Black flies are tiny, biting gnats, often called “buffalo gnats.” Females require a blood meal to produce eggs. These small insects are known for swarming, especially around the face and head. Their painful bites are caused by a scissor-like mouthpart that lacerates the skin to feed on blood. The resulting wounds can be slow to heal and cause discomfort due to the anticoagulant in their saliva.
Immediate Personal Protection
The most immediate strategy for avoiding black fly bites is to apply an effective personal repellent to your exposed skin and clothing. Repellents containing DEET (N,N-diethyl-meta-toluamide) are widely considered the standard for protection against biting insects, including black flies. Picaridin, a synthetic compound derived from the pepper plant family, offers a comparable level of effectiveness and is a good alternative, as it does not dissolve plastics or synthetic materials like DEET can.
For those preferring a botanical option, Oil of Lemon Eucalyptus (OLE), which contains the active ingredient p-menthane-3,8-diol (PMD), is registered by the Environmental Protection Agency (EPA) and is effective. While OLE can provide protection similar to low concentrations of DEET, it may require more frequent reapplication to maintain its efficacy over several hours. You can also treat clothing with permethrin, an insecticide that repels and kills black flies on contact, but this product should never be applied directly to the skin.
Beyond chemical protection, adjusting clothing and activity timing offers a strong physical defense. Black flies are attracted to dark colors and carbon dioxide, so wearing light-colored clothing can make you a less appealing target. Opt for long sleeves, long pants, and a hat, ensuring cuffs are tightly secured to prevent flies from crawling underneath the fabric.
Outdoor activities should be timed to avoid the black fly’s peak activity, which typically occurs during the early morning hours (9:00 and 11:00 AM) and again in the late afternoon (4:00 to 7:00 PM). Black flies are also less active when the air temperature drops below 50°F or when there is a strong wind, as they struggle to fly in breezy conditions. Seeking out dry, sunny, or windy locations during the midday hours can provide a natural reprieve from swarming activity.
Managing Flies in the Home and Yard
Creating physical barriers is the most reliable way to prevent adult black flies from entering your living spaces. Ensure that all window and door screens are fully intact, as even small tears can allow these tiny gnats to slip through. Utilizing air conditioning is a passive control method, as it allows you to keep doors and windows closed, eliminating entry points and maintaining an environment less hospitable to the flies.
Since black flies can travel up to 10 miles from their breeding source, efforts to eliminate them in a large yard are generally ineffective. Unlike mosquitoes, adult black flies are not typically drawn to static light or traditional fly traps in a way that provides meaningful control. Instead, focus on using physical exclusion measures, such as fine-mesh netting, to cover outdoor eating areas or porches during peak season.
Manipulating the immediate environment using wind can also deter them from congregating near your home. Strategically placed fans on patios or decks can generate enough air movement to prevent black flies from landing and swarming, as they are poor fliers in even moderate breezes. Because they are drawn to odors and visual cues, keeping the yard free of compost piles or other strong scents that might attract general gnats and flies can also reduce the overall nuisance population near the house.
Addressing the Source: Larval Control
The most effective, long-term solution to managing a black fly population involves targeting the aquatic larval stage at its source. Black flies breed exclusively in flowing water, such as streams, rivers, and irrigation ditches, where the larvae attach themselves to submerged rocks and vegetation. The presence of black flies often indicates clean, unpolluted water, as the larvae are filter feeders that require moving water to bring them organic matter for sustenance.
Larval control is primarily accomplished through the application of Bacillus thuringiensis israelensis (Bti), a naturally occurring, spore-forming soil bacterium. Bti is a microbial insecticide that is highly selective, meaning it only targets a narrow range of insects, specifically the larvae of black flies and mosquitoes. It is considered safe for humans, mammals, birds, fish, and most other aquatic organisms, making it an environmentally acceptable choice for water treatment.
The control agent works when black fly larvae, which are actively feeding on particles in the flowing water, ingest the Bti spores. In the alkaline digestive system of the larva, the Bti releases protoxins that become activated, rapidly destroying the lining of the midgut. This action causes paralysis and mortality in the targeted larvae before they can mature into biting adults.
While large-scale Bti application programs are typically managed by municipal or state agencies using aerial or boat spraying, homeowners may be able to treat small, contained water sources on their private property. The Bti product must be applied directly to the flowing water where the larvae are present and often requires continuous or weekly treatments to be effective throughout the black fly season. Treating a small stream or irrigation channel with a commercially available Bti product can significantly reduce the local adult population.