The black fly (family Simuliidae) is a common biting insect that acts as a significant nuisance to anyone spending time outdoors in Maine. Only the female black fly bites, requiring a blood meal for egg development, while the males feed exclusively on flower nectar. These small, dark insects are notorious for swarming around the head, attracted to the carbon dioxide exhaled by mammals. Their bites can cause localized irritation and swelling, and their sheer numbers during peak activity make them a persistent concern.
Defining the Season’s Timeline
The black fly “season” in Maine has evolved significantly, transitioning from a brief late spring event to a presence that can span much of the warm weather months. Historically, the season was confined to a few weeks from the end of May into early June. Now, improvements in water quality, which allow more of the state’s 40-plus species of black flies to thrive, and warmer weather patterns have lengthened their activity window.
The initial surge, or primary season, generally begins in late April or early May, immediately following the snowmelt and the subsequent rise in water temperature. This initial population peaks from late May through mid-June, particularly in the southern and central parts of the state. The life cycle accelerates once the water thaws, as the larval stage requires clean, fast-moving water.
While the most concentrated swarming may subside by early July, different black fly species emerge throughout the summer, sustaining a population until the first hard frost. This continuous presence means outdoor activities must account for black flies until the autumn chill. The variability of the season is directly linked to annual weather patterns, with wetter and warmer years extending the period of high activity.
Geographic Hotspots and Severity
The concentration and severity of black flies are highly dependent on local geography and the availability of their specific breeding habitat. Since larvae must develop in clean, running water, the highest populations are consistently found near streams, rivers, and lake outlets. Consequently, the heavily wooded, inland areas of Maine, especially the Northern and Western regions, experience the most intense and sustained infestations.
Conversely, coastal regions and large, open bodies of water generally have lower black fly populations. The insects prefer to remain near their running-water breeding sites, and strong winds common along the coast help disperse the poor fliers, reducing local density. When a female black fly bites, the reaction can range from minor irritation to localized swelling and itching.
Mitigation and Protection Strategies
Employing a combination of chemical and physical barriers is the most effective approach to managing black flies during their active periods. Repellents containing DEET are highly effective, but studies show that concentrations over 30% do not significantly increase protection time and carry a greater risk of skin irritation. Picaridin is an effective alternative active ingredient.
Clothing can be treated with Permethrin, a contact insecticide meant for fabric only, not direct skin application. Combining permethrin-treated clothing with a DEET or Picaridin application on exposed skin creates a formidable barrier against biting pests.
Wearing appropriate clothing acts as a physical defense, as black flies cannot bite through fabric. Long-sleeved shirts and pants, tucked in at the cuffs, are highly recommended. Since black flies are attracted to dark colors, wearing light-colored clothing is preferable, and a head net provides immediate relief. Behaviorally, avoid outdoor activity during peak biting hours (late morning and early evening) and stay away from wooded stream banks.