prevent flies

Flies, mosquitoes, and other bugs are nuisances, causing irritation, discomfort, and health risks for horses and humans alike.

By Heather Smith Thomas, The Horse: Your Guide To Equine Health Care

Flies and mosquitoes are nuisances, causing irritation and discomfort as they feed on horses. They can also be health risks, spreading West Nile virus, encephalomyelitis viruses, equine infectious anemia, vesicular stomatitis, and other diseases. In this article we will share some tips on eliminating these pests and give you some new ideas and insight on what works, and what doesn’t, in the war on flies and mosquitoes.

Eliminating the Source

Lee Townsend, PhD, professor emeritus of entomology at the University of Kentucky, says the ideal situation for controlling insects is to get rid of breeding sites. Often this is impractical or impossible, but it’s worthwhile to identify and remove or alter as many as you can.

Breeding sites for some insects might not be on your farm. “Face flies and horn flies breed in fresh cow manure. If your horses live near cattle herds, you can’t control the breeding sites and are just dealing with the flies that come to your horses,” says Townsend. “Horse flies and deer flies breed in wetlands or along stream banks that may be miles away. Some of the mosquitoes that are problems for horses can fly several miles before they feed and may also be carried long distances on wind currents.”

Stable flies can also come to your farm from somewhere else. In a Florida study, stable flies were marked and released, and strong winds carried them 135 miles, says Bill Clymer, PhD, BCE, a livestock parasitologist based in Amarillo, Texas. “But most stable flies come from your own place or within a mile or two radius,” he adds. He recalls a farm in Colorado that had a serious problem with stable flies that were coming from a pole barn a mile away. Alfalfa hay had been stored there, but the hay had been taken out, leaving a thick mat of old alfalfa leaves. A blowing rain got the hay leaf litter wet, and millions of stable flies were breeding there. The stable fly problem was solved by merely cleaning out the barn and destroying the breeding site.

You can reduce populations of stable flies, mosquitoes, and house flies by reducing or eliminating breeding sites on your farm (standing water that collects in old tires or cans in junk piles, and wet organic material such as manure and old bedding). Manure, wet straw, and wasted hay should be removed daily from stalls and pens and spread thinly for quick drying or composted in a covered pile. Properly composted organic material gets hot during breakdown, and the heat kills fly eggs and larvae. Covering the pile with black plastic helps keep flies off and creates more heat under the plastic–which deters breeding and speeds the compost process.

Also make sure there’s good drainage in stalls, pens, and pastures, with no moist areas created by leaking faucets or water systems.

Clymer says there are three ways to control flies–sanitation, sanitation, sanitation. Unfortunately, there are no “wonder drugs” in the marketplace that will control the various pests that are a problem for horses. Therefore, owners must concentrate on pest management and not just killing the pest that is the immediate problem. “The stable fly got its name because it’s found primarily around stables, breeding in horse manure, though it also breeds in any decaying matter like grass clippings, old bedding, old round bales left on the ground, etc.,” he says. Often, just getting rid of breeding areas will eliminate the problem.

Housing can also help control attacks by pests. Horse flies and deer flies won’t enter deep shade or a building; they prefer sunlight. Other types of biting insects attack outside at night, and the horse can be protected by being brought indoors. This strategy can help reduce insect attacks, says Greg Johnson, PhD, retired professor of Entomology at Montana State University, especially if you have screens on windows and doors (with fine mesh to keep out gnats).

Repellents and Insecticides

You can’t eliminate all breeding sites, so your second line of defense is to create a protected “island” of animals within a vast sea of insects. Johnson says there are many types of products, with various ways to apply them–spot-ons, roll-ons, sprays, wipes, towelettes to rub on the horse, etc. “Chemical companies are responding to the needs of horse owners for easy treatment, such as products you can apply around the eyes and sensitive areas where you don’t want to spray,” he says.

Dennis French, DVM, MS, Dipl. ABVP, head of the University of Illinois’ Department of Veterinary Clinical Medicine, says there are many products available, but only a few active ingredients; the various products just contain different formulations or percentages. Some inactive ingredients merely act as carriers. Others such as piperonyl butoxide are sometimes added as an active ingredient to give a synergistic effect, augmenting the insecticide’s action and strength, explains French. Some spot-ons are longer-acting and also seem to work for some horses that are allergic to sprays.

“Horses protected by insecticide/repellent still attract insects because flies/mosquitoes use animal movement, body warmth, or exhaled carbon dioxide to find a blood meal,” says Townsend. “You can’t change this by any treatment put on the horse; you’re just trying to short-circuit the insects’ hunting process at the last minute.”

Flies continue to approach and might even land on the horse before being forced away by the protective product (which might kill them after they leave). Some insects are always challenging the treatment, so as soon as it starts to break down, they start to feed again. In the meantime, the horse is still being annoyed by the insects even when they don’t settle long enough to feed.

Even the best of treatments applied to the horse are effective for only a few days. “There are two main groups of insecticide used in equine products,” says Townsend. “One is natural pyrethrins derived from certain chrysanthemum flowers. While very safe for use on animals, they are broken down by sunlight quickly and may only give protection for a few hours.”

The other group is the permethrins (synthetic pyrethrins), which are more stable and last longer, giving protection for three to five days. Permethrin is highly toxic to many insects and arthropods but is one of the least toxic insecticides for use on mammals. It is a synthetic pyrethroid like those used on vegetable and fruit crops to prevent insect damage.

“Many of these products, particularly the pyrethroids, are not very soluble in water, but if the horse is working and sweating, or walking through streams, it tends to wash off” and must be applied more often, says Townsend. Some products contain sunblock that helps minimize breakdown of the insecticide’s active ingredient by sunlight, and some contain material that helps it bind to the hair shaft longer, but moisture from sweat or weather still tends to wash it off.

If you are working with a horse daily, you have the opportunity to apply insecticide regularly, but pasture animals or horses you see only on weekends might not get adequate protection. You need to monitor them to see how effective the products are and how long they protect the horse.

Various products have a wide range of concentrations. “It may be as high as 40% of active ingredient in products used as a spot-on, compared to one-tenth of a percent in a wipe or spray,” advises Townsend. “When a horse is working hard and sweating, you may want to use a lower concentration of insecticide and more frequent application. Those with lower percentage won’t provide protection as long, but you can safely apply them more frequently. If you are working with the animal daily, this might be the best strategy.”

Read labels before using a product. “The differences are in the concentration or strength, or additives like a sunblock, aloe vera, or some other soothing material, or coat conditioner,” says Townsend. “By reading labels, you can make better choices for your purposes, and in some cases look for a more economical approach to fly control–simply by knowing what’s there and not letting a brand name entice you.”

Johnson agrees that reading labels is important, noting, “Some products say you should not retreat before 14 days. Some say you should not treat foals less than 12 weeks old.

“Many products boast a long list of flies they kill, but I am skeptical,” he adds. “I know the rates, type of chemical, and application methods used will not kill horse flies, deer flies, stable flies, or horn flies. Horn flies in many regions have become resistant to these insecticides, and the products don’t have much effect on horse flies and deer flies. Some of the products also claim up to 14 days protection against mosquitoes and biting flies, but I haven’t seen any hard evidence to support that claim.”

He also advises owners to use an insecticide designed for the fly you are trying to control. “If it’s a stable fly, many chemicals on the market will not kill stable flies on the horse,” says Johnson. “It’s hard to get insecticide to the location where stable flies are feeding. Stable flies feed on the legs and chest because the skin is thinner there. Keeping these areas covered with insecticide can be a challenge.”

Cliff Hoelscher, PhD, retired professor of international agriculture at Texas A&M University, says it’s important to apply a product to deter mosquitoes all over the horse’s body; mosquitoes will feed anywhere they can get a blood meal. “For mosquitoes and stable flies, you need to apply it frequently (daily),” he says. “People want to spray horses for mosquitoes and have it last for a week, but that doesn’t work.”

Some products are merely repellents; flies don’t like them and tend to leave without biting, but they are not killed. “One repellent often used for humans and horses is citronella, but it only lasts a few hours,” says Townsend. “If a horse is working hard and sweating, you lose the effectiveness very soon. If you can get by with it, it may be helpful, but you should always monitor the horses to see how well a product is actually working–and also keep checking for possible skin irritation.”

Spraying the Premises

Some sprays have long-lasting residual effect (for several weeks) and are designed to be applied on fly resting areas such as bushes, barn rafters, fences, etc. Others are fogs or mists that can be used daily, expelled into the barn air, or aimed at areas where flies congregate. Fog sprays (cans of insecticide that spray continually until empty) can be used inside a barn with windows and doors closed (and no animals inside). The building should be well-ventilated before you bring horses back in.

“If you have lots of house flies, these might be effective since these flies tend to enter buildings,” says Johnson. When using any of these sprays, always follow label directions to ensure the safety of your horses.

Stable flies take a blood meal, then go roost somewhere to digest it. Only a small percentage of the stable fly population is actually on the horse at any one time. Clymer has good results controlling them on his ranch using the newer synthetic pyrethroids and trying to prevent possible breeding sites. “Instead of using sprays on horses, I normally use a residual spray on walls and other areas where flies congregate,” says Clymer. “This product lasts a long time and can be very effective in a barn or shed until it gets a layer of dust on it. The stable fly has already bitten the horse, but if you kill the fly, you prevent its reproduction. House flies, horn flies, and even stable flies have developed resistance to some of the older insecticide sprays, and you might have to change products to get good results.”

There are also automatic, timed sprays that spray inside the barn as often as needed. Tubing from pump to nozzles is secured to stall partitions or rafters, with nozzles every 12 to 15 feet. Less elaborate systems dispense insecticide from an aerosol bottle with an automatic timing device powered by flashlight batteries, putting out spray every 15 minutes.

“Make sure spray systems keep working properly,” says French. “The lines must stay clean and the reservoir tank must be clean. Most of these spray systems are situated above the horses. If you have horses in the barn all the time, there can be problems if there’s any bacterial contamination in the tank. Horses may develop significant eye lesions from the mist settling down on them. We’ve seen some conjunctivitis and/or eye ulcers associated with contaminated spray systems,” he says.

“Insecticide sprays can help in the short term to reduce adult flies and mosquitoes, but the real key is to try to see where they are breeding and eliminate those sites as much as you can,” says Townsend. “If your stable and paddocks are producing large numbers of insects, you need to see what you can do to stop this.”

Biological Controls

Some horse owners prefer not to use insecticides on their horses (using “natural” repellents instead), or they try to find alternatives to insecticides around the barnyard to control insect populations.

Biological control options include parasitic wasps that lay their eggs in immature (pupal) stages of several species of flies. The wasp eggs hatch quickly and use the fly pupae as food. Identify the target fly before you use wasps to make sure this will work. These wasps can help control house flies and stable flies, but not horse flies or face flies. The adult wasps are harmless to animals and humans because they do not sting or bite.

“These work best if you have fairly restricted breeding sites and can put out enough wasps to deal with a fly population,” says Townsend. In dry climates and dry years, you will get more fly control with parasitic wasps than in wet years with many breeding sites. The amount of wasps needed is usually based on the number of horses you have. Suppliers recommend releasing wasps at the start of fly season before flies are numerous and putting out more wasps (weekly or monthly) through the summer, spreading them on manure in corrals and barns. A stable/paddock area with one or two horses would need about 5,000 parasitic wasps each month. Each operation is a different situation and may require different numbers for effective control.

“Even though number of horses is a way to figure the number of wasps needed, you may have only a few horses and many breeding sites–and the recommended number of wasps might not be able to keep up with the fly population,” says Townsend.

There are also some differences in fly parasites. You need to determine which ones will be most effective in your situation. Some types feed just on house flies or on stable flies, while others feed on both. Don’t buy unspecified blends of wasp species, as some are not very effective for fly control.

Clymer says several companies sell parasitic wasps; only a handful are actual insectaries. He recommends that you only buy from reputable companies that understand the pest management concepts and supply the correct species of parasitic wasp.

Clymer also recommends that we not forget about the importance of dung beetles in the constant battle against horse parasites and manure accumulation. In a stall situation, dung beetles will probably not have much impact, but for horses on pasture, they can dramatically reduce both internal and external parasites, as well as bury lots of manure. There are more than 90 species of dung beetles in North America and nearly all regions have several species. They, like the flies, are only active during the warmer months of the year. Dung beetles are not currently being marketed in the United States, but they can move into an area over time if the conditions are right.

They can also be collected and relocated. However, their transfer across state lines may be regulated. It is also very important, if you are going to collect dung beetles and relocate them, that they be able to adapt to the new location. The avermectin horse dewormers can cause high mortality in immature dung beetles and could prevent dung beetles from being established. Moxidectin dewormer is environmentally friendly and does not kill immature dung beetles, notes Clymer.

Some horse owners use nematodes as a biological control tool. These are microscopic worms that live in the soil and destroy the larval stages of flies, termites, and other pests. They release bacteria that is harmless to mammals, birds, and earthworms, but kills fly larvae–which are then fed upon by the nematodes. These tiny fly-killers can be ordered from several companies; you add water to activate them, then place them in manure piles, in stall corners, and other sites where flies might be a problem.

“There are some things to keep in mind when using these,” says Townsend. “The soil must be moist. Nematodes don’t survive in dry conditions. Put them into moist bedding or manure piles. Also be aware that these are living organisms and have limited shelf life. If you mix up the dose rate you want to use and have extra, you can’t keep them to use later.

“Like parasitic wasps, these seem like a good idea, and for the right type of situation they might be,” says Townsend. “But generally it takes a variety of control methods–an integrated approach–to reduce a fly problem. Nematodes might be useful for house fly and stable fly control where breeding sites are accessible. After you’ve done as much as you can to eliminate the sites, you could use wasps or nematodes as a way to catch up.”

You can reduce adult flies with spray (residual insecticide that stays on sprayed surfaces) on parts of your property such as vegetation or barn walls where flies rest, then use nematodes or parasitic wasps in breeding sites, along with good sanitation. “If you are careful, you won’t kill wasps or nematodes with sprays,” says Townsend.

Clymer says these various methods of control are all useful. “The more tools you use, the better control you will have,” he says. “You can’t rely on any one of them to do the whole job, and none of them will replace good sanitation.”

Some horsemen use apple cider vinegar externally or internally to repel flies (up to a half cup can be mixed with the daily grain) and feel it makes a horse less attractive to insects. “There may be some benefit to this, but it’s something you’d have to try out and monitor to see if it works,” says Townsend.

Fly Traps and Bug Zappers

Devices that kill flies on contact or trap them will kill some, but not others. “There’s a new one that’s just been developed for horse flies and deer flies, but it hasn’t been evaluated yet for effectiveness,” says Johnson. “Years ago I used panel traps for stable flies. These are made with flexible panels covered with sticky material. Sunlight is reflected off the panels and attracts the flies. Mosquito magnets catch a lot of mosquitoes, but we’re not sure how much effect any of these traps have on actual populations. I doubt that one trap would be sufficient; you might need two or three or more on a premise, and that gets expensive.”

House flies are attracted to fly electrocutors, baits, and traps, but stable flies and horse flies are not, says Townsend. The latter are blood feeders and attracted to the odor, shape, and form of the horse. “Some mosquito magnets produce warmth and CO2 and attract biting flies, but it’s hard to say what percent they kill,” he adds. “In large, open areas without many sources of blood meals for blood sucking flies and mosquitoes, a trap will be more useful than where there’s vegetation or buildings and many other things that block out what’s attractive about the traps.”

French says new horse fly traps work well. “A black ball attracts the flies; they go up under a canopy to get to it, and when they decide this isn’t where they want to be, they fly straight up into a jar that captures them,” he says. There are also some propane mosquito traps that work fairly well.

Traps can be part of an overall program, but you shouldn’t rely on them alone.

Physical Protection

Before insecticides, horse owners resorted to augmenting the horse’s own defenses (mane and tail) by providing extra swishing power. Long fringe was added to halters, bridles, and harness, so the horse could brush flies off his face and body more easily. Today there are some sophisticated physical protections, including improved fly shakers, masks, bonnets, and body sheets.

Some masks protect just the eyes, while others cover ears and cheeks as well. Some have fleece-lined ear openings and edges to keep flies from crawling in under them. Fly shaker strips/fringe can be attached to halter, bridle, or harness, and some of these are impregnated with repellent or insecticide. There are also fly boots and fly leg wraps (with mesh weave to allow air circulation) to protect the legs so horses won’t have to stomp continually to try to dislodge flies. Fly sheets cover the body with a fine mesh (allowing air flow and sweat evaporation) that keeps flies from biting–without making the horse too warm.

Some leg wraps and fly sheets are permeated with permethrin that is effective for more than a year. This type of protection is a spin-off from military use of permethrin-impregnated uniforms that protect soldiers from disease-spreading insects. The treated fabrics kill and repel most flies, as well as spiders, ticks, and mosquitoes, and they stay effective through many launderings.

“When insects land on this material, it’s irritating to them and they tend to leave before biting, even if they are not killed by the insecticide,” says Townsend. “One drawback to pyrethroids is that humans and horses can develop skin sensitivity to them. If insecticide is in contact with the skin for a long time (i.e., repellent horse clothing), watch for signs of irritation or redness. Some horses are more sensitive than others. Don’t use these products long-term without monitoring.”

Feed-Through Larvicide

Various types of feed-through larvicides have been around for years. Owners can feed them in horses’ grain daily; the product them passes through the digestive tract to end up in the manure, where it inhibits development of fly larvae. No adverse effects have been noted in trials conducted on pleasure horses, broodmares, or breeding stallions.

Entomologists are in agreement that the use of pesticides directed at the control of flies in the larval stages (i.e., larval sprays or feed-throughs) can significantly increase the speed of insects developing resistance to those products. Some of the products, such as cyromazine, currently on the market have been around for years, and resistance has developed. When resistance is an issue, it is often necessary to change the class of chemical to get effective control. Some products can also produce cross-resistance to other classes of compounds.

Horse owners should remember that there are lots of other areas that flies will develop (besides manure); therefore, this should not be relied upon solely as a means of fly control.

An earlier product that worked against larval stages of flies in horse manure was an organophosphate pellet. Equine products no longer use organophosphates, because they are more toxic to horses (especially with long-term use) than some other insecticides. “Some horses developed liver problems when organophosphates were continually used as a feed-through larvicide,” says French. “There were also problems when the manure is left in a pasture; it remains there for a long time without being broken down, and dung beetles were also inhibited by organophosphates.”

If the only house flies and stable flies on your farm are produced from your own horses’ manure (rather than migrating in from somewhere else), they can be readily controlled by using feed-through products, he says.

“One question frequently asked is if using the larvicide will affect parasitic wasps,” he says. “Yes, it will. The wasps won’t have anything to feed on if fly larvae are killed by the feed-through.”

There are many products and strategies you can use to combat insect pests on your property. The most effective approach for your particular operation depends on many individual factors and will utilize several of the products and management options available. Consult with your veterinarian or local extension agent to map out the ideal strategy that will minimize insect problems for you and your horses.

 Adverse Reactions

Some horses develop skin sensitivity to certain products, even on first use. “Be very cautious when using any product; if a horse has an allergic reaction, stop using it,” says Bill Clymer, PhD, BCE, a livestock parasitologist based in Amarillo, Texas. Don’t use it again on that horse or he might react more seriously the next time. When trying a new product, put a small amount on one area of the body to see if the horse has a sensitivity reaction before applying it all over the body. If problems persist, contact your local veterinarian.

“Even when you put it on a small area, the horse will likely have a reaction over his whole body if he’s sensitive to it,” says Dennis French, DVM, MS, Dipl. ABVP, head of the University of Illinois’ Department of Veterinary Clinical Medicine, because it opens the gateway for the reaction. Smaller is better when testing. If the horse reacts, use warm, soapy water to remove the product, then rinse well. “The soap tends to bind to the product and helps take it off. Rinsing alone won’t get it all off,” explains French.

“One reason that pour-on insecticides (like those used on cattle) will never be registered for use on horses is that horses are more apt to react to these,” says Clymer. “You may have only 1-2% of horses that react, but it might be a serious reaction, with swelling and blistering of the skin,” he says.

Setting Fowl Against Flies

Laboratory trials have documented that Muscovy ducks remove adult house flies at least 30 times faster than commercial bait cards, coiled fly paper rolls, fly sheets, or fly traps. “Ducks in cages took only 0.6 hours to remove 90% of the population of flies, compared to 15.3 hours for the most effective commercial device,” says Dennis French, DVM, MS, Dipl. ABVP, head of the University of Illinois’ Department of Veterinary Clinical Medicine. “The ducks survived for more than 12 weeks in pens with calves without injury or feed supplement. Ducks ingested an average of 25 house flies per 15-minute observation period,” he says. Ducks might be beneficial if a person didn’t mind the nuisance of having them around the barn.

Insecticide Application Safety

Some horses are skittish when you use aerosol products. When spraying the legs, Cliff Hoelscher, PhD, retired professor of international agriculture at Texas A&M University, suggests having the horse behind a fence panel where he can’t kick or strike. “If we have a stable fly problem, I use a slatted board between me and the horse so I can spray the legs without being kicked,” he says. “Here at Texas A&M University, we have a special stall where we can put two horses at a time to treat, without any danger to ourselves.”

Always follow label directions. Bill Clymer, PhD, BCE, a livestock parasitologist based in Amarillo, Texas, recommends reading the label every time you use a product, even if you’re using it every day–to make sure you haven’t gotten it mixed up with something else. Label information is usually written in fine print, but the safety precautions and dosages are written larger and are easily read.

Hoelscher recommends wearing rubber gloves, a long-sleeved shirt, and long pants (or some type of coverall that covers arms and legs) when using a spray. “Ideally you should wear a mask, but even if you wear a hat or cap, this will reduce exposure to face and eyes,” he says. “Cover up well when spraying, then wash your hands or bathe after you finish; this will greatly protect you from overexposure.” 

Reprinted with permission from The Horse: Your Guide to Equine Health Care/ Find more veterinarian-approved horse care information at

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