Black flies are small, dark, biting insects that can make spending time outdoors miserable during their active season. Females require a blood meal to produce eggs, using specialized mouthparts to cut the skin and feed on pooling blood. These flies are most active during the daylight hours, often peaking in the late morning and again in the late afternoon. Dealing with swarms requires a combination of chemical and physical methods to enjoy time outside.
Synthetic Repellents for Direct Application
The most powerful tools for preventing black fly bites are synthetic chemical repellents applied directly to the skin and clothing. N,N-Diethyl-meta-toluamide, known as DEET, is highly effective against a wide range of biting insects. Repellents containing DEET must be applied correctly to block the flies, which often crawl under clothing to bite.
Applying repellent around the seams of clothing, such as the cuffs, ankles, and necklines, helps prevent black flies from crawling into covered areas. A newer synthetic option, Picaridin, provides an alternative that is highly effective against biting flies, often outperforming DEET. Picaridin offers protection without the strong odor or potential to damage synthetic materials that DEET possesses.
For maximum protection, look for Picaridin concentrations around 20%, which can provide up to eight hours of protection against flies. Treating clothing and gear with Permethrin is another synthetic option. Permethrin is a repellent and insecticide applied only to fabrics, not skin, that binds to the material and remains effective through several washings or for up to six weeks. This treatment creates an invisible, long-lasting barrier that repels and kills black flies on contact.
Natural and Area Deterrents
Several plant-derived compounds offer proven, though often shorter-duration, protection against black flies. The most effective natural repellent is Oil of Lemon Eucalyptus (OLE), which contains the active ingredient p-menthane-3,8-diol (PMD). The Centers for Disease Control and Prevention (CDC) recommends OLE as an effective option against biting insects, including black flies.
Repellent products containing OLE, typically at a concentration of 30%, can provide a mean protection time of about two hours. While effective, OLE products require more frequent reapplication than their synthetic counterparts to maintain a continuous barrier. Other essential oils, such as citronella, peppermint, and cedarwood, may offer some localized, temporary relief.
These oils are sometimes incorporated into area deterrents like outdoor candles, coils, or diffusers. These products release a concentrated scent or smoke that can disrupt the flies’ ability to locate a host in a small, localized space, such as a porch or patio. Black flies are not attracted to light, meaning electric bug zappers are ineffective against them.
Protective Clothing and Physical Barriers
The simplest and most immediate defense against black flies involves using physical barriers to prevent them from reaching the skin. Wearing long-sleeved shirts and long pants significantly reduces the available surface area for biting. For the most complete coverage, pants should be tucked into socks or boots, and buttoned shirts should be avoided in favor of those with zippers, which eliminate small entry points.
The color of clothing matters, as black flies are strongly attracted to dark colors. Choosing lighter-colored attire, such as whites, grays, or light yellows, makes the wearer less visually appealing to the flies. In areas with severe infestations, a head net is a highly effective physical barrier to protect the head and neck.
These specialized nets, often made of fine mesh, prevent the flies from swarming the face, ears, and eyes, which are common targets. Some outdoor enthusiasts opt for full mesh jackets or shirts, which are designed to be worn over regular clothing, providing complete coverage and ventilation while physically blocking the insects.