Horses and Horseflies

Paul Westfall

Granville County Extension Agent

With the taste of summer that we have enjoyed instead of a “normal” spring, thoughts turn to things like growing grass in pastures, getting ready to cut hay, and enjoying the spring blossoms.  We also may start to think about controlling some external parasites.  One of those parasites in particular is really hard to control and causes great distress to horses, dogs, cats, cows, and people.  That insect is the horsefly.  I’ll include deer flies and other species of large, biting flies in the same category.

There just are not many ways to effectively control these types of flies as they only stay around the horse long enough to bite through the skin and get a meal, then they fly off.  They just don’t stay around long enough for contact insecticides to be effective.  The Agricultural Chemicals Manual says that permethrins may give some protection for short periods of time.  Unfortunately, most of the horseflies are still around after that short period of time.

I was visiting a horse farm recently and discovered a horse fly control method that the farm owner states has dramatically reduced the number of horseflies and deer flies bothering their horses.  That method is to use a horse fly trap.  Even though I’ve been an Extension agent for a while, I had never seen one of these traps. I thought I would share the basic concept.

After a bit of research, I’ve found that there are actually two different kinds of traps – one is kind of a pyramid and the other is the type I will describe because I’ve seen it in action on a farm.  Both types are commercially available, but I think this one is easier to build at home.  The biting fly trap requires four metal T posts and a couple of yards of black canvas or rubberized canvas or tarp material.  This is used to create a large, dark solid area between the posts.  Black plastic can probably be used but is not recommended, as it will have to be replaced frequently due to the wind or horses ripping and tearing the plastic.  A couple of trays containing water are mounted below pieces of safety glass or similar material that are placed at about a 45-degree angle above the trays.  The trays contain water with dish soap. I’m told that plexiglass won’t work in this trap.  It reflects light differently than glass and the flies evidently think that a sheet of plexiglass is solid and won’t fly into it.  They don’t see the glass and try to fly through the clear area.

The theory is that the dark plastic or canvas pieces contrast with the surroundings, mimicking a large animal standing still waiting to be bitten.  The clear glass represents zones where the horsefly would normally fly while selecting a site to land and feed.  The fly tries to fly through the angled glass and bounces into the dishwater, gets wet, can’t fly away, and remains in the tray for disposal.  An aquarium net is recommended to remove the dead flies 3 or 4 times per week.

I can’t say how the pyramid traps work, but I have seen the results of this type being placed in a sunny area of a pasture or in a lane next to a pasture known for horsefly activity.  The horses are content to be there.

One note of caution – some owners don’t like to have the trap in the pasture with the horses.  They tend to lean on the posts, bending them, or it is possible for a horse to step on a sharp edge of a post at ground level if the posts aren’t pounded far enough into the ground.

I have to say that I am not endorsing any particular brand of trap, just saying that one may be something worth trying.  A trap can be constructed at home using materials that are likely already on hand on most farms, so the cost shouldn’t be too high.  No insecticides are used, just soapy water, so the horses should be safe on the environmental front, too.

I’ve posted a couple of pictures.  One shows what the assembled trap looks like, and the other shows the glass installed in a catch pan.