Michigan’s combination of rivers, wetlands, and Great Lakes shoreline creates ideal habitat for several species of biting flies. Depending on where you are in the state and what month it is, you could encounter black flies, deer flies, horse flies, or stable flies, each with different peak seasons, habitats, and biting behaviors. Here’s what to expect and how to protect yourself.
Black Flies: May Through September
Michigan is home to over 65 species of black flies, with the highest concentrations in the northern part of the state where clean rivers and streams are plentiful. These small, stout, humpbacked flies breed exclusively in clean, well-oxygenated moving water, so you’ll find them heaviest near rivers and streams rather than still ponds or lakes.
Peak black fly season runs from mid-May to mid-June, but with so many species staggering their life cycles, you can encounter them well into August and early September in the right areas. Each adult fly lives only two to three weeks, but successive waves of hatching keep the population going through the warm months. Only the females bite. Males feed on nectar and leave you alone.
Black flies behave differently from mosquitoes in a few useful ways: they don’t come indoors, they’re only active during daylight hours, and they can’t bite through clothing. Their bites, however, cause painful itching and swelling that can last several days or even weeks. The National Park Service recommends head nets during black fly season at places like Pictured Rocks National Lakeshore, where swarms can be intense in spring.
Deer Flies and Horse Flies: Mid-Summer Biters
As black fly numbers taper off, deer flies and horse flies pick up. Deer flies are most active in June and July, while both species can appear anytime from May through September. You’ll find them near swamps, marshes, pond and stream banks, and forest edges. Deer flies in particular patrol the borders of broken woodlots looking for a meal.
These flies bite very differently from black flies or mosquitoes. Females have broad, blade-like mouthparts that slash the skin open rather than piercing it with a needle-like probe. The result is a deep, painful wound that bleeds considerably. The flies then lap up the blood with sponge-like structures on their mouths. They also inject a substance that keeps the blood flowing, which contributes to the bleeding and irritation at the bite site. You’ll know the difference immediately: a deer fly or horse fly bite hurts on contact.
Deer flies are recognizable by their brightly colored eyes and patterned wings. Horse flies are larger, drab brown or black, sometimes with light spots, and have clear wings. Both fly in fast, noisy circles around you before landing to bite. At places like Pictured Rocks, deer flies tend to stay on shore while horse flies will follow you right into the water.
Stable Flies on Great Lakes Beaches
Stable flies are the main biting fly on Michigan’s Lake Superior and Lake Michigan beaches from mid to late summer, especially on warm, humid days with a south wind. They look like small house flies but deliver a sharp, painful bite, typically targeting your legs and ankles. Unlike most biting flies, both males and females bite.
Their larvae develop in decaying vegetation that washes up along the shoreline, which is why beaches are their primary territory. On bad days, stable flies can be numerous enough to drive people off the beach entirely.
The critical thing to know about stable flies is that insect repellent does not work on them. DEET, picaridin, and other standard repellents have no meaningful effect. Your best defense is physical: loose-fitting long pants and thick socks when they’re active. If you’re planning a beach day in July or August and the wind is blowing from the south, be prepared for the possibility that stable flies will be out in force.
What Works (and Doesn’t) for Repellent
Repellent effectiveness varies dramatically by fly species. DEET works well against black flies and biting midges but does not work well against deer flies, horse flies, or stable flies. Picaridin performs similarly to DEET for mosquitoes and biting midges, and some products are labeled for biting flies and chiggers, but no repellent truly works well on deer flies, horse flies, or stable flies.
Permethrin, which is applied to clothing rather than skin, creates a toxic contact effect on ticks and some insects. Treating your shirts, pants, and socks with permethrin can add a layer of protection, though it works more as a contact irritant than a true repellent. For deer flies and horse flies specifically, physical barriers like long sleeves, hats, and head nets are more reliable than any chemical you can spray on your skin. Light-colored clothing also helps, since dark colors and movement attract these flies.
How Fly Bites Feel and How to Treat Them
Black fly bites often don’t hurt much at first but develop into intensely itchy, swollen welts that can persist for days or even weeks. Deer fly and horse fly bites hurt immediately and bleed more because of how the flies cut the skin open. Stable fly bites feel like a sharp pinch, usually concentrated around the ankles.
For any fly bite, wash the area with soap and water first. Any break in the skin is an entry point for bacteria, so cleaning the site promptly matters. Apply a cold compress (ice wrapped in a thin towel) to reduce swelling and pain. Over-the-counter anti-inflammatory pain relievers can help with discomfort. For itching, topical options include hydrocortisone cream, antihistamine cream, or pure aloe vera.
Avoid scratching. Fly bites that are scratched open heal slower and are more prone to secondary infection. If a bite becomes increasingly red, warm, or swollen over several days rather than improving, that could signal an infection that needs medical attention.
Disease Risk Is Low but Not Zero
North American black flies do not transmit disease to humans, and stable flies aren’t known to transmit disease to people either. Deer flies, however, can theoretically transmit tularemia, a bacterial infection sometimes called “rabbit fever.” In practice, this is extremely rare in Michigan. Only 6 cases of tularemia were reported in the state between 2011 and 2020, though the number has ticked up slightly, with 8 cases between 2021 and 2023. Most tularemia cases in the U.S. occur in the South Central region, not the Midwest. The primary concern with fly bites in Michigan remains pain, swelling, and itching rather than infectious disease.
Timing Your Outdoor Activities
If you’re planning a trip to northern Michigan, the Upper Peninsula, or the Lake Michigan shoreline, knowing the seasonal overlap of biting flies helps you prepare. May and early June bring the heaviest black fly activity, especially near rivers. Late June and July are peak deer fly season. Mid to late summer brings stable flies to beaches and horse flies to wooded lakeshores. By late September, most biting fly activity has ended for the year.
Black flies and deer flies are daytime feeders, so dawn and dusk don’t provide the relief they do with mosquitoes. Early morning and late evening can be slightly less intense, but don’t count on it. For beach trips, wind direction matters more than time of day. A north wind keeps stable flies down, while a warm south wind brings them out. Checking the forecast before heading to the shore can save you a miserable afternoon.