Horse flies, members of the Tabanidae family, are a common summertime nuisance known for their painful bite. The female fly uses blade-like mouthparts to slice the skin open, creating a small wound from which to lap up blood. This mechanism causes immediate discomfort and often results in a raised, irritated welt. Understanding the factors that draw these insects to humans is the first step in effective repulsion.
Understanding Horse Fly Attraction
Female horse flies rely on sensory cues to locate a host. They are highly visual hunters, primarily attracted to large, dark-colored objects that stand out against the horizon. This preference stems from the fact that dark surfaces reflect horizontally polarized light, mimicking the appearance of large mammals they typically feed on.
Movement is another significant visual trigger. Once close, the flies switch to chemical and thermal detection methods. They are drawn to exhaled carbon dioxide (CO2) and the heat signature emitted by a warm body. Chemical compounds in sweat and body odor also make a person an attractive target.
Recommended Chemical Repellent Options
The most reliable method for repelling horse flies involves using EPA-registered chemical repellents applied directly to exposed skin and clothing. \(\text{N},\text{N}\)-Diethyl-meta-toluamide, commonly known as DEET, is a highly effective synthetic repellent against biting flies. Products containing DEET are generally recommended in concentrations up to 30%, which provides extended protection. Higher concentrations are not proven to offer substantially better protection and should be avoided.
Picaridin (or Icaridin) is a synthetic compound that offers comparable efficacy to DEET but is often preferred because it is nearly odorless and feels less oily on the skin. A 20% concentration of Picaridin can provide up to 12 hours of protection against biting flies and is safe for use on adults and children over two months old. Unlike DEET, Picaridin does not typically damage plastics or synthetic fabrics.
Oil of Lemon Eucalyptus (OLE), which contains the active ingredient p-Menthane-3,8-diol (PMD), is a plant-based, EPA-registered option that can also repel biting flies. While effective, OLE-based products may need more frequent reapplication than DEET or Picaridin for sustained protection. Some OLE products are not recommended for use on children under three years of age, so consulting the product label is always necessary.
When applying any repellent, safety guidelines must be followed strictly. Repellents should never be applied to cuts, wounds, irritated skin, or near the eyes and mouth. For children, apply the product to your own hands first, then rub it onto their exposed skin, taking care to avoid their hands, which may transfer the chemical to their face.
Physical Prevention and Natural Alternatives
Physical barriers offer an effective way to minimize exposed skin and reduce the visual cues that attract horse flies. Wearing long-sleeved shirts and long pants made of thick material can prevent bites, as their powerful mouthparts may be able to penetrate thin fabrics. Clothing color is also a major factor, as light-colored clothing is significantly less attractive to horse flies than dark colors like black or dark blue.
Reducing rapid movement can also help, particularly in areas where horse flies are prevalent, as fast motion is a strong visual cue for a potential host. Additionally, avoiding outdoor activities near marshy areas, ponds, and fields during the flies’ peak feeding times, typically the mid-day heat, can limit encounters.
Various non-regulated essential oils, such as citronella, lavender, and peppermint oil, are often cited as natural alternatives. While anecdotal evidence exists, these natural options generally provide only short-term or limited protection when applied topically. Lavender oil offered the best repellent effect against horse flies compared to some other plant-based oils, but even this requires very frequent reapplication for any sustained benefit. For reliable, long-lasting protection, regulated chemical repellents remain the superior choice.