Tack and feed store displays are overflowing with fly spray bottles. There are numerous options available with different active ingredients, formulations and brand names. As you reach for the shelf, here are some tips to help you choose and use these products wisely so your horse doesn’t get bugged this summer.

How Fly Spray Works

Although fly spray is a familiar product to horse owners, you may not have given much thought to how it works or what’s in the bottle.

“Fly repellents work by either repelling flies or killing them upon contact,” says Erika T. Machtinger, Ph.D., assistant professor of entomology at Pennsylvania State University in State College, Penn. “Repellents contain active ingredients that emit an odor or taste that flies find unpleasant and will try to avoid. Some fly repellents work by combining both repelling and killing properties. These repellents contain a combination of active ingredients that repel flies and contain insecticides that kill them upon contact.”

Why Fly Control Matters Flies are not just a pesky annoyance to horse owners and their animals—they can also pose significant health risks, says Machtinger. Depending on the species of fly, they can transmit pathogens that can cause serious diseases and conditions in horses. Biting flies have also been linked to adverse reactions, including hypersensitivity and itching. In addition, Machtinger points out that flies can also impact a horse in other ways. “High fly pressure can lead to pain, irritation, and changes in grazing behavior, such as reduced grazing time and lower forage intake,” she says. “This can result in reduced energy for growth, reproduction, and body condition maintenance. The negative impact of flies on horse behavior can lead to serious consequences, including injury or loss of condition.”

Active Ingredients

Most companies will list features and benefits on the front of the label, says Casey White, an entomologist and senior director of technical services and innovation for Central Life Sciences, Farnam’s research and development division.

“If it’s something that kills and repels, it’ll say ‘kills and repels’ or ‘insecticide and repellent.’ Often, it’ll call out major species of insects or arthropods, such as ticks, that the product is effective against.”

The small print is where you should pay attention to the listed ingredients, including the number and type of active ingredients and their percent concentration. The active ingredients are what give the product its killing and repelling power.

Active ingredients can be either natural or synthetic. Pyrethrins are a natural insecticide derived from the chrysanthemum flower. These offer knockdown benefits but break down quickly when exposed to sunlight. Pyrethroids are synthetic versions designed to have longer-lasting action; these include permethrin, cypermethrin, tetramethrin, and others.

Essential oils such as citronella, geraniol, eucalyptus, thyme, cedar oil, lemongrass, rosemary oil, and clove oil are natural options to repel insects. Fatty acids are another type of natural repellent, including octonoic, nonanoic, and decanoic acid.

Other ingredients include synergists, which work to amplify the active ingredients to provide greater killing power and/or longer-lasting protection. One of the most common is piperonyl butoxide (PBO). Others include butoxypolypropylene glycol and n-octyl bicycloheptene dicarboximide (MGK 264).

Comparing Products

Looking at the list of active ingredients and their percentages is one way to compare products and select the one that you think will work best for your needs.

“Formulas that use newer technology and a higher level of active ingredients cost more to make,” says Amy Cairy, vice president of marketing for W.F. Young, makers of Absorbine. For these reasons, you can expect many of the best-selling fly sprays to come with a higher price tag, she notes.

Along with comparing the levels of active ingredients, it’s also important to look at the recommended application rate, says White. A product may look like it’s half the concentration of a comparative product, but if you have to apply twice as much of it, he points out that you’re essentially applying the same amount of active ingredient—just in a more dilute form.

Fly Spray Formulation

In addition to the active ingredients, a product’s formulation is another difference to consider. Most fly sprays can be categorized as oil- or water-based, and there are pros and cons to each type.

Oil-based sprays have staying power, but can attract dirt and dust and may be irritating to horses with sensitive skin. If you see “contains petroleum distillate” on the list of ingredients, that’s the tipoff that it’s an oil-based spray.

Water-based sprays are non-irritating and less of a magnet for dust, but will likely need to be reapplied more frequently.

Many horse owners prefer all-natural formulas using essential oils and plant extracts as an alternative to chemical sprays.

Long-lasting sweat and water-resistant formulas contain ingredients that help the product stick to the hair shaft. These are designed to hold up well to rain and sweat under normal turnout and working conditions, says Cairy.

However, she notes that if you rinse your horse with a hose, sponge him off, or bathe him with shampoo, you are removing the product from the horse’s hair and should reapply the fly spray afterward.

Coat conditioners and sunscreen are other bonus ingredients you might find in a fly spray.

Choosing the Right Fly Spray

The right fly spray can depend on several factors.

“First, consider the type of flies that are prevalent in your area and choose a spray that targets those specific types of flies,” says Machtinger. “Next, consider your horse’s individual needs, such as any skin sensitivities or allergies. It’s also important to consider the spray’s effectiveness and how long it will last, and finally the ease of application and whether it requires dilution or not.”

The local climate and weather conditions, such as heat and humidity, as well as what you are doing with your horse also influence your choice of fly spray. You may want to have a few different kinds in your kit with different features and formulations. The fly spray you use right before you go in the show ring might be different than the one you’d use to turn your horse out in the field.

“For example, if you have a horse that is easily bothered by a wide variety of flies or is reactive to insect bites, you’ll want to find a long-lasting, broad-spectrum fly spray that both kills and repels insects,” says Cairy. “Or if you live where it’s hot and humid or have a horse that is prone to heavy sweating, you’ll want to reach for a fly spray that binds to the horse’s hair shaft and won’t sweat off.”

DIY Fly Spray Homemade fly spray recipes abound on the internet. If you’ve been tempted to whip up your own batch of fly spray to save a buck, here are a few things to consider before you do it yourself. “The cost for a manufacturer to develop a formula and obtain a registration from the Environmental Protection Agency (EPA) can easily exceed $250,000 and take upwards of two years to complete, given the extensive testing required to prove safety and efficacy,” says Cairy. She advises caution to anyone considering making fly spray at home. “Horses have very sensitive skin and can have severe reactions to certain ingredients, especially essential oils,” she says. “In addition, mixing certain ingredients together can cause adverse reactions on the skin, including chemical burns, especially when the horse is turned out in the sun or when other products like shampoos or coat sprays are applied.” This may put your mind at ease after wondering if spending the money is worth it. “Fly sprays from trusted companies have a long track record of safety and efficacy, and while they may be more expensive to purchase, they generally provide the best protection and overall value for you and your horse,” says Cairy.

Not Enough

One of the biggest fly spray mistakes is simply user error. Either not applying enough, not using the product according to the directions, or selecting the wrong type of product for the insect species you are dealing with.

How do you know if you’re applying the recommended amount of fly spray? Cairy suggests counting sprays and measuring how many it takes to reach the recommended amount, then using that for future reference. (About 45 sprays is what she has found it takes to reach a typical 2-ounce application.)

The Right Way to Apply Reading the product label and applying according to the manufacturer’s instructions is essential for any fly spray to work correctly. Even if you have been using the same fly spray for years, it’s always a good idea to refamiliarize yourself with the information on the label, says Casey White, an entomologist and senior director of technical services and innovation for Central Life Sciences, Farnam’s research and development division. ◆ Apply to a clean horse. The product needs to adhere to the hair shaft to provide effective protection, and a layer of dirt gets in the way of that. “If you apply fly spray to a horse that is covered in dirt and loose hair, the fly spray won’t easily get to the hair shaft and will likely fall off when the dirt and loose hair falls off your horse,” says Cairy. ◆ Cover the entire body, including head, legs and face. Missed spots can become a target for flies. (Avoid spraying the face; always use a cloth to apply around the eyes and nostrils.) Using a towel, applicator mitt, or brush is the most effective way to ensure coverage across the entire animal, says Erika T. Machtinger, Ph.D., assistant professor of entomology at Pennsylvania State University in State College, Penn. She recommends using disposable gloves to protect your skin from contact with the product and following all label precautions. ◆ Be sure you are using enough product. It is crucial to follow the label application rates for the fly spray to deliver optimum performance. ◆ Test new products. It’s a good idea to spot-test new products, especially if you know your horse is sensitive. Apply to a small area, such as on the shoulder, and monitor for any reaction before you make a full application. It’s not common, but it can happen, says White.

Pest Resistance

If you find that your go-to fly spray doesn’t seem to be as effective, there may be a reason for that.

“In addition to the method of application, the effectiveness of these products can depend on the toxicity of the active ingredient to flies, and local pest resistance,” says Machtinger. “Flies can quickly develop a resistance to the active ingredients in many fly sprays (or already are resistant to them), reducing or in some cases eliminating their effectiveness.”

Pesky house flies are among the most affected by insecticide resistance. Fortunately, Machtinger reports, newer fly spray formulations using natural ingredients such as fatty acids and plant extracts have proven to be effective and longer-lasting in laboratory tests. The 2019 published study she led also suggests that formulation differences among pyrethroid products can significantly affect their efficacy.

“We do see some resistance to fly sprays, particularly permethrin,” says Cairy. “When horse owners start thinking, ‘My fly spray isn’t working the way it used to,’ we recommend that they rotate to one with a different formulation.”

When rotating products, look for something with an active ingredient that works differently than what you have been using, such as a different chemical class, advises White.

“Many of the on-animal sprays have the same mode of action, which makes incorporating other fly management tools even more important.”

Other Fly Management

There are many other things horse owners can do to help keep fly populations down, including good sanitation and manure management, and taking steps to eliminate fly breeding habitats and disrupt their life cycle.

“Scatterbaits, fly traps, feed-through larvicidal products and larvicides applied directly to fly development areas are other options that should be incorporated into a good integrated pest management (IPM) program,” says White.

It might seem like a fly is just a fly, but it’s important to try to identify what fly species you are dealing with, because they aren’t all created equal when it comes to fly control.

“To prevent flies on horses, it’s important to first understand which species of fly is the pest,” says Machtinger. “Each species will have different methods of control that are effective.”

She emphasizes the need to use a variety of tools in combination, such as good stable management and hygiene, fly masks and sheets, and species-specific fly traps.

Flies are a nuisance, but armed with the right knowledge and tools, it’s possible to fight back.

Further Reading
◆ Hoof Problems Caused by Fly Stomping
◆ Flies, Worms and Yucky Things
◆ Using Parasitoids for Fly Control
◆ Understanding Fly Spray Labels

This article about fly spray appeared in the July 2023 issue of Horse Illustrated magazine. Click here to subscribe!