A DIY horse fly trap exploits the same instincts that drive these pests toward your animals: attraction to dark colors, heat, and movement. The most effective homemade designs use a dark, sun-warmed object as bait and a funnel-style collector that takes advantage of the fly’s natural tendency to fly upward after landing. You can build one for a fraction of what commercial traps cost, and a well-placed trap can make a noticeable dent in the horse fly population around a barn or pasture.
Why Horse Flies Target Dark, Warm Objects
Horse flies have specialized vision that functions like thermal imaging, allowing them to detect heat signatures from a distance. They’re drawn to large, dark-colored objects warmed by the sun because those objects look, to a horse fly, like a potential blood meal. A dark car, a tractor, or a black beach ball sitting in a field all register the same way a dark horse does. The flies also track large moving objects in fast flight, which is why they seem to chase you relentlessly on a summer day.
Beyond heat and color, horse flies respond to polarized light reflected off shiny surfaces. The higher the degree of polarization, the more attractive the surface. This is why glossy or shiny dark materials work better than matte ones in trap designs. Understanding these cues is the whole foundation of a good DIY trap: you’re building a decoy that looks and feels more appealing than the real animals nearby.
The Ball-and-Funnel Trap
The most proven DIY horse fly trap is based on a design originally called the Manitoba trap, developed by researchers who tested various shapes and colors. Their key finding: the most attractive target is a glossy black (or red) sphere roughly 24 inches in diameter, positioned about three feet above the ground. The flies approach the ball, land on it, and then instinctively fly upward, where they enter a cone-shaped funnel leading into a collection container they can’t escape.
To build one, you need:
- A dark, inflatable ball. A black exercise ball or beach ball works well. The 20- to 24-inch range is ideal. Glossy surfaces outperform matte ones.
- A cone or funnel. A large inverted cone made from window screen material, fine mesh netting, or even a repurposed umbrella frame covered in mesh. The wide end faces down around the ball, and the narrow end opens into a collection container above.
- A collection container. A clear plastic bottle, jar, or small bucket with a lid placed at the top of the cone. Drill or cut small ventilation holes (too small for flies to exit). The sun heats the container, and flies that enter cannot find their way back down through the narrow funnel opening.
- A support structure. A single post, a tripod of PVC pipes, or even a sturdy garden stake. The ball should hang or rest at roughly waist height, about three feet off the ground, with the cone and collector assembly positioned above it.
The concept is simple. Sun heats the dark ball, creating a thermal and visual signal. Horse flies land on the ball, find no blood meal, and do what they naturally do when leaving a host: fly upward. The cone channels them into the collection container, where they dehydrate and die. No chemicals, no electricity.
The Sticky Bucket Trap
If you want something even simpler, a sticky bucket trap skips the funnel entirely. You coat a dark bucket or ball with insect adhesive so that flies stick on contact. Specialty horse fly trap glues are made from natural minerals, resist water and heat up to about 158°F (70°C), and can last up to 20 weeks outdoors. Apply the glue in a thin layer, about 1 to 2 millimeters, to the outside of a dark-colored bucket or directly onto a suspended ball.
One advantage of this approach is that it selectively catches horse flies. These glues don’t attract pollinators like bees and butterflies. The visual and thermal cues that draw horse flies (large dark surfaces, heat) are not the cues that attract beneficial insects, which respond to flower colors, UV patterns, and floral scents. That said, you’ll still want to hang the trap away from flower beds and beehives to minimize any incidental contact.
The tradeoff is maintenance. Sticky traps need re-coating once they’re covered in flies and debris, while a funnel-style trap just needs its collection container emptied periodically.
Do Chemical Attractants Help?
You might wonder whether adding a scent lure makes a trap more effective. The research is mixed. Carbon dioxide (a component of cow breath) attracts biting flies broadly, but you’d need impractical quantities like a CO2 cylinder or dry ice to sustain it in the field. Acetone, the solvent in nail polish remover and also found in cow breath, can produce small to moderate increases in catch rates for some horse fly species. It’s cheap and easy to dispense by soaking a cotton ball and placing it near the trap.
Octenol, a compound found in cattle sweat, also boosts attraction for certain species but is harder to find and more expensive. Ammonia vapor shows some promise but hasn’t been tested enough to rely on. For most DIY setups, the visual and thermal cues of a dark, sun-warmed ball do the heavy lifting. Chemical lures are optional additions, not requirements. If you want to experiment, a small dish of acetone refreshed every few days near the base of the trap is the simplest option.
Where to Place Your Trap
Placement matters more than trap design. A perfectly built trap in the wrong spot will catch almost nothing. Position traps in full sun so the dark target heats up throughout the day. Horse flies are most active during the hottest parts of summer afternoons, and a sun-warmed ball radiates the thermal signature they’re searching for.
Place traps where flies are already a problem: near small pastures, along fence lines, close to riding arenas, or at the edges of trails where horses pass regularly. Keep the trap close enough to these areas to intercept flies but far enough that horses, cattle, or dogs can’t knock it over, chew on it, or get tangled in it. A distance of 15 to 30 feet from where animals stand or graze is a practical starting point.
If your property has a consistent breeze direction, consider placing traps upwind of where animals gather. Horse flies often approach from downwind (following scent plumes), so a trap positioned between the wind source and the herd can intercept them on the way in. Multiple traps spread across a property will always outperform a single trap, especially on larger acreages.
Building Tips That Improve Performance
A few details separate a trap that catches a handful of flies from one that fills its collection container in a week. First, keep the dark surface clean and glossy. Dust, cobwebs, and bird droppings reduce the visual contrast and polarization that attract flies. A quick wipe with a damp cloth every few days helps. Second, make sure the funnel has no gaps where flies can escape sideways. Screen or mesh should overlap at seams, and the transition from funnel to collection container should be tight, with only a small opening (roughly 1 to 2 inches in diameter) for flies to pass through.
Third, empty the collection container before it gets so full that dead flies block the entrance. A container with some ventilation holes keeps air flowing and prevents moisture buildup that can rot the catch and create odors. Finally, if you’re using a ball, secure it against wind. A swinging ball can actually be slightly more attractive since horse flies track movement, but if it swings wildly it may pull apart your funnel assembly. A short tether or a ground anchor keeps things stable.
Some people paint their ball with a high-gloss black spray paint to maximize the shiny, polarized surface. Others wrap a bucket in black trash bags and replace them when they get dull or torn. Both approaches work. The key principle stays the same: dark, glossy, warm, and positioned where horse flies are already hunting.