dark, moving objects that stand out against the horizon. Their visual system perceives dark colors as a large silhouette of a potential host animal.
Female horse flies are also drawn to thermal signatures, seeking out the body heat radiating from warm-blooded animals. They prefer sunlit and warmer targets, which explains their daytime activity pattern. The combination of visual attraction to dark objects and preference for warmth makes a dark shirt on a sunny day an irresistible target.
Beyond sight and heat, horse flies detect chemical signals in the air. They are attracted to carbon dioxide emitted from breath and sweat, which contains compounds like ammonia. These chemical cues signal the presence of a host, guiding the fly to the immediate area.
Personal Protection Strategies
Selecting the right clothing is the first line of defense against horse flies, as they are visually attracted to dark contrasts. Opting for light-colored clothing, such as white, tan, or light gray, reflects heat and makes you less visible. Wearing thick, tightly woven fabrics that cover exposed skin—including long sleeves and long pants—provides a physical barrier the fly’s mouthparts cannot easily penetrate.
Topical repellents offer another layer of protection by confusing the fly’s olfactory system. Repellents containing p-Menthane-3,8-diol (PMD), derived from oil of lemon eucalyptus (OLE), are effective options. Picaridin is another synthetic compound that demonstrates good efficacy against biting flies.
While DEET provides some protection, its effectiveness against horse flies varies compared to its use against mosquitoes. Permethrin-based products are designed to be applied to fabric only, offering a long-lasting deterrent effective through multiple washes. Strategically timing your outdoor activities can also reduce encounters, as horse flies are most active during the middle hours of the day.
Environmental and Area Control Methods
Area control focuses on reducing the local population by exploiting the fly’s natural behavior. Traps are a highly effective, non-chemical method, such as pyramidal or canopy-style traps (Manitoba traps). These systems use a large, dark, heat-absorbing object to mimic an animal, attracting flies based on visual and thermal preferences.
Once a fly lands on the dark decoy, its instinct is to fly upward toward the light, where it is funneled into a collection container. Placing these traps in sunny, open areas where flies are abundant maximizes effectiveness. Sticky traps using a dark or blue panel can also intercept flies in high-traffic areas.
Habitat modification is a long-term strategy that targets the fly’s life cycle. Larvae develop in semi-aquatic environments, such as the muddy edges of ponds, marshes, and saturated vegetation. Eliminating or draining standing water, improving soil drainage, and fencing off marshy zones reduce available breeding sites. Proper management of manure and organic material, ensuring it dries quickly, also removes potential larval habitat.
Protecting Livestock and Domestic Animals
Horse flies cause stress, reduced weight gain, and mechanical transmission of diseases in livestock. Animal-specific topical insecticides are the primary chemical defense, often containing pyrethrins or synthetic pyrethroids like permethrin. These products must be applied frequently, sometimes daily, because the flies are intermittent feeders and the insecticides break down quickly in sunlight.
Physical barriers include fly sheets, masks, and leg wraps, which shield the animal from bites. Some barriers use striped patterns, similar to the natural defense of zebras, to disrupt the fly’s visual attraction. Providing animals with access to a stable or shaded shelter during peak daylight hours is an effective environmental control method, as horse flies prefer bright, sunny conditions.
In barns, high-velocity fans can be installed to create air movement that deters horse flies from landing. While traps are effective, placing them near animal housing, rather than directly next to it, draws flies away from the livestock. Specialized pour-on treatments, which differ from sprays, may also be used in cattle and horses for longer-lasting protection.