The enjoyment of a backyard or patio can quickly be ruined by the persistent presence of biting flies, such as stable flies, deer flies, and biting midges. These insects inflict painful bites and can make outdoor activities unbearable. Successfully managing a biting fly population requires a multi-pronged strategy combining immediate personal protection with long-term environmental control to reduce the number of flies in your outdoor space.
Identifying Common Biting Flies and Their Habits
Recognizing the fly species responsible for painful bites is the first step toward effective control. Deer flies and horse flies (family Tabanidae) cause painful bites using scissor-like mouthparts to draw blood. Their larvae develop in moist soil, mud, and water edges, so these flies are typically found near wooded areas and wetlands. Stable flies look similar to house flies but have a distinctive piercing proboscis, primarily attacking ankles. They are common near livestock, breeding in decaying organic matter like wet straw, spilled feed, and manure.
Black flies are small and humpbacked, requiring fast-moving water sources like rivers or streams for larval development. They are daytime biters known to swarm around the head and face. Biting midges, also called “no-see-ums,” are tiny, sometimes less than a millimeter long, allowing them to pass through standard window screens. Their larvae thrive in damp, organic-rich environments such as coastal salt marshes and mud flats.
Immediate Personal Protection and Repellent Solutions
When immediate relief is needed outdoors, topical repellents are the most practical defense. The most effective synthetic ingredient is DEET, with concentrations between 20% and 30% providing several hours of protection. Picaridin is another highly effective synthetic option, often preferred because it is non-greasy and does not damage plastics. For a plant-based alternative, Oil of Lemon Eucalyptus (OLE), containing para-menthane-3,8-diol (PMD), is recommended and shows effectiveness comparable to low-concentration DEET.
Physical barriers offer a non-chemical means of protection in areas with high fly density. Wearing light-colored clothing is beneficial, as many biting flies, particularly horse and deer flies, are visually attracted to dark, moving objects. Long sleeves, pants, and head nets provide an effective barrier against persistent biters. Strategically placed fans can also clear a patio or deck area, as the weak flying ability of stable flies and midges prevents them from navigating against a strong breeze.
Active Population Control Methods
To actively reduce adult fly populations, specific trapping devices are required. For stable flies, which are attracted to visual cues and heat, specialized sticky traps should be placed low to the ground near breeding areas. Since stable flies are not attracted to sugary baits, visual and heat-mimicking traps are necessary. Deer flies and horse flies are attracted to large, dark, moving objects and are effectively managed with traps like the H-trap, which uses a dark, heat-absorbing ball beneath a funnel to lure and capture them.
Baited traps using fermentation or ammonia-based lures are effective for filth-breeding flies but less successful against blood-feeding species like stable or horse flies. Insect light traps, or bug zappers, use ultraviolet light to attract and electrocute flying insects. However, they are often less effective against primary biting flies and may unintentionally kill beneficial insects. Carbon dioxide (CO2) traps mimic the breath of a host and can attract black flies and midges, drawing them away from populated areas for capture.
Eliminating Outdoor Breeding Sites
The most sustainable way to reduce biting fly populations is to target larval habitats before they mature into biting adults. Stable flies rely on moist, decaying organic matter, so maintaining a clean and dry outdoor environment is paramount. This involves regularly turning compost piles, removing grass clippings and spilled feed, and ensuring manure piles are either properly composted or spread thinly to dry quickly.
For agricultural properties, feed-through larvicides containing insect growth regulators can be mixed into animal feed. These specialized larvicides pass through the animal, preventing fly larvae from developing in the manure and breaking the stable fly life cycle. For biting midges, whose larvae live in saturated soil, maintaining proper drainage and addressing standing water is crucial.
While horse and deer fly breeding sites in sensitive wetlands are difficult to eliminate, managing organic debris at the edges of ponds and streams can help reduce their numbers. In specific areas where water cannot be drained, larvicides such as Bacillus thuringiensis israelensis (Bti) can be applied. Bti targets the larvae of midges and black flies without harming other wildlife.