By Margaret Evans

Everyone knows the seasonal annoyance of flies. For horses they can be a real tail swatting, foot stomping, head shaking, skin twitching aggravation. But flying insects such as midges, gnats, horse flies, deer flies, black flies, face flies, house flies, mosquitos, and others are more than a nuisance – they can cause serious skin irritations and can also carry diseases.

The female horse fly needs a blood meal to produce eggs. Her mouth is equipped with sawing edges to pierce the horse’s skin and cause blood flow, and the anticoagulant in her saliva stops the blood from clotting. Photo: Flickr/Jonathan Bliss

Midges, black flies and horse flies can cause sweet itch (pruritus) along the mane, back and tail causing horses to rub furiously and inflame the sites further. Infected mosquitoes cause West Nile virus. The potentially fatal equine infectious anemia (EIA or swamp fever) virus is spread by horse flies, deer flies, stable flies, and mosquitoes when they take a blood meal from an infected horse and then settle on another. Equine encephalomyelitis, an infectious disease that affects the horse’s brain, is transmitted by mosquitoes (the vector) after it has picked up the virus from reservoir hosts such as birds or rodents. A horse’s skin is also at risk from allergic dermatitis, mechanical (from scratching and rubbing) dermatitis, the transmission of fungal infections and, in more severe cases, bacterial infections. The skin disruption that flies cause draws in more flies, adding more complications.

How to get the bugs to bug off demands multiple tricks of the trade. Basically these include topical sprays, fly masks and sheets to provide a barrier, and stringent environmental management to make barns and pastures as inhospitable as possible to the insect’s egg-laying needs.

Horse flies feed on the nectar of plants and plant exudates such as resins, gums or oils (substances that ooze from a plant). But the female needs a blood meal in order to consume the protein required to produce eggs. While the male fly’s mouthparts are quite weak, it is the female fly that has the cutting and sawing equipment to do the job - and cause grief to horse and human hosts in the process.

The female’s mouthparts are made up of a bundle of cutting stylets, or blades, and some of them have sawing edges. The fly’s muscles can move these cutting blades side to side to pierce the horse’s skin and enlarge the wound to cause blood to flow. Her saliva contains an anticoagulant to stop the blood from clotting. She laps up the blood with a spongelike part in her mouth and she will visit multiple hosts to obtain sufficient blood. Sometimes the saliva can trigger an allergic reaction in a horse causing hives or breathing problems. The constant aggravation of flies can cause horses to become stressed, interrupt grazing, lose weight, and in dairy cattle can reduce milk output.

Battling the Bugs

Pest management is essential, starting with hygiene in and around the barn. According to an online report Fly Control around Horses by Dr. Roger Moon and Betsy Gilkerson Wieland, University of Minnesota, flies will lay 50-150 eggs every few days in moist, organic debris including manure, old hay around feeders, straw or soiled bedding. Maggots hatch from the eggs and feed on the bacteria growing in the debris. They mature into pupae and then into winged adults. The entire life cycle can be completed in just two weeks and the adult fly will live one to three weeks, depending on weather. Flies need a moisture range of 35 to 70 percent for larval development and, for every fly you see, half a dozen are in the development stage.

Manure as well as rotting hay or straw should be removed and composted well away (at least 500 metres) from where animals are housed or contained. Use an aerobic composting system that generates heat; a system that does not get hot enough is a breeding ground for flies.

The use of fly predators also offers effective natural fly control. These are tiny parasitic wasps that do not bite or sting. They live their entire life cycle on or near manure where the fly pupa are found and destroy the next generation of flies in that pupa stage. They target stable flies and house flies and can reduce fly populations by up to 90 percent.

House flies and face flies are two of the most common flies found around horses. They feed on moist secretions from the eyes, nose, vulva and prepuce, and can be extremely irritating to horses. More than a nuisance, flies can cause uncomfortable allergy-like reactions and carry diseases. Photo: Canstock/Pakhnyushchyy

Use products that reduce manure odours in the stalls and aid in keeping the floors dry. Keep the grass cut around the barn – flies will look for resting areas out of direct sunlight and long grass is ideal. By extension, remove all rotting leaves, grass cuttings, decaying fruit on the ground (windfall apples) or wasted berries in season, all of which become places where eggs will be laid.

Some flies and mosquitoes seek standing water in which to lay eggs. Remove or overturn any container that will collect rainwater, fix leaky waterers that puddle on the ground, and fix dribbling taps. Keep feed tubs and water buckets scrubbed to remove algae and scour out the water trough regularly. Remove empty flower pots and tires where rain collects.

If possible, use properly installed fans to blow air over stalled horses as this will deter flies and mosquitoes. Circulating air makes it harder for flies to navigate and land on the animals.

Consider using fly traps such sticky traps and replace them frequently as they quickly become clogged with dead flies and dust.

If possible, make your barn fly-predator friendly by encouraging bats and birds to nest in the rafters. Barn swallows are aerial insectivores that feed on flying insects. This group of birds, which includes swifts, swallows, flycatchers and goatsuckers, has declined alarmingly in population across Canada. According to the Government of Ontario, barn swallows declined 65 percent between 1966 and 2009 where they are now listed as a threatened species. Bird Studies Canada reports that the Canadian population of barn swallows has declined 80 percent since 1970. Reasons include pesticides, migration hazards, and loss of nesting habitat when old barns are removed for development. In fact, right across North America the barn swallow has actually seen a 95 percent drop in population in the last 40 years, a statistic mirrored by many species of song birds.

Barn swallows feed on flying insects, but their population in North America is in sharp decline. Give them access to a ledge or cross-beam to build their nests in your barn and they’ll do their part to keep the insect population under control. Photo: Canstock/SteveByland

To help swallows in your barn, give them access to the interior and provide a ledge or cross beam high up where they can build their cup-shaped nests out of mud. A source of wet mud outside will give them the tools to do the job and in return your horses will be rewarded with a beautiful bird that efficiently eats the tormenting biting insects.  

Bats are legendary as voracious insect predators and will eat half their weight in insects every night. They prey on mosquitoes, crickets, grasshoppers and flies, and a little brown bat may catch up to 600 insects an hour. You might want to consider installing a bat house high in a nearby tree or on a suitable location on your barn roof to encourage residents.

The little brown bat has glossy brown fur and weighs between four and eleven grams. The only mammals that can fly, bats will eat half their weight in insects every night and are most active within two or three hours after sunset. Photo: Wikimedia/Don Pfritzer

A bat house hung from a tree or attached to the side of a building will entice bats to nest on your property.

Products such as topical sprays, wipes and roll-ons work temporarily to stop insects landing on the horse’s skin and to augment other control methods and debris management.

“Pyrethrum or Resmethrin fogs and space sprays can be used to kill adult flies indoors, but relief will be temporary because these insecticides break down quickly,” says Moon. “Pyrethrum contains pyrethrins that are extracted from certain cultivated chrysanthemums. Resmethrin is a synthetic pyrethrin. Longer lived pyrethroid and organophosphate residual premise sprays can be applied indoors and outdoors, and will be most effective if applied to fly perching areas.”

The active ingredients in fly repellants for horses are the botanical pyrethrins from certain species of the chrysanthemum plant. They are poisons that work by penetrating the skin or shell covering of an insect and paralyzing the nervous system. But the insects’ enzymes kick in to detoxify the pyrethrins so repellent manufacturers add synergists to the mix that delay the work of the enzymes ensuring that the pyrethrins do the job. Some formulas contain the synthetic insecticide permethrin which also attacks the nervous system. But there is always a need for more effective products, especially considering that climate change may see an influx of more biting insects from the south.  

“[That] will depend on what kind of fly, where you live, and how the climate changes,” says Moon. “Nobody is predicting large changes or increases northward from the tropics to northern temperate latitudes. Rather, changes are likely to be more incremental. All Canadian provinces are projected to become warmer and wetter so to the extent that happens horse owners should see more mosquitoes and other aquatic biting flies for longer parts of the year. The same will probably happen with leg biting stable flies, which develop in moist, warm, decomposing organic matter. In the opposite direction, forecasts are for warmer, but drier weather patterns in the desert southwestern US, so fly abundance there should decrease outside of irrigated pastures.”

Moon said that Lyme disease in humans has spread into eastern and central provinces and states, and experts think that is due to spread of the black-legged tick that spreads the Lyme disease bacterium.

“Veterinarians argue about whether horses get Lyme disease,” he says. “Eastern equine encephalitis cases have increased slightly in the northeastern region, but I am not certain that is due to changes in vector abundance.”

Many horse owners prefer natural products to chemical insecticides for their horses. Garlic, apple cider vinegar, peppermint and citronella are bases for some success with fly repellents. In 2014, Health Canada (HC) made a decision to ban the use of oil of citronella despite the fact that its own expert panel argued that fly sprays with citronella were safe and that the science research that HC was relying on was flawed. In the wake of a furious pushback, HC reversed its ban in 2015 allowing for the use of the very popular oil.  

Physical barriers for horse protection from flies include fly sheets, fly masks made of see-through mesh to protect horses’ eyes, ear nets and mesh leg coverings. Interestingly, white, or white-based summer/fly sheets, seem to work better than dark materials. And, anecdotally, some people notice that white horses are less bothered by flies than dark (bay and black) horses.

Some people have observed that white horses are less bothered by flies than dark coloured horses, and science may back this up. Photo: Canstock/Perutskyi

Physical barriers such as fly sheets and masks help to protect horses against all types of flies. Photo: Canstock/Panama

There is some science behind this.

At Lund University in Sweden, Suzanne Akesson, an evolutionary ecologist, wanted to know whether zebra stripes were attractive to tabanids, the insect group that includes tsetse flies as well as horse flies and deer flies. Some research had already been done using experimental coloured squares and it had been shown that the tsetse fly preferred black landing surfaces rather than white or striped surfaces. Her team, which included researchers in Hungary, also knew that flies use reflected polarized light from a horse’s coat as a signal to find a host. These light waves from dark colours are oriented in the same direction similar to light reflected from pools of water that flies seek to lay eggs.

At a Hungarian horse farm, they tested the attractiveness of tabanid flies on solid black or white squares, black and white striped squares, and similarly marked life-size plastic horses. They also set up tests with varying widths of stripes. Clear glue was used to trap the flies when they landed.

The study showed that stripes were the least attractive to flies and the narrower the stripe width the more the flies avoided the pattern.

The reason was that stripes reflect multiple light patterns that created confusion for flies compared to the desired uniform light patterns of solid dark colours. The study was published in the Journal of Experimental Biology.

Following through on that research, there are now fly sheets, neck sheets and fly masks available in zebra-striped patterning that, by most accounts, have excellent ratings and which can be ordered through your local tack store or online.  

Flies are, quite simply, part of the lifestyle of having horses. And at least flies are seasonal.

“Biting mosquitoes, tabanid flies and stable flies are worst in summer while blackflies can be worse in spring,” says Moon. “All of those insects [will] cause the horse misery.” He urges horse owners to do what they can to lighten that misery out of compassion for their horses.

More by Margaret Evans

Main photo: Canstock/DeepGreen