Attack of the Flies

It's that time of the year again when the flies come out.

The winter snows have melted, the sun is shining, and birds are singing. You can finally work horses in the outdoor arena. But as the horse begins to work up a sweat, a familiar ritual begins. Biting insects are attacking him, and in return, he is distracted and ignoring the rider. What can you do?

Common Invaders

There are several types of flies that farm managers must deal with. Face flies are common in pastures, particularly in the eastern part of the country. Around the barn, stable and house flies are the most common and are very similar in appearance; small, black, and annoying. But if you could view them from the side, you’d note that the stable fly has a bayonet-like proboscis sticking out from its face which it uses to penetrate the skin. They draw blood primarily from the horse’s front legs, and horses react to the painful bites by stomping their feet and swishing their tails. In contrast, the house fly feeds on urine and manure and will not normally bother the horse.

Other biting insects include mosquitoes, black flies, horn flies, and bots. These nasty creatures can interrupt lessons and training sessions if not controlled. More importantly, all of them carry various diseases. Face flies, explains Dr. John Campbell of the University of Nebraska’s Entomology Department, “are capable of transmitting the eyeworm to horses, while both the stable fly and house fly can transmit a nematode parasite. This parasite enters the body through a wound and causes ulcerative sores, typically found on the shoulders, chest, and neck. Mosquitoes are carriers of both horse and human diseases such as West Nile Virus, equine infectious anemia and encephalitis.” Obviously, these insects must be controlled.


Of course, sanitation is the first step in controlling fly populations. In addition to manure management (see sidebar) there are several other actions you can take. House flies, notes Dr. Phillip E. Kaufman of the Entomology and Nematology Department at the University of Florida, are attracted to molasses for its sugar content, so keep all food sources covered. Adds Campbell, flies will breed in spilled grain if it is moist. So it is imperative that wet, mushy grain behind feeders be removed frequently. In addition, “find out where the flies are resting,” suggests Campbell. “They’ll feed but then they’ll rest in a shaded area. If you can find that spot, you can treat it with residual insecticide. Residual insecticides are products that will last as long as they don’t get wet or aren’t in direct sunlight. They are not something you’d put directly on the horse.”

Unfortunately, even with the cleanest of barns, flies can still be a problem. The reason? In addition to their amazing reproductive capacity, they may be coming from other farms. The stable fly can travel two to three miles, so they may be migrating from a cattle farm down the road. Therefore, you may have to enlist the aid of chemical or natural agents.

Chemical Solutions

Pyrethrin-based sprays are natural sprays derived from the African chrysanthemum flower. They have low toxicity and degrade quickly. However, because they degrade in sunlight, they are not effective over a long period of time. The synthetic version, pyrethroids, do not degrade as quickly and so tend to repel flies longer. Organophosphates are more toxic than the pyrethrin compounds, but they normally don’t break down as fast. All are effective.

Whether you are using sprays on individual horses or via a barnwide system, it is very important that you rotate products, at least once per season. And that means rotate by the chemical family, not the brand name. “There is tremendous resistance with house flies and it has been shown that they can build resistance in as few as 20 generations,” says Campbell.

For best results, advises Kaufman, follow application directions exactly. Products carry the maximum concentrations of insecticides allowed by the government, so if you dilute the spray more than recommended, you’ll be lessening its effectiveness, not prolonging its use.

All-natural products have become popular in the last several years. Do they work? Campbell feels they have some effect, but not a lot. In order to get products labeled, he explains, companies only have to show that they have a 20 percent effectiveness rate. Labels on products that have received this low rating will say “aids in” repellency. Campbell advises purchase of products that say “controls” flies, as these sprays have received a higher effectiveness rating.

Zapping Mosquitoes

Mosquitoes have become a real concern with the rise of West Nile Virus. They breed in stagnant water, so drain all standing water on your property. For ponds and other large areas, you can purchase small, solid patties containing bacteria that, although harmless to humans and other living organisms, will kill both mosquitoes and black fly larvae when ingested. Before using, check with your extension office, as some states have restrictions on the use of these patties.

Fly Parasites

Fly parasites, or eliminators, are small, gnat-sized nocturnal insects that are harmless to horses and humans. They destroy fly populations by depositing their eggs into fly pupae, thus insuring that very few flies make it to adulthood. These parasites kill many nuisance flies, including stable, house, and face flies. It is most effective to start a fly predator program early in the season, before flies take hold. If flies are already a problem, then it may take 30 days or more before you notice a difference (the life-cycle of the typical fly). And because flies breed faster than the predators, you will need to continually replenish your supply.

Companies that sell these tiny stingless wasps will ship them to you on a regular basis. The frequency and size of the shipment will depend on the number of horses you have and the degree of fly infestation. Costs vary, but on average, for 20 to 25 horses, expect to spend between $400 and $600 per season. “Make sure,” cautions Campbell, “that you use the right predators. If you live in New York and purchase predators from California that are native to California, they won’t do very well in New York.”


Sticky traps such as the ones hung from walls are very common and inexpensive. Do they work? Kaufman’s research group conducted trials on five dairy farms to see just how effective these traps are. In ten weeks, they caught 140,000 stable flies and 900,000 house flies, but yet, they saw no statistical reduction in the number of flies on the calves. However, because they do eliminate some flies, Kaufman feels they are worth using.

Chemical traps, which typically use pheromones as attractants, tend to work better on house flies than stable flies, explains Campbell, because stable flies are blood feeders and are more attracted to your horses than to the attractant in the traps. Still, some traps are effective on both, and Campbell suggests purchasing products that specifically mention stable flies.

Dealing with flies is a constant battle. There is no magic bullet which will eradicate them all, but with good management and a little help from pesticides, they can be controlled.






"*" indicates required fields

The latest from Stable Management, the #1 resource for horse farm and stable owners, managers and riding instructors, delivered straight to your inbox.

Additional Offers

Additional Offers
This field is for validation purposes and should be left unchanged.