Black Flies in Ontario: When They Come Out and How to Stop Them

If you have ever walked out to your patio in early June and been chased back inside by a swarm of tiny biting insects that draw blood when they bite, you are not alone — and you are not dealing with mosquitoes. You are dealing with black flies. This guide covers when they come out in Ontario, where they breed, and how professional treatment controls them as part of a mosquito barrier spray program.

What Are Black Flies?

Black flies (family Simuliidae) are small, dark, hump-backed biting flies that emerge in massive numbers each spring across Ontario. They are sometimes called "buffalo gnats" or "turkey gnats," though Ontarians almost universally call them black flies. There are more than 60 black fly species in Canada, with several common to the GTA — including Simulium venustum and Simulium decorum, two of the most aggressive human-biting species.

Adult black flies are 1 to 5 millimetres long — much smaller than mosquitoes — with a distinctive humped thorax that gives them the look of miniature houseflies. They are dark grey to black, with clear wings and short antennae. Unlike mosquitoes, only the female black flies bite. Females need a blood meal to develop their eggs, while males feed on plant nectar.

When Do Black Flies Come Out in Ontario?

Black flies in Ontario follow a predictable seasonal pattern driven almost entirely by water temperature. Larvae develop in flowing streams over the winter, and adults emerge once the water and air reach the right temperature window.

The Black Fly Calendar in the GTA

• Late April – Early May: First emergence. Numbers are still light. Mostly a problem near major waterways.
• Mid-to-Late May: Population explosion. The first major waves emerge as streams reach 13°C–15°C.
• Early June – Late June: Peak black fly season across the GTA. The worst weeks of the year for outdoor activity near rivers, ravines, and forested properties.
• Early July: Populations begin to decline rapidly as flowing water warms past optimal larval temperature and host species shift their behaviour.
• Mid-July onward: Most black flies have completed their adult stage. Pressure drops to background levels for the rest of the summer.

This is very different from mosquito season in the GTA, which builds slowly through May and peaks in July and August. Black flies are an early-summer problem; mosquitoes are a mid-to-late-summer problem. Many homeowners who book their first professional mosquito treatment in mid-May get the dual benefit of cutting black fly pressure during the peak weeks.

Black Flies vs Mosquitoes: How to Tell the Difference

Both are small biting insects, but the differences matter when deciding how to deal with them.

Where Do Black Flies Breed?

This is the single most important fact about black flies, because it determines who is at risk: black flies breed exclusively in clean, flowing water. Larvae attach themselves to submerged rocks, branches, and vegetation in streams and rivers, where they filter-feed on organic particles in the current. They cannot develop in standing water — which is the opposite of mosquitoes.

That means properties near the following waterways see the worst black fly pressure each spring:

• Credit River and tributaries — affecting much of Mississauga, Oakville, Brampton, Caledon, Halton Hills, and Georgetown
• Humber River system — affecting Etobicoke, Vaughan, Woodbridge, Kleinburg, and parts of Brampton
• Don River and ravine network — affecting much of central Toronto, North York, and Thornhill
• Rouge River — affecting Scarborough, Markham, and Pickering
• Sixteen Mile Creek — affecting Milton and Oakville
• Bronte Creek — affecting Oakville and Burlington
• Red Hill Creek and Stoney Creek — affecting Hamilton and Stoney Creek
• Niagara Escarpment streams — affecting Ancaster, Dundas, Burlington, and the Halton Hills corridor

Black flies are strong fliers and can travel several kilometres from their breeding sites in search of hosts. If your property is anywhere within a few kilometres of a clean, flowing waterway — and most of the GTA is — you will see black flies during the spring emergence. Toronto homeowners with properties backing onto Don Valley ravines, Humber River corridors, or High Park feel this acutely each year.

Why Black Fly Bites Are Different

Black flies do not pierce the skin like mosquitoes — they cut it. Their mouthparts are designed to slice a small wound and lap up the pooling blood, similar to a horsefly but on a much smaller scale. The bite is often felt sharply when it happens, unlike a mosquito bite which usually goes unnoticed until the welt forms.

The aftermath is also distinct:

• A small, bleeding puncture wound
• Localized swelling that can persist for days
• Intense itching that often peaks 24–48 hours after the bite
• Small bruise-like marks at the bite site for some individuals
• In cases of multiple bites, "black fly fever" — fever, headache, swollen lymph nodes, and nausea

Black flies in Ontario are not known to transmit human pathogens, so the medical risk is low. The quality-of-life impact, however, is severe. A single bad black fly week can drive families indoors for weeks, and the bites themselves can take longer to heal than mosquito bites. For pets — particularly dogs with thin ear leather and exposed bellies — black fly bites can cause hot, painful welts that linger.

Black Fly Bite: Why They Hurt & How to Treat Them

A black fly bite hurts in a way a mosquito bite usually does not. Because the fly cuts the skin rather than piercing it, you often feel a sharp pinch the moment it bites — and the small wound tends to bleed for a moment afterward. The fly’s saliva contains anticoagulants and mild anaesthetic compounds that keep the blood flowing and mask the bite, which is exactly why the after-effects — swelling, redness, and itching — can be more intense and longer-lasting than a mosquito welt.

First Aid for a Black Fly Bite

Prompt care keeps a black fly bite from turning into a bigger problem. The basics:

• Wash it right away. Clean the bite with soap and cool water to remove residual saliva and lower the infection risk from the open wound.
• Cool the swelling. Apply a cold compress or ice wrapped in cloth for 10–15 minutes to bring down inflammation.
• Calm the itch. An after-bite itch relief product, hydrocortisone cream, or an oral antihistamine can take the edge off the itch and swelling. Check after-bite itch relief on Amazon.ca →
• Do not scratch. Scratching reopens the wound and is the main way black fly bites become infected. Keep it covered if you cannot leave it alone.

Signs of Infection or Allergy — When to See a Doctor

Most black fly bites are harmless and heal on their own, but watch for these warning signs and seek medical care if they appear:

• Spreading redness, warmth, or pus around the bite — signs of a secondary bacterial infection.
• Black fly fever after multiple bites — fever, headache, swollen or tender lymph nodes, and nausea.
• A large, hot, hard swelling that keeps growing over 24–48 hours rather than settling.
• Any sign of a severe allergic reaction — hives spreading across the body, facial or throat swelling, or difficulty breathing. This is a medical emergency; call for help immediately.

How Long a Black Fly Bite Takes to Heal

A typical black fly bite is itchiest 24–48 hours after it happens and settles within a few days, but it can take a week or more to fully heal — longer than a mosquito bite, and often leaving a small bruise-like mark or scab. Keeping the wound clean, cool, and unscratched is the single biggest factor in how fast it clears and whether it scars.

Where Black Flies Hide on Your Property

Black flies do not breed in your yard, but they rest in your yard between feeding bouts. They prefer:

• The shaded undersides of shrubs and hedges
• Dense ornamental plantings
• Tree canopies, especially on the leeward (downwind) side
• Tall grass at property edges
• Cool, damp, sheltered corners of fences and structures

This resting behaviour is exactly what makes professional barrier spray effective against them. The treatment targets the same surfaces black flies use for cover.

Why Property-Edge Treatment Matters

Black flies arrive from outside your property and cannot be eliminated by source reduction the way mosquitoes can. You cannot drain a river. What you can do is treat the rest sites and feeding zones in your immediate yard environment so that the black flies that arrive do not stay long, do not feed in numbers, and do not establish a comfortable population during the peak weeks.

Professional barrier spray for mosquitoes — applied to vegetation along property edges, treelines, fences, ornamental beds, and the underside of shrubs — collaterally controls black flies during the same May–June window when both insects are pressuring your yard. A properly timed first treatment in mid-to-late May provides peak coverage right when black fly emergence is heaviest.

What Homeowners Can Do (Beyond Treatment)

Black fly control is mostly about reducing exposure during peak weeks. A few things help:

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• Avoid scented products. Floral perfumes, scented sunscreens, and hair products attract black flies.
• Wear light colours. Black flies are strongly attracted to dark blue, brown, and black. Light grey, beige, and white are far less attractive.
• Cover up. Long sleeves, pants, and a hat block most bites — black flies are notorious for crawling into hairlines and behind ears.
• Use DEET or picaridin repellent on exposed skin. Picaridin (also sold as icaridin) is odourless, non-greasy, and does not damage plastics or gear the way DEET can, which makes it popular for all-day spring wear. Check picaridin repellents on Amazon.ca → See our full picaridin vs DEET comparison to pick the right active.
• Treat your clothing with permethrin. Permethrin-treated clothing is highly effective for outdoor workers, hikers, and anyone spending long hours near waterways during the May–June emergence — it kills black flies on contact and lasts through several washes. Check permethrin clothing spray on Amazon.ca → Read our permethrin guide for Canada for safe application.
• Stay away from waterways during peak emergence. The closer you are to a river or stream in late May and early June, the heavier the pressure.
• Eliminate yard rest sites. Trim back overgrown shrubs and dense vegetation along the property edge.
• Schedule a barrier spray. A professional treatment in mid-May provides the strongest coverage during the worst three weeks.

How to Get Rid of Black Flies

Getting rid of black flies works differently than getting rid of mosquitoes, and understanding why comes down to one thing: where they breed. Mosquitoes lay eggs in standing water, so you can knock their numbers down by dumping saucers, cleaning gutters, and draining anything that holds water. Black flies do the opposite — their larvae attach to rocks and vegetation in clean, fast-flowing rivers and streams, feeding on particles in the current. You cannot drain a river, and there is no yard chore that removes their breeding site. That means “getting rid of black flies” is really about protecting yourself and making your yard an unwelcome place for them to rest and feed.

1. Personal Protection First

• Cover up and go light. Long sleeves, pants, and a hat block most bites, and black flies are far less drawn to light grey, beige, and white than to dark blue, brown, and black.
• Use repellent. DEET or picaridin on exposed skin, and permethrin-treated clothing for long hours near waterways during the May–June emergence.
• Skip the scents. Floral perfumes, scented sunscreens, and hair products all attract black flies.

2. Make Your Yard Less Hospitable

• Cut the rest sites. Trim overgrown shrubs, hedges, and dense vegetation along property edges where black flies shelter between feeding bouts.
• Open up shaded, damp corners. The cool, sheltered pockets near fences and structures are exactly where they linger.
• Time it to the season. The pressure is heaviest in late May and June, so your yard prep and any treatment matter most in that window.

3. Treat the Rest Zones with Barrier Spray

Because you cannot touch the breeding source, the most effective control is a professional barrier spray applied to the surfaces black flies use for cover — leaf undersides, treelines, fences, and ornamental beds. It will not stop every fly drifting in from a distant river, but it dramatically reduces the population that settles, feeds, and bites in your yard. A first treatment timed for mid-to-late May delivers peak coverage right as emergence spikes.

How BuzzSkito Helps With Black Flies

Our standard mosquito barrier spray service includes black fly control as part of the treatment. We focus the application on the resting habitats both insects share — under shrub canopies, along fence lines, in dense ornamental beds, on the underside of broad leaves, and across the lawn-bed transition zone. The Health Canada–approved residual formula remains active for up to 30 days, keeping pressure low through the peak black fly window even with rain events in between.

For properties near waterways — Credit River backyards in Mississauga, Don Valley ravine homes in North York, Humber-adjacent properties in Etobicoke and Vaughan — we recommend booking the first spray of the season for mid-to-late May. That single timing decision is the difference between a tolerable spring and a wash-out spring.

Get a Free Quote

If you are tired of dodging black flies in May and June, we can help. Book a free yard assessment and we will walk through your property, identify rest zones and likely pressure sources, and give you an honest quote for seasonal coverage. We service 19 cities across the GTA — same-week scheduling for new customers during the spring rush.

Related Guides

• The Ultimate Backyard Mosquito Control Guide (GTA Edition)
• When Does Mosquito Season Start in the GTA?
• Mosquito Control Near Water: Ravines, Ponds & Waterfront Properties
• 12 Ways to Prevent Mosquitoes in Your Backyard
• Picaridin vs DEET: Which Repellent Works Best in Canada?
• Permethrin in Canada: Yard & Clothing Spray Guide
• Bug-Protective Clothing: Jackets, Head Nets & Suits for Canada
• Best Screen Tents & Gazebos for Canadian Backyards
• Toronto Mosquito Control
• BuzzSkito Mosquito Control Services