Biting flies, including mosquitoes, deer flies, horse flies, and stable flies, can quickly ruin time spent outdoors. The female of the species requires a blood meal to develop her eggs, driving their relentless search for a host. Protection relies on understanding how these pests locate a target and utilizing a combination of chemical and physical defenses to disrupt their hunt.
Understanding How Biting Flies Find Us
Biting insects track down a host using a multi-stage sensory process. Initial long-range detection is triggered by the plume of carbon dioxide (CO2) we exhale. This gas signals the presence of a warm-blooded animal up to 50 meters away, initiating their upwind flight toward the source.
As the fly gets closer, it integrates other sensory cues. Specific body odors, such as lactic acid and volatile compounds in human sweat, become stronger attractants at medium range. Once within a few feet, the insect uses thermal sensors to detect body heat, pinpointing a precise landing spot for the bite.
Visual cues also play a significant role, particularly for deer flies and horse flies. These insects are strongly drawn to dark colors, high contrast, and movement, making a person wearing a black shirt an easier target to spot. Repellents primarily work by interfering with the chemical detection stage, masking the odors that confirm the host’s presence.
The Most Effective Synthetic Repellents
The gold standard for topical protection remains N,N-Diethyl-meta-toluamide, commonly known as DEET. It is effective against a wide range of biting pests, including mosquitoes and various flies. The concentration of DEET directly correlates with the duration of protection, not the level of efficacy.
For instance, a product with a 24% DEET concentration can provide approximately five hours of protection, while a 10% concentration may only last about two hours. Concentrations higher than 50% do not offer a significantly longer period of effectiveness. Users should apply just enough to cover exposed skin and wash it off when returning indoors, being mindful that DEET can damage some synthetic fabrics and plastics.
A highly effective alternative is Picaridin (also known as Icaridin), a synthetic compound. Picaridin offers protection comparable to DEET but has several distinct advantages. It is virtually odorless, feels less greasy on the skin, and does not damage gear, plastics, or synthetic materials.
For maximum duration, Picaridin formulations are most effective at a 20% concentration, providing protection for up to eight hours against flies. Permethrin is a synthetic insecticide intended for treating clothing, tents, and gear, never for direct skin application. Permethrin bonds to the fabric and, once dry, works as a contact killer and repellent, often remaining effective through several wash cycles.
Utilizing Plant-Based and Natural Alternatives
For those preferring naturally derived options, Oil of Lemon Eucalyptus (OLE) is the only plant-based ingredient recommended by public health organizations for its proven efficacy. The active component is p-Menthane-3,8-diol (PMD), often synthesized for commercial products. OLE/PMD products offer protection comparable to low-concentration DEET formulations, typically lasting up to six hours. Note that OLE extract is chemically distinct from pure, unrefined lemon eucalyptus essential oil, which lacks rigorous testing for safety and effectiveness.
Other essential oils, such as citronella, geraniol, and cedarwood oil, are common in natural repellent products. While they possess repellent properties, their duration of effectiveness is significantly shorter compared to PMD or synthetic options. These volatile oils require frequent reapplication when used topically to maintain a protective barrier. Studies on geraniol, for instance, showed higher repellency rates against mosquitoes and sand flies than citronella.
Physical Barriers and Environmental Control
Non-chemical methods provide an important first line of defense against biting flies. Clothing choices significantly influence visibility, as flies are attracted to high contrast and dark colors that absorb heat. Wearing light-colored garments, such as white, khaki, or light gray, makes a person less conspicuous to species like horse and deer flies.
Tightly woven fabrics, such as denim or certain synthetics, also serve as a physical barrier that biting insects cannot penetrate. Fine-mesh netting over hats or face screens can provide immediate relief in areas where flies are numerous.
Environmental strategies focus on disrupting the fly’s life cycle and habitat. Stable flies breed in moist, decaying organic matter, so diligent removal of wet hay, silage, and manure is crucial for population control. Specialized traps designed for deer and horse flies often use visual lures, such as a large black object or fiberglass panel, which the flies mistake for a host. These traps capture the flies when they follow their attraction to the heat and color.