Fly Sprays for Horses, What You Don’t Know Can Hurt You!



People often ask me, “What is the best fly spray?” That is not an easy question but if you ask 50 people, you’ll get 50 different answers! Let’s see if I can demystify the question some.

To start with there are probably 200 or more brands of fly sprays. Each touting its efficacy with claims of “24/7 protection,” “Kills and protects for up to 17 days!” “Repels and controls biting flies for up to 8 hours!” There are claims on every bottle. Let’s look at those claims and how they can put such outrageous things in print. First and foremost, a company must do clinical trials  before a product is released to the public. Most companies hire a testing facility where they put a white square in a box full of flies and spray it with their product. The box is closed and weatherproofed so the flies have every opportunity to land on said square. Then they set the timer and see how long it takes for a fly to land. The product is fresh as a daisy the whole time it is in there so it can take days or weeks before the fly population decides to land there, there are no blood hosts living in the box, and there are plenty of other places for them to rest! Now the product has been “proven” to be effective for two weeks. Reality check. The first time your horse rolls in the dirt most of what you put on in the morning is now on the ground. Bathe, ride, work your horse sweaty, or a dozen other environmental happenings and your fly spray is gone. So be aware that claims on the bottle do not reflect real-life situations. Another fun fact is the types of flies they claim to repel and kill are those that land on the white square or die on the white square. In real life, the B-52’s don’t die after they bite your horse, they just leave a trickle of blood and a hearty thank-you for the meal.

The next thing to know is what’s in it. You can claim whatever the test facility “proved” for your product but the sin of omission is a grey area with labelers. There are many things that can affect your main ingredient. For instance, the UV rays from the sun can make your active ingredient inert in a matter of minutes, especially the chemical-based kill ingredients. Therefore a sunscreen in the mix would indicate a longer-lasting product. A waterproof (no such thing) formula will last longer but there is nothing you can’t wash off your horse. If it was truly waterproof, gosh we wouldn’t have to spray ever again! Again, in controlled experiments it probably sheds water like a duck. Note the following excerpts taken from the MSD sheet on permetherin:  (

Pyrethrins and permethrins:

· are unstable in the presence of light, heat, moisture and air

· are hydrolysed by oxygen and/ or sunlight

So it pays to look into the label further than the claims and see just what is in it and any chemical-based product with sunscreens in it will be preferred over those without.

The ingredients found in fly spray are the important items on the label, not who made it, how fancy the label is or how outrageous the claims. There are two classes of fly sprays, I call them “Knockdowns” and “Herbals.” The Knockdowns will contain permetherins, cypermetherins or pyrethrins. The remaining active ingredients perform the “repel” function or “supersize” function and are generally piperonyl butoxide (your super sizer), citronella, and other scent repellers. The key to efficacy is reading the ingredient labels. Here’s a perfect example:

Ultrashield EX contains: .50% permetherin, .10% pyrethrins, 1% piperonyl butoxide, and 98.40% inert ingredients (generally alcohol and/or water) It sells here for $24.91 in the RTU spray quart bottle.

Bronco contains: .03% prallethrin (a synthetic pyrethroid) .10% permetherin, .50% piperonyl butoxide and 99.367% inert ingredients. It sells here for $6.85 in the RTU spray quart bottle

So why the huge disparity in price? Ultrashield has higher-quality ingredients and much more of them as well as a healthy dose of super-sizer. Bronco is essentially a bottle of water that they waved a fly repellent over the top. In our tests at several different farms we found Bronco to be as effective as a bandaid on a severed artery. The bugs literally licked the stuff as it came out of the bottle. The Ultrashield worked on the one or two hour trail rides and the first half of the day in turnout but by 5:00 the horses were running for the gate. We didn’t find any dead bugs around the horses either, yet it will kill it if you spray the bug directly (so maybe carry a holster with you and zap them as they buzz you?) But even armed with this information from the label and multiple on-site tests with real horses, people will swear to you that Bronco works great. Some people just feel better thinking they are doing the right thing by offering a few minutes of protection. There are a couple of skin and hair moisturizers that people swear by as well and you know what? They do work on mosquitos and gnats because they are mostly made of marigold oil (pyrethrin’s home) and the scent does discourage those types of flies (and many of your friends!).

On to the herbals. They consist of two types, the ones that actually are herbal and those that sneak under the federal guidelines and put a bunch of flowers on the label to make you think they are herbals. The true herbals are essential oils such as thyme, cedar, lemongrass, rosemary, citronella, clove and geraniol. These are very effective on mosquitos and gnats (just like their smelly cousin Avon Skin So Soft) but are fairly useless when it comes into July and August and the B-52 bombers and their ugly red-headed step-children, the deer fly, come out to play on trail riders and pastures. The “fakers” in the herbal world are those that carry a large quotient of pyrethrins. In large quantities, it is a neuro-toxin to flies, in small quantities it acts as a repellent. Pyrethrins are gathered from marigold flowers, hence the “natural” labeling. But as I say, poison ivy and uranium are naturals, too, and not necessarily something I want to spray on my horse. If you want a true herbal, stick with the ones that are made of essential oils and keep your trail riding to the evening hours or early mornings during the peak of biting-fly season.

Your choices of brands may be hundreds but your main ingredients remain countable on two hands. Therefore, knowing the type of bug you want to repel will help you choose the right spray. Gnats, no-see-ums, and mosquitos all respond well to scent-type citronella or geraniol-based sprays. Big biters respond well to permetherins. The higher the percentage of permetherins the more efficient the spray. ProZap makes a product called War Paint in a stick form and it contains a full 7% permetherin, thereby making it a favorite for bellies and necks in pastured horses. Of course I would be remiss if I didn’t warn you that permetherin comes with a stern EPA listing as “likely to cause human cancer.”  That said, you should never inhale it, spray around children and wash your hands after working with your horse. Also, be a dear and inform your farrier and vet so they are aware of the carcinogen sprayed all over your horse.

A little word about the “super-sizer” piperonyl butoxide. It’s not a repellent or kill ingredient at all but instead it is added to the premium brands because it causes the permetherin to work harder and faster. Check out the warning on that fun product below:

A 2011 study found a significant association between piperonyl butoxide in personal air collected during the third trimester of pregnancy and delayed mental development at 36 months. Children who were more highly exposed in personal air samples (≥4.34 ng/m3) scored 3.9 points lower on the Mental Developmental Index than those with lower exposures. The lead researcher stated, “This drop in IQ points is similar to that observed in lead exposure. While perhaps not impacting an individual’s overall function, it is educationally meaningful and could shift the distribution of children in the society who would be in need of early intervention services.

One other alternative to sprays against the miserable stable fly (you know the one… looks like a house fly but bites the ankles of you, your horse and dog incessantly) is the fly predators. These little buggers eat the larval stage of this fly (a carrier for EIA and a host of farm animal diseases) and if done religiously, in two seasons you will have almost no stable flies.

The final choice is yours. Do the health risks of the premium brands outweigh your horse’s comfort? Or can you do alternative horsekeeping such as night turnouts, dusk and dawn trail riding at peak season, sheets and hoods, and good housekeeping in the barn? Some folks worry about West Nile virus but herbals do a great job keeping the mosquitos at bay. Ultimately, your home area will lay down the final verdict on which one works best because you may have different environmental influences and different bugs than your friend who might live only two miles away! When I’m asked which one is best, I shorten the spiel above and tell people to always start with the herbals to be safe and then if you really can’t live with the bugs, work your way up to the “canned cancers.”

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About hossjudge

Author - mystery novels that include horses Tack shop owner NEHC Large J judge in both divisions Clinician, coach, 4-H leader for 35 years. All around horse nut!
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5 Responses to Fly Sprays for Horses, What You Don’t Know Can Hurt You!

  1. cashsmom says:

    Good info that everyone that uses bug spray should know.

  2. Pingback: Friday News & Notes from FLAIR Nasal Strips : EVENTING NATION

  3. Pingback: Fly Sprays: Canned Cancer vs Essential Oils | A House on a Hill

  4. Tim John, DVM says:

    Well researched post about the realities of fly sprays. One other point that should be made is that some pyrethroids are also endocrine disruptors (estrogen primarily). There is an alternative to the “knockdown” and the “herbals” that uses fatty acids — Ecovet Fly Repellent. As the owner of a large veterinary hospital in WA, I can attest to it’s effectiveness. Full disclosure — my company markets this product.

  5. berney2007 says:

    I’m way late to the party, but glad to have found this random blog post. It was helpful. I just bought a young mare after a few years without anybody, and want to check out the newest stuff. Funny stuff. Good writing.

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