Fly Protection Uncovered

Every season has its trials when it comes to horse-keeping. Although we might pine for sunny skies when we're battling spring mud, the turning of the seasons quickly reminds us of one of winter's little blessings -- no buzzing, biting, annoying flies! When those squadrons of pests descend on us from above, driving us and our horses to distraction (not to mention spreading filth and disease), it's pretty easy to start wishing for winter again.

Fortunately, we're not defenseless. Ever since flypaper was invented (in 1861, for you trivia buffs), humans tirelessly have applied themselves to the Battle of the Bugs -- and while we might never win the war, we've certainly triumphed in some of the skirmishes. These days, we have quite an arsenal at our disposal to attack and repel flies. From electric bug zappers, to pheromone traps, to parasitic wasps that feed on fly pupae, we've explored all sorts of ways and means of getting rid of flying insects.

The cornerstone of any fly counter-aggression program in any barn or stable is a bottle of fly spray. When we're planning to go on a trail ride through the woods, we squirt the stuff on our horse's coats (and spray the human version on our clothing and exposed skin). When we turn our horses out in the field on a buggy afternoon, we circle their eyes with roll-on fly repellents and tie slow-release insecticide tags on their halters. Although nothing's perfect in keeping the bugs off our equines, fly repellent formulas do help. But with their chemical smells and labeling lists of barely pronounceable ingredients, it's not unreasonable to wonder just how toxic these products really are. Are they truly safe for your horse? For you? For the environment?

Good News & Bad News

Almost all of the fly repellents currently on the market that are formulated for horses are based on a chemical called pyrethrin. The good news, for those who are suspicious of chemicals, is that pyrethrin is extracted from the dried flowers of the genus Chrysanthemum (most commonly the African painted daisy, Chrysanthemum cinerariaefolium, which is grown as a crop in Kenya, India, and a few other equatorial regions). The broad-spectrum bug repellent properties of these plants have been recognized as far back as 400 BC. (Archaeologists identified powdered flower petals in Iran that date back to this period that probably were used for louse control.) When pyrethrin is applied directly to insects, it has a powerful "knockdown" (instant kill) effect, and what's more, it quickly decomposes in sunlight, leaving no harmful residues. As chemicals go, pyrethrin is about as environmentally friendly as you could ask for.

Part of what makes pyrethrin such an effective chemical is its dual action: it acts as a neurotoxin to paralyze and kill bugs with which it comes in direct contact, and it has repellent properties because insects find it severely irritating. Most flying insects are highly sensitive to pyrethrins, which means you don't have to apply heavy concentrations of "daisy juice" for it to work. At the same time, it's quite non-toxic to most mammals. Fish and other forms of aquatic life are very sensitive to its effects, so you should be careful never to dump pyrethrin in a water source. Birds are also somewhat sensitive to the compound.

Naturally, there are a couple of down-sides to pyrethrin. The first is that the supply of the substance tends to be a little erratic. Pyrethrin-producing flowers are only harvested in a few select parts of the world, so in poor growing years, prices can soar or the chemical can become unobtainable. Karen Norkaitis, director of scientific affairs for W.F. Young Inc. (makers of Absorbine liniment and fly repellents such as SuperShield and UltraShield), confirms, "Natural pyrethrins can be hard to source because the crop varies from year to year. We're facing a massive shortage right now, in fact."

The second problem is it's very biodegradable. Because pyrethrin's chemical composition dissolves rather quickly into its component atoms when it's exposed to sunlight, its efficacy, on its own, is limited to the space of an hour or two. So, manufacturers have come up with all sorts of clever ways to lengthen its lifespan, such as adding a PABA sunscreen to the fly spray formulations. Horse people might assume the sunscreen is there to protect the horse's skin from ultraviolet rays, but in fact it's there to protect the pyrethrin! Almost invariably, fly spray manufacturers also add a synergist -- a chemical that works in concert with pyrethrin to create a longer-lasting protectant effect. Piperonyl butoxide, or PBO, a derivative of the sassafras tree, is one popular synergist, which has quite a bug-repellent effect all its own. Combined with pyrethrin, PBO can help make a fly spray last up to a few days, so you don't have to re-apply the product so often.

Chemists have improved on nature by formulating a vast array of synthetic versions of pyrethrin. These "pyrethroids," as they're called, avoid the problem of fluctuating availability, and are designed to have a more lasting effect than natural pyrethrin. They, too, are biodegradable, albeit less rapidly. There are literally hundreds of synthetic pyrethroids of varying strengths, and they have improved significantly on pyrethrin's length of action, sometimes remaining effective for up to a couple of weeks (if they're not washed off by rain or sweat). But with increased strength comes increased risk of toxicity, by the same neurological action that paralyzes the flies they are meant to target.

Fly spray formulations that claim broad-spectrum efficacy (killing a wide range of fly species) plus very long-lasting action are likely to be made with one of the more powerful pyrethroids, such as cypermethrin, and they usually carry more stringent warning labels and instructions for safe application. Milder formulations don't claim to be as long-lasting, and will need to be re-applied more often, but generally have a larger margin of safety.

"Resmethrin," says Norkaitis, "is the synthetic closest to (natural) pyrethrin in terms of longevity and strength. Permethrin is longer-lasting, and also acts as a contact insecticide; its action is a little stronger. And cypermethrin is even more potent, and also very expensive, and more toxic to the animal."

The big problem with synthetic pyrethroids is that many species of flies now demonstrate resistance to them. Jerry Butler, PhD, a medical and veterinary entomologist with the Entomology and Nematology Department at the University of Florida, has done extensive studies on fly repellents and insecticides. He notes, "Permethrin used to last three days. Now it's only effective for a couple of hours. As resistance has built up, the repellent action has been lost first." So, it's not your imagination; your fly spray probably is less effective than it used to be. But interestingly, no resistance has yet been noted for natural pyrethrin.

In response to these resistance problems, fly spray manufacturers have been forced to combine ingredients in order to provide that initial fly-killing effect and a residual, long-lasting repellent action. It's common to see fly sprays which contain both natural pyrethrin and a synthetic pyrethroid, plus PBO.

"PBO is always added," says Butler, "because it helps shunt the enzymes that resistance produces. In other words, when PBO is present, the fly can no longer denature the pyrethroid. PBO works extremely well in conjunction with natural pyrethrin, but less well in combination with the synthetic pyrethroids."

Citronella is another common fly spray additive. An extract from Cymbopagon nardus, which is a grass related to the culinary herb lemongrass, citronella oil is what gives many fly repellents that citrusy odor. Its main action is to repel mosquitoes (against which it can be quite effective).

Fly repellent sprays frequently contain other ingredients, too. Aloe and lanolin, for example, serve as coat conditioners and help reduce any irritant effect the formula might have on the horse's skin. They also help protect the repellent from washing off as the horse starts to sweat, according to Norkaitis. Butler notes that there's a potential problem with some of the "'extra" ingredients added to fly sprays to make them soothing to the coat and agreeable to the senses. Very often, he says, they serve as fly attractants, counteracting the repellent effects of the active chemical!

"Horses are a very attractive target for a fly in the first place," he notes, "and very often commercial fly repellents have as many ingredients that attract as they do ingredients that repel. The cream formulations (which are designed to protect open wounds from flies and maggots) have particularly big problems in this regard."

Often, Butler adds, the repellent ingredient washes off the horse first when he begins to sweat, "and the lanolin and other attractants are what's left on the coat!"

What about DEET (N, N-diethyl-meta-tiluamide), the chemical so commonly used in human fly repellents? For some reason, this compound hasn't found its way into formulations designed for equines, although Butler knows of at least one Florida-based company that is preparing to launch a DEET-based product. The Environmental Protection Agency has noted that exposure to high concentrations of DEET might pose "a limited health hazard," and it's been demonstrated that high concentrations don't have significantly more repelling action than moderate amounts, anyway. (Don't waste your money, therefore, on DEET-based bug sprays promising "military strength." Even the Army is now using a more dilute formulation, applied more frequently, and getting better results.) Studies on DEET at the University of Florida have revealed that in very low concentrations, DEET can actually be a fly attractant! It appears, then, that there would be a rather limited protocol for applying a DEET-based repellent to a horse, neither too much, nor too little.

How Safe Are They?

Discussions of DEET beg the question of pyrethroids as well: How safe are they, really? That they are natural compounds might be comforting at first glance, but as Butler points out, "Some of the most toxic materials in the world are natural." When you saturate your horse's skin with a fly spray, hoping to get enough repellent action to enjoy a one-hour trail ride, are you putting his health at risk? What about when the wind changes as you're spraying, and you get a lungful of the product yourself?

Pyrethroids, whether synthetic or natural, are neurotoxins. It's rare for compounds that essentially are poisonous to one species to be completely harmless to another, but in the case of pyrethroids, we can at least say they're "pretty safe." Since pyrethroid-based fly sprays first arrived on the market in the 1970s, they've been used successfully on horses and humans with remarkably few reports of adverse health effects, other than the occasional allergic reaction (which might as easily be caused by another ingredient in the formulation as by the pyrethroid). Norkaitis notes, "All of our fly repellent formulas go through regular toxicity testing with the EPA, which involves use on animals. Evidence of allergic responses has to be reported to the government."

Butler remarked that in regions where high heat and humidity are a problem, the application of fly sprays can cause edema (fluid retention throughout the body, making the horse look puffy and bloated). He suggests that water-based formulations probably are safer for horses with a history of that sort of reaction, rather than the oil-based preparations that stick to the coat better, but might have a stronger irritant effect.

On the whole, though, "it's fair to say the risk (of health problems) is low when the products are used as directed," says Dr. Jim Donald, of the Reproductive Toxicology Department of the California EPA in Sacramento, Calif. The ever-watchful EPA reviews the safety quotient of pyrethroids about every five years, and continues to approve them, says Butler. This is both an indication of their safety margin, and of the fact that there are very few other compounds available that are as effective!

However, that's not to say that there are no concerns. A few of the synthetic pyrethroids, including resmethrin, were flagged by the EPA in 1994 as having possible "reproductive and developmental toxicities." The warning is intended for humans, not horses, and it's based on studies with rats. When rats ingested resmethrin at a level of 25 mg/kg of body weight per day, orally, there was evidence of impaired fetal growth in pregnant females and a risk of fetal death. But it's important to note that the studies were based on rats ingesting or inhaling pyrethroids, or having them injected intravenously. Donald confirms, "There is no data cited from dermal exposure." It's generally accepted that systemic toxicity from inhalation and dermal absorption is very low (which is one of the reasons pyrethroids are used in the treatment of head lice). In humans, at least, it's been demonstrated that pyrethroid metabolites are processed rapidly by the kidneys and excreted.

Nonetheless, this notation from the EPA has been enough for the California branch to flag resmethrin as a chemical with possible toxic effects, under its Proposition 65. The Proposition, a voter-passed initiative unique to California, requires the governor to make available a list of chemicals known to have carcinogenic properties (pyrethroids have none, reassuringly) or a hazard of reproductive toxicity. Once a chemical makes this list, products containing that chemical might be required to bear a warning label, depending on the concentration of the toxin in the formulation. (No warning is required if the dosage in the product is one one-thousandth the amount required to produce a toxic effect.)

"The intent of Proposition 65 is just to inform the public and identify hazards," says Donald. "We don't have any latitude to disagree with the EPA once it identifies a chemical as having reproductive toxicity." (For more information on Proposition 65, visit

W.F. Young Inc. considers the California Proposition 65 list significant enough that it is re-formulating its products containing resmethrin for the California market. But one does have to take the EPA findings with a grain of salt. In order to risk any toxic effect equivalent to that found in rats, an average-sized horse would have to ingest over 11 grams of resmethrin, something he's very unlikely to do voluntarily, or even by absorption through his mucous membranes. Consider that the concentration of active pyrethroid in the average fly repellent spray is likely to be only 2-3% at best, and often mere fractions of 1%, so the risk is remote.

As for PBO, it's suspected of causing chronic liver and kidney damage in test animals at very high doses (over 1,800 mg/kg of dermal exposure in rabbits). As an enzyme inhibitor, it's suspected of causing convulsions, hyperexcitability, skin irritation, and prenatal damage in humans exposed to extremely high doses in their environment (such as from an occupational chemical spill). At the concentrations found in fly repellent sprays, it has a good margin of safety, and unlike pyrethroids, it's not particularly toxic to fish.

Still, it's important to read a fly repellent's warning label before you start spritzing it all over your horse. Some recommend you wear gloves; all will recommend you avoid spraying it near your horse's eyes, mouth, and nose (you can safely apply repellent to the face by spraying it on a soft towel, then wiping it on). There proably will be recommendations as to how often you should re-apply the product. If you haven't used that particular formulation before, it's a good idea to spray a little on a small patch of skin first to test for any adverse reaction before you saturate your critter with it. Of course, you should limit your own exposure to the spray as much as is practical; avoid getting it on your less-protected skin, and try not to inhale as you spray.

Keep in mind the toxicity pyrethrins have to various kinds of aquatic life. Avoid contaminating any water source, and don't spray it near your hay bales or grain bags.

Herbal Alternatives

Concerns about the public impression of "chemical" formulations have encouraged a number of manufacturers to come out with fly sprays based not on pyrethroids, but on other "natural" products, usually herbs. There are a number of herbs reputed to have a fly repellent action; sometimes the reputation is deserved, sometimes not. For the consumer looking for a natural alternative, such products are immediately attractive. After all, doesn't "essential oil of pennyroyal" sound a whole lot more inviting than "piperonyl butoxide?"

The big question, of course, is efficacy. Do herbal preparations really do the job? Norkaitis notes of her company's pyrethroid-free, PBO-free SuperShield Green fly spray, which was launched in 1999, "We haven't done any direct comparisons with our SuperShield formulations, but we have compared it to other herbal formulas, and have found it to be very effective." SuperShield Green contains six herbal extracts and essential oils with repellent properties, she says: citronella, lavender, eucalyptus, pine needle, tea tree, and pennyroyal (which contributes to the scent of the product). Aloe, witch hazel, and lanolin are added to soothe insect bites and other minor skin irritations. Norkaitis notes that consumers shouldn't expect herbal fly sprays to last as long as formulations with PBO in them. "We've shown efficacy for four to six hours, though, " she says.

In his examinations of what really repels flies, Butler has looked at an estimated 4,000 herbal formulas, and tested them against various species such as mosquitoes, horn flies, stable flies, house flies, ticks, and no-see-ums ("one of the hardest bugs to repel," he says). "Some of them are pretty effective," he notes. "Pennyroyal, lavender, and wormwood come to mind, although we tried pure wormwood oil on horses and quickly learned they don't like it much!" (Wormwood has a particularly pungent odor, which is masked in some preparations by including other, sweeter-smelling herbs.) As for the various commercial herbal formulas that are available, Butler notes that most companies rely on folklore more than they rely on double-blind studies. "They put it out on the market and see if it works. If the customers come back, there's at least a perception that it's effective."

The same goes for the dozens of "homemade" fly spray recipes now readily available on the Internet. Most of these contain essential oils of the herbs already named, with the possible addition of basil, bay leaf, cloves, tansy, cedar, black pepper, rue, rosemary, southernwood (related to wormwood), santolina, spearmint, neem (a tree which grows in India), sassafras, or thyme. As a rule, these are mixed up with olive oil or another emollient, plus an emulsifier (to spread the oil droplets) such as a commercial dishwashing liquid. Most of these recipes will have some mild repellent action, although how long they will last, and against what species of bugs it will be most effective, is hard to say. "The best suggestion I can make," says Butler, "is to try (a herbal preparation) side-by-side with a pyrethrin-based formula, on two horses in the same conditions. That will tell you better than anything."

What about the legendary Skin-So-Soft, the Avon bath oil that has gained such a reputation for fly repellency? Despite its manufacturer being prevented by law from making any claims about Skin-So-Soft's bug repelling action, a great many horsepeople swear by it, attributing both efficacy and gentle action to the oil. But does it really work? "It's not completely bogus," says Butler. "It works very well, especially against no-see-ums and black flies, for about 10 minutes. It has very short-term activity. There are about five essential oils in Skin-So-Soft, three of which are probably good repellents. The problem is in getting it to stick around! On top of that, the other two essential oils in the product work as attractants, so you really get a mixed effect.

"The product's reputation is based on a little activity and a lot of imagination," Butler concludes.

How about apple cider vinegar? Historically, it has been used both as a topical fly repellent (wiped on the coat), and as a feed additive that is supposed to make the horse's flesh less appealing to biting flies. There's little evidence that the latter approach is effective -- most nutritionists suspect you would have to feed many cups of vinegar per day to effect any change in the horse's overall flavor. As for the topical application, it's possible that would have some effect, he says. "Acetic acid, or vinegar, is a repellent at high enough concentrations, as is lactic acid. But at low doses, it acts as an attractant."

Encouraging News

For those who feel their fly repellent options are severely limited, there's a ray of hope emerging from Butler's University of Florida lab. After 15 years of testing those 4,000 compounds, Butler believes he has identified a natural plant extract that will rival DEET and pyrethroids in its broad-spectrum effectiveness against many species of bugs. The compound, called geraniol, is derived from lemongrass, and it is responsible for the distinctive flavor in Thai food. It's chemically related to citronella and looks to be a more potent version of that chemical (repellents in the same chemical family also are found in citrus peels).

"Geraniol is used in the perfume industry; that's where we found it," Butler explains. It has been labeled "generally regarded as safe" by the EPA, and provides almost four hours of protection against a wide range of biting insects, including mosquitoes, no-see-ums, stable flies, horn flies, and even ticks and fire ants. Recently patented, geraniol is likely to be available soon in an equine fly repellent formulation.

Meanwhile, we should use fly repellent products responsibly, whether they originate from an herb garden or a chemical plant.

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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