Black flies are common across Maine, particularly in wooded and watery environments. These small, dark insects, sometimes called “buffalo gnats,” are found near clean, fast-running water, which serves as their breeding grounds. While Maine is home to many species, not all feed on human blood. Their presence indicates healthy, unpolluted waterways.
Peak Seasons and Activity Factors
Historically, black fly activity in Maine was concentrated from late May through the first couple of weeks of June. However, due to climate shifts, their presence can now extend throughout the entire summer. Black fly larvae develop in clear, flowing streams, hatching after the spring thaw when water temperatures begin to rise. Adult black flies typically become active when the air temperature reaches at least 50 degrees Fahrenheit.
These insects are most active during daylight hours, often peaking in the morning and late afternoon. Strong breezes tend to disperse them, and they are less prevalent on bright, sunny days. Black flies prefer cloudy, humid conditions and may become more active just before a storm. Adult black flies can travel several miles from their breeding sites in search of a blood meal.
Understanding the Nuisance
Only female black flies bite, as they require a blood meal to nourish their developing eggs. They use blade-like mouthparts to cut a small hole in the skin, feeding on the blood that seeps out. The resulting pain, swelling, and itching are reactions to the fly’s saliva, which contains anticoagulants and pain-killing compounds. A black fly bite often appears as a reddened welt with a distinct, visible wound at its center.
Black flies are attracted to the carbon dioxide humans exhale, as well as sweat, certain fragrances, and dark-colored clothing. They frequently swarm around exposed skin, especially targeting the head, neck, and ears. While their bites can be painful and irritating, black flies in Maine are not known to transmit diseases to humans.
Minimizing Encounters
Wearing appropriate clothing provides an effective defense against black flies. Opting for light-colored clothing, long sleeves, long pants, and a hat can reduce exposure, as black flies are attracted to dark colors and can crawl under loose garments. Tucking pants into socks and wearing fine-mesh head nets offer additional protection, particularly in areas with high black fly populations.
Insect repellents containing active ingredients such as DEET or picaridin can be used. Picaridin is often favored over DEET, which some sources suggest may be less effective or even attract them. Permethrin-treated clothing can also provide a barrier, but this chemical should only be applied to fabric, not directly to skin.
Timing outdoor activities can also help reduce encounters. Avoiding dense wooded areas or locations near streams during peak activity times, such as late morning and early evening, can also contribute to a more comfortable outdoor experience. Engaging in continuous movement, such as while rafting or hiking, can also help individuals outpace these persistent insects.