How Fly Spray Works: Why It Matters to You and Your Horse

You may be one of those horse owners who stays faithful to a product for years. Or you may be in the camp that likes to experiment with different products.

One thing is certain, the shelves—both online and at your local retailer—are brimming with a variety of fly repellent products.

You don’t need a science degree to have a basic understanding of how fly sprays work. In just a few minutes, you can learn the how and why behind choosing products that work best for your horse.

How they work

You might think the goal of any fly spray is just to keep flies away, but it’s not that simple.

The active ingredients—both natural and synthetic—in fly sprays have different properties and don’t work the same way.

“The active ingredients and chemicals may result in different types of repellency,” says Casey White, entomologist and Senior Director of Technical Services and Innovation at Farnam's Research and Development facility in Dallas.

“If you’re a horse owner who doesn’t want to see flies land on or stay on your animal, look for a product that says ‘repels and kills,’” advises White, noting that most fly control products in the Farnam portfolio will kill and repel.

If the label says “kills and repels,” this means that whether the insect is contacted by the product during application or comes in contact with it on the animal, the pest will be driven away and die.

Tactile repellency

Entomologists often refer to tactile repellency as “hot foot” repellency. That’s because when insects land on a horse sprayed with a product containing these active ingredients, they experience an irritating sensation, and quickly identify that’s not a good place to be.

Just by landing on the sprayed animal, the pests will likely leave and then die because they’ve come in contact with those active ingredients.

Among active ingredients that offer tactile repellency are:

• Cypermethrin
• Permethrin

“You may see flies on the horse for a brief period, but they’ll be repelled to move away from the treated horse and likely go away and die,” says White.

Spatial repellency

Ingredients with spatial repellency discourage flies from landing in the first place.

They create somewhat of an invisible barrier around the sprayed horse, causing blood-feeding pests like mosquitoes, midges and certain flies to look for other places to obtain a blood meal.

Among active ingredients that offer spatial repellency are:

• Citronella
• Other plant-based essential oils

“Most horse owners using a spray as a repellent value the fact that the product drives pests away, instead of letting them land and feed on or annoy the horse,” says White.

Closer look at ingredients

Product labels don’t distinguish between spatial and tactile repellency characteristics, but now you know there’s a difference between active ingredients.

By law, every fly spray label must list the product's active ingredients, which are registered with the Environmental Protection Agency (EPA).

Active ingredients are those ingredients with insecticide (killing) and repellent properties.

Active ingredients can be both natural and synthetic. Because synthetic active ingredients last longer than natural active ingredients, the most effective fly repellent products often contain a combination of both natural and synthetic active ingredients.

One of the most common active natural ingredients are pyrethrins, which are derived from a species of chrysanthemum.

Pyrethroids are synthetic forms of pyrethrins. Some common pyrethroids are permethrin, cypermethrin, deltamethrin, tetramethrin, and resmethrin.

“Inactive” ingredients serve as the base of any fly spray and these ingredients aren’t required to be listed individually.

Natural angle

But wait, isn’t citronella a natural ingredient?

That it is. In fact, citronella is just one common ingredient found in repellents that use natural ingredients instead of chemicals.

Natural ingredients typically include essential oils such as citronella, geraniol, eucalyptus, thyme, cedar oil, lemongrass, rosemary oil and others.

Just realize that natural repellent sprays don't contain insecticides, meaning they are better used to repel insects vs. kill them.

Water-based vs oil-based

Fly sprays are either made with a water or oil base, depending on the ingredients used. If you see "petroleum distillates” listed among active ingredients, it's oil-based.

Oil-based products adhere to the hair coat quickly and provide fast pest knockdown. However, if you’re trying to keep your horse’s coat spotless, just know that these products are more likely to attract dust and dirt.

Water-based products tend to be less irritating to skin and don’t attract dirt and dust, but White points out that water-based repellent products break down more easily.

To help water-based repellents last longer and stay effective, the formulation typically includes chemical agents known as synergists, which you’ll see on the ingredients list as Piperonyl Butoxide (PBO), Butoxypolypropylene Glycol, and N-Octyl Bicycloheptene Dicarboximide (MGK 264). Try saying those fast three times!

White explains that when a product has claims like “weatherproof” and “won’t sweat off,” this is due to both the combination of active ingredients and the formulation.

Keep in mind that bathing isn’t the same as the horse being rained on or getting sweaty.

“Any time you are bathing a horse, it’s going to diminish the effectiveness of the product. Even if a product has weather-resistant or sweat-resistant claims, you’re likely going to remove the product if you use shampoo or high volume of water,” says White.

He recommends reapplying fly spray after bathing, but it’s important to let the horse completely dry first.

Read that label

Okay, so the label on your fly spray doesn’t count as exciting reading, but those directions are key to how well a product works. Take a minute to actually read them.

“If you’re buying a product to protect your horse and expecting it to meet the claims on the label, you can’t expect it to perform unless you use it according to label directions,” says White.

One of the most common mistakes horse owners make is not applying fly repellents according to directions.

“Many people will just spray their horse a dozen times and think they’ve done it justice. This may kill any flies that are actually sprayed, but to get the residual rate that’s stated on the label, you have to apply according to the label,” he notes.

White explains that a huge amount of research goes into dosage and application rate to ensure that label claims are met. Following label directions is important for both safety and to ensure the product works as stated.

Application rate

In case you wondered, application rates can vary significantly.

For example, some product directions specify applying 1 ounce per 100 pounds. For an average 1,000-pound horse, that translates to over 1 cup of spray per application. Other products specify applying 1 ounce to the entire horse.

It’s easy to see how just a few quick spritzes across the horse with either product isn’t going to apply enough to work according to label directions.

A straightforward way to know you’re applying the right amount is to do a little research of your own when using a new product. Count the number of sprays as you spray the product into a measuring cup to the amount stated on the label for protection and efficacy.

You don’t have to measure again, just spray that approximate number of times when applying the product to your horse.

Using multiple products

There are times you may want to use more than one product.

For example, let’s say you rely on a spot-on for on-going protection for your pastured horse.

A spot-on is applied to strategic locations on the animal according to label directions. Thanks to the oil-based carrier, after application the product disperses across the animal’s body to provide protection and full coverage over time.

“The spot-on product works well on horses that are turned out on pasture and not being ridden frequently,” says White. “If you go out to ride on the weekend, you might want to spray with a repellent product before riding, depending on insect activity.”

Natural repellents tend to smell good and are very effective for a short duration. This can make them an ideal choice specifically while riding.

Congrats! You just became a mini expert in the science of fly repellents. In addition to making smart choices the next time you shop, you’ll never look at a fly spray label the same way again.