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The two traps that actually work against biting flies:
Meet Ontario’s Two Biting Flies
If a fast, tenacious fly has ever ruined a lakeside hike or a canoe portage, it was almost certainly a deer fly or a horse fly. Both belong to the family Tabanidae — the group entomologists literally call “horse flies and deer flies.” They share the same basic biology: only the females bite, because they need a protein-rich blood meal to develop their eggs. The males are harmless flower visitors, feeding on nectar and pollen and never touching you.
The quickest way to tell them apart is size and eyes. A deer fly is roughly the size of a housefly, has clear wings crossed by dark bands or smudges, and carries dazzling green-and-gold patterned eyes. A horse fly is a heavyweight — up to three centimetres — with a thick body, mostly clear wings, and huge solid dark eyes. Deer flies circle your head; horse flies tend to plant themselves on your back or legs and bite before you notice.
Why the Bite Hurts So Much
A mosquito is a delicate surgeon — it slides a thin stylet into your skin so gently you often do not feel it. Tabanids are the opposite. The female’s mouthparts work like tiny scissors and a sponge: she slices the skin open, then laps up the blood that pools in the wound. That cutting action is exactly why a deer fly or horse fly bite delivers a sharp, immediate sting, frequently bleeds, and leaves a raised, itchy welt that can last for days.
The good news for Ontarians is that these flies are not major disease carriers here the way mosquitoes (West Nile) and blacklegged ticks (Lyme) are. Horse flies can mechanically move some livestock pathogens, and North America records rare cases of tularemia spread by deer flies, but for the average person the real cost is the pain of the bite and the occasional infection or allergic reaction from scratching it.
Why Deer Flies Dive-Bomb Your Head
The maddening head-circling behaviour is not random. Deer flies home in on the tallest, warmest, darkest, moving part of a target — and when you are walking, that is your head. They track the carbon dioxide in your breath, your body heat, your motion, and dark colours, then orbit the crown of your body waiting for a chance to land. This is precisely why a sticky patch worn on top of a hat works: the flies commit to the highest point and stick fast. It is also why runners and hikers on humid trails get swarmed while people sitting still in the shade are bothered less.
The Tabanid Life Cycle
Understanding where these flies come from explains why they are hard to spray away. Female deer and horse flies lay clusters of eggs on vegetation overhanging moist soil, pond edges, marshes, and stream banks. The larvae drop into the damp ground and live there as predators, eating other small invertebrates, sometimes for one to three years depending on the species and climate. When mature, they crawl to drier soil, pupate, and emerge as adults in early-to-mid summer.
Two facts fall out of this. First, the breeding habitat is usually a wetland you do not own and cannot legally drain — which is why there is no “remove standing water” silver bullet the way there partly is for mosquitoes. Second, the adults are powerful fliers that range widely from where they hatched, so the flies bothering your patio may have developed hundreds of metres away. Control has to focus on intercepting and deterring adults, not eliminating a nearby nursery.
How to Treat a Bite
- Clean it. Wash with soap and water to lower infection risk from the open cut.
- Cool it. A cold compress for 10–15 minutes eases the pain and swelling.
- Calm the itch. An over-the-counter antihistamine or hydrocortisone cream helps. Do not scratch — scratching is what turns a bite into an infection.
- Watch it. See a doctor for spreading redness, pus, a fast-growing welt, fever, or any strong allergic reaction (widespread hives, facial swelling, or trouble breathing).
Most bites settle within a few days. Because the wound is a genuine cut rather than a pinprick, keeping it clean matters more than it does with a mosquito bite.
Yard and Personal Control That Actually Helps
No single tactic clears biting flies, but stacking several makes a real difference around an Ontario home:
- Blue sticky ball traps. Hang a blue coroplast sphere coated in sticky glue so it sways in the breeze in a sunny, open area near where you sit — the movement and dark colour draw deer and horse flies, which stick and cannot escape.
- Hat patches. A sticky deer fly hat patch on the crown of a light-coloured hat traps the flies that circle your head on trails and in the garden.
- Dress light and cover up. Tabanids prefer dark, moving targets, so light colours, long sleeves, and long pants cut both attraction and access.
- Time it. Activity peaks on hot, sunny, windless afternoons. Work outdoors in the early morning, on cool or overcast days, or when there is a breeze.
- Repellents. DEET and picaridin (icaridin) help, though biting flies are somewhat less deterred than mosquitoes — reapply as directed. Follow the label and Health Canada guidance.
- Barrier spray on resting sites. Adults rest on shaded vegetation, fence lines, and shrubs between feeding. A professional barrier treatment to those exact zones reduces the resting population lingering near your patio, on top of its main job against mosquitoes and ticks.
Traps like a blue sticky ball are especially worth it for cottages, acreages, and homes backing onto ravines or water. Compare them alongside the other backyard options in our best mosquito trap in Canada guide, since the same open, sunny placement rules apply.
Deer Flies, Horse Flies, and the Bigger Picture
Biting flies are the summer nuisance that no product fully solves, but they are also the least dangerous of Ontario’s common biters when it comes to disease. The realistic goal is to reduce contact — trap the ones that circle you, dress to make yourself a poor target, avoid the hottest windless afternoons near water, and knock down the adults resting in your yard with a barrier spray. Stack those and the difference on your deck is obvious, even though the flies keep drifting in from the wetlands beyond your fence.