Horseflies, large and agile insects belonging to the family Tabanidae, are a common summertime nuisance due to their aggressive biting behavior. Only the females bite, requiring a protein-rich blood meal to produce their eggs, which they procure using blade-like mouthparts that slice the skin. These painful bites are immediately noticeable, unlike a subtle mosquito bite, and the persistent annoyance can significantly disrupt outdoor activities for people and livestock. Horseflies are also capable of mechanically transmitting serious diseases, such as tularemia and Equine Infectious Anemia (EIA). Controlling these pests requires a layered strategy that addresses both immediate protection and long-term population reduction.
Immediate Personal Protection
When moving into areas with high horsefly activity, the most immediate defense involves the strategic use of topical repellents and protective clothing. Repellents containing Picaridin are often more effective against biting flies than DEET. A 20% Picaridin formulation provides up to eight hours of protection and is favored because it is odorless, non-greasy, and does not damage plastics or synthetic fabrics.
While DEET is a proven repellent, its primary function is to confuse the insect’s olfactory senses, making it harder for the fly to locate a host. For maximum defense, consider treating clothing with Permethrin, a synthetic insecticide that repels and kills insects on contact. This treatment is intended for fabric only and is highly durable, often remaining effective through several dozen wash cycles.
Horseflies are drawn to large, dark, moving objects that absorb heat. Wearing light-colored, tightly woven fabrics, such as long-sleeve shirts and pants, makes the host less visible to the fly. This preference for dark colors is known as polarotaxis, where the flies are attracted to the polarized light reflected by dark surfaces.
Horsefly activity is highest during daylight hours, particularly from late morning through mid-afternoon, often peaking between 11:00 a.m. and 2:00 p.m. Limiting time spent outdoors near known habitats, such as marshes, ponds, and wet woodlands, during this window can minimize exposure. Since these flies avoid shade, seeking shelter in a covered or heavily shaded area during peak hours can offer temporary relief.
Trapping and Active Control Methods
Targeted trapping is a highly effective, non-chemical method for actively reducing the adult horsefly population, relying on their instinctive attraction to heat and movement. Commercial and DIY traps exploit the female fly’s need to find a large, warm-blooded mammal. These traps typically feature a dark, heat-absorbing object, like a black plastic ball or bucket, suspended in a sunny, open location.
The dark surface mimics the thermal signature of a host, drawing the female horsefly in for a blood meal. The most specialized traps, such as the funnel-style H-Trap, use a dark ball hanging beneath a conical hood. When the fly lands on the dark surface and fails to feed, its natural escape response is to fly vertically upward, where it is guided by the hood into a collection container from which it cannot escape.
Another successful technique involves coating a similar dark object with a non-toxic, heat-resistant, sticky glue. This method, often called a sticky trap, captures the fly upon landing. For safety, sticky traps should be placed away from where livestock or non-target wildlife, such as birds, might accidentally come into contact with the adhesive surface.
The use of insecticides for horsefly control is generally limited due to the flies’ behavior. Adult horseflies do not spend extended periods resting on surfaces, which makes residual insecticide sprays on barn walls largely ineffective. However, space sprays, fogs, or mists containing pyrethrins can provide a quick knockdown effect in enclosed areas. This is a temporary measure, as new flies can quickly migrate into the area.
Eliminating Attractants and Breeding Sites
The most sustainable method of long-term control is to disrupt the horsefly’s life cycle by eliminating or modifying their breeding habitats. Female horseflies lay their eggs on vegetation or objects that overhang moist environments, with the larvae dropping into the damp soil or water below. The larvae then live in wet mud, stagnant water, or decaying organic matter, making moisture control paramount.
Implementing proper drainage is essential for managing the environment. This includes fixing leaky water troughs and pipes, and filling in low-lying spots in pastures that collect rain and become boggy. Harrowing pastures to break up manure and aerate the soil also helps to dry out potential breeding sites.
While horsefly larvae do not typically breed in dry, composted manure, other nuisance flies do, making proper manure management a necessary step in a comprehensive control plan. Removing soiled bedding and manure from stalls daily and storing it in covered piles ensures the material heats up sufficiently to kill any developing larvae. This process significantly reduces the overall fly population on a property.
Physical barriers can also provide a fly-free zone for animals and people. Installing fine mesh screens or hanging curtains, often with a mesh size of 16×16 squares per inch, across barn and stable openings prevents large horseflies from entering. These barriers are effective because the large-bodied horseflies are not adept at navigating small openings.