In most of the north, black flies arrive in spring, in noticeable numbers around mid-May in many areas.1EcoSpark. Life After Water Series: Life of the Black Fly The season isn’t tied to a calendar date. It follows the warming of rivers and streams, so it shifts from year to year and from place to place.
What Sets Off Black Fly Season
Black flies spend the winter as larvae (the young, worm-like stage), living in cold streams and feeding on organic matter in the water.2Nature Up North. April Showers Bring May Black Flies To get through the cold, the larvae slow their growth and metabolism.3Mosquito Magnet. Where Do Mosquitoes, Midges & Black Flies Go in Winter? When temperatures rise in early spring, they turn into pupae and then into winged adults.4Nature Up North. April Showers Bring May Black Flies
Specific water temperatures act as the starting signal. Development begins when river water passes certain temperatures, around 4–5°C (about 40°F) and again near 10°C (50°F), depending on the species.5PubMed Central. The Relationship between Water Temperature and the Development Cycle Beginning and Duration in Three Black Fly Species Streams most often reach those temperatures in March and April.6PubMed Central. The Relationship between Water Temperature and the Development Cycle Beginning and Duration in Three Black Fly Species A warm March and April brings an earlier hatch, and a cold spring delays it.7PubMed Central. The Relationship between Water Temperature and the Development Cycle Beginning and Duration in Three Black Fly Species
A late snowmelt keeps streams cold and pushes the hatch back, while an early thaw can bring biting flies in early May. Locals often judge the start by when leaves and lilacs come out, since those also track spring warmth.
There is also a gap between the hatch and the biting. Larvae need several weeks of growth after the water warms, then comes the pupal stage, and only then do adults rise to the surface.
The same biology explains why black flies stay near running water. Clean, fast streams, lake outlets and brownish forest streams can produce huge numbers. Ponds and still water are mosquito habitat, not black fly habitat, so living near a river or stream means more flies.
Black flies don’t all start at once. Species that begin developing at the lower temperature threshold tend to appear first, and those that need warmer water follow. The first adults you notice are usually a few flies circling your head near a stream or trail, and heavy swarms come days or weeks later, once more of the population has finished developing.
How Long Black Fly Season Lasts
In many northern areas, black flies typically appear in mid-May and die off once spring creek runoff is over.8EcoSpark. Life After Water Series: Life of the Black Fly In a typical year, the worst stretch runs from mid-May into June. Many northern species produce only one generation a year, which explains the single, concentrated spring burst.9PubMed Central. Ratio changes in blood-feeding and voltinism in black flies (Diptera: Simuliidae) over a latitudinal gradient in North America north of Mexico
Farther south, more species produce several generations a year, so flies can keep appearing into summer.10PubMed Central. Ratio changes in blood-feeding and voltinism in black flies (Diptera: Simuliidae) over a latitudinal gradient in North America north of Mexico Some reports suggest seasons are getting longer as temperatures and rainfall rise, though firm data are limited.11Substack. When Will Black Fly Season End?
Even in one place the season tends to feel like waves. Different species hatch at different water temperatures, so their peaks don’t line up, and each species emerges over a few weeks, so the peaks overlap. Numbers can drop and then surge again after rain.
Several signs show it is winding down. Streams drop and slow as snowmelt ends, and hot, dry weather reduces the number of biting flies. Swarms thin out in late June across much of the north, and fishing and hiking get easier in July. The end usually comes gradually rather than on a particular day.
A wet spring keeps streams running longer, which can extend the season, and dry years can shorten it. Plan trips with a few weeks of leeway rather than a fixed date.
Even in a long season, the flies are rarely equally bad everywhere. A trail far from running water can be quite bearable while a stream bank a mile away is miserable. The flies that remain in summer are often a different set of species than those in May. They may be less intense, but they can still bite, so repellent and head nets stay useful in damp, shady spots.
How Long a Single Black Fly Lives
Adult females usually live one to three weeks, depending on the species and the weather.12Metropolitan Mosquito Control District. Black Fly Life Cycle They survive best in cool, wet weather.13Metropolitan Mosquito Control District. Black Fly Life Cycle Some estimates put the adult lifespan as long as three to six weeks.14Black Hills Corporation. Black Fly Control
The season outlasts each fly because new adults keep emerging from the streams for weeks. Replacements arrive constantly, so killing the flies around you doesn’t end the swarm.
Only females bite. Both sexes feed on nectar, but females also seek blood to produce their eggs.15EcoSpark. Life After Water Series: Life of the Black Fly The blood supplies protein for developing eggs, so bites come in a short window of her life. After feeding, she lays her eggs in or near running water. They hatch into larvae, and the cycle continues in the same streams. Because each female bites during only part of a life lasting weeks, the number of flies you meet depends more on how many are emerging at that moment than on how long any single fly survives.
Worst Times of Day and Weather
Light levels are the main thing that controls when black flies fly, and wind speed comes second.16PubMed. Effect of weather conditions on the flight activity of Nigerian blackflies (Diptera:Simuliidae) Humidity matters least.17PubMed. Effect of weather conditions on the flight activity of Nigerian blackflies (Diptera:Simuliidae) Calm, mild-light parts of the day bring the most flies: the transition hours around dawn and dusk, and overcast days, can produce plenty of activity. Bright, hot midday sun is less attractive to them than mild light, so on a warm sunny afternoon you may notice fewer flies than on a cool, cloudy one.
Cold and wind keep them down. In one study, fewer black flies were active on cold mornings.18CrossRef API / Journal of Avian Biology. Does weather affect biting fly abundance in avian nests? In the same study, morning wind reduced their numbers, though evening wind did not.19CrossRef API / Journal of Avian Biology. Does weather affect biting fly abundance in avian nests? A cold, breezy morning is the best time for outdoor work in fly season. Small flies struggle to fly in a breeze, and a breeze also disperses the breath and odor plumes they follow to find you.
You can plan your day around this. Get yard work done during breezy midday hours, and expect swarms on still mornings and toward evening. Black flies rarely come indoors or bite at night, unlike mosquitoes. Open, windy spots like docks are better than sheltered woods.
Rain is a common point of confusion. Light rain doesn’t always stop them, because they like cool, damp conditions. Heavy rain and strong wind do keep them down.
On a bad morning or evening you notice flies circling your head and landing around your hairline, ears and neck rather than a single bite here and there. They are small and quiet, so many people feel the bite before they realize flies are present. Watch for specks landing on your hands, arms and clothing. Black flies often crawl before they bite, so they can get under collars and cuffs without being noticed.
Why Timing Differs by Region
In Maine, the far north runs about two weeks behind the coast in spring.20Maine.com. Maine Black Fly Season: Timing & Trip Planning A trip to northern woods in late May can hit the peak while the coast is easing off. Elevation works the same way, with higher, colder streams warming later.
Ask local outfitters or campgrounds about this year’s conditions. In mountain and northern areas the season can run into July, while southern and coastal areas peak earlier. The same species can be weeks apart in two towns.
A place still holding snow or cold streams is behind one with leaves out. Coastal areas warm earlier than inland areas at the same latitude, so a trip that is fine on the coast can hit peak flies a short drive inland. If a trip crosses a range of heights, low valley streams may already be past peak while high-elevation streams are just starting to produce flies.
Some places simply have far more flies. Larvae need current to bring food past them, so streams with a steady flow and rocks to cling to are better habitat than slow, silty or stagnant water.
A big, productive stream can supply flies to a wide area, which is why a campground near a good stream can have far more flies than one a few miles away. How many are around on a given day, though, depends on the season and weather rather than on the stream alone.
Avoiding Bites During Peak Season
Females find hosts by size, shape, color, carbon dioxide from breath, body odor, movement, warmth and humidity.21Pennsylvania Department of Environmental Protection. Biology Light-colored clothing and fewer exposed areas reduce bites, since dark colors such as blue and black attract them.
For a swarm, choose loose, long-sleeved shirts and long pants in a light color. Because black flies crawl into collars and cuffs, close the gaps: tuck shirts into pants, tuck pants into socks or boots, and close collars and cuffs. Elastic bands at the wrists and ankles can help keep crawling flies from reaching skin. A head net is worth carrying if you will be near running water in peak season, since many bites land around the face, ears and hairline.
DEET or picaridin repellents can help. Apply repellent to exposed skin, following the label, and add some to the edges of clothing, such as the neck, cuffs and ankles, where flies crawl in. Reapply after heavy sweating or swimming, since the protection fades. Repellent reduces bites rather than eliminating them, so use it together with clothing and good timing.
What Black Fly Bites Do to You
Black fly saliva contains substances that stop blood clotting and stir up the immune system, causing local and sometimes body-wide reactions.22PubMed Central. Kawasaki Disease with Retropharyngeal Edema following a Blackfly Bite People bitten many times at once can develop blackfly fever, with headache, nausea, swollen glands and a mild temperature.23Incognito. Blackfly Bites: Why They Hurt More Than Mosquito Bites Severe allergic reactions are rarely reported.24PubMed Central. Kawasaki Disease with Retropharyngeal Edema following a Blackfly Bite
A bite is often not noticed until afterward. Typical signs are a small bleeding spot, then itching and swelling that can last for days. Bites often cluster around the neck, hairline and ears, or wherever clothing leaves a gap.
They differ from mosquito bites in a few ways. Black flies cut the skin instead of piercing it, so the bite may bleed. Swelling often appears later and lasts longer.
For care, wash the bites and use cold packs. Anti-itch creams or antihistamines help, and avoiding scratching prevents infection. See a doctor for spreading swelling, trouble breathing, or high fever.
Disease risk depends on location. In tropical regions, some black flies spread river blindness, a parasitic disease.25Oxford Academic (Journal of Medical Entomology). Black fly (Diptera:Simuliidae) salivary secretions: importance in vector competence and disease North American black flies are not a disease concern for people.