Pest Control: The Death Squad

Choosing a fly, mosquito, or tick formula for your horse or barn is a lot like selecting a hand lotion: You can pick from extra moisturizing, scented, hypoallergenic, etc., or packaged in tubes, jars, bottles, but the bottom line is they are all still lotions. Ditto the equine insecticides. There are plenty of them out there, but the vast majority are pyrethrins and their synthetic derivatives, pyrethroids. Sure, there are some non-pyrethrin/pyrethroid formulas (herbals and a couple of other chemical varieties), but choices are pretty limited.

Flower Power


Pyrethrins and pyrethroid insecticides offer dual protection by repelling and killing blood-feeding insects.

"Pyrethrins are naturally occurring botanical insecticides, obtained from members of the chrysanthemum family," explains Nancy C. Hinkle, PhD, Medical-Veterinary Entomologist of the University of Georgia. Pyrethroids are synthetically developed compounds (permethrin and cypermethrin, among others) similar to pyrethrins. The most important differences between pyrethrins and pyrethroids, at least for the consumer, are that pyrethrins break down rapidly in sunlight and offer limited residual activity, whereas pyrethroids are more photostable (not subject to change under exposure to light) and provide extended residual effect. Pyrethrins and pyrethroids come in a number of topical formulations, available as sprays, powders, wipes, spot-ons, and premise misters, all of which provide the consumer with a choice of options that work best for him or her.

Pyrethrins and pyrethroid insecticides offer dual protection by repelling and killing blood-feeding insects. States Ed Schmidtmann, PhD, a research entomologist with the USDA's Arthropod-Borne Animal Diseases Research Laboratory, "As repellents, they interfere with the insects' sensory nervous system and prevent them from blood feeding. As insecticides, they quickly interfere with the transmission of nerve impulses in the insect, and that's rapidly fatal."

Pyrethroids owe their dominance in the marketplace primarily to their quick knockdown ability and improved, long-lasting effectiveness over older insecticides, along with low mammalian toxicity. In a recent study, Schmidtmann found that permethrin applied as a whole-body spray reduced mosquito blood feeding from treated heifer cattle by 79-88% at four days after treatment, with reductions of 61-68% at 11 days post-treatment.

"We saw even greater repellency against black flies feeding on heifers," he notes. "In another study with ponies treated with a permethrin fly-wipe, we found the blood feeding of black flies was reduced by 97% and 87% at one and seven days post-treatment, respectively." (Schmidtmann also notes that in comparing spot-ons with spray treatments, there was little difference in efficacy.)

Pyrethrins and pyrethroids also enjoy an excellent safety record with low toxicity and few adverse side effects. "These products remain on the surface and don't enter the bloodstream," explains Ralph Williams, PhD, professor of entomology at Purdue University. "They may get tied up in the skin oils, but they do not penetrate the skin, and thus are very safe."

Although some of the pyrethroid formulations have produced minor skin irritations in breeds that tend to have sensitive skin (such as Thoroughbreds), the risk is slight.

Despite label claims, though, how well and how long these products perform largely rests on individual circumstances. Says Roger Moon, PhD (entomology), professor of veterinary entomology at the University of Minnesota, "Effectiveness depends how fast the product evaporates or erodes. On a horse that is sweating or moving through dense brush or wet grass, pyrethrins and pyrethroids will erode faster than on a horse standing out in a dry paddock or in an arena. When used as a premise spray, longevity is reduced if treated surfaces become coated with dust, as this cuts down on the insects' exposure, or if surfaces become wet and the insecticidal residue is washed off."

Adds Williams, "Even though the label might say these active ingredients last up to several days or a week, these are under ideal conditions. In normal conditions, a horse receives about one day's protection because of the nature of the horse and the conditions."

While pyrethroids have ruled for close to 20 years, there is concern about mounting resistance to these chemicals. Reports Jerry F. Butler, PhD (entomology), professor emeritus at the University of Florida, "Resistance levels have been increasing in the last 15 years. As selection pressures have been put against things like houseflies, stable flies, and horn flies, the repellent action is what leaves first. In the beginning, around 1984-85, a repellent wipe-on would work for a week; now it's down to about four hours as a repellent."

Old and New Again

Once a commanding force among insecticides, organophosphates, such as coumaphos, dichlorvos, malathion, and tetrachlorvinphos, comprise an older group of insecticides that function as nerve poisons, killing the insect by inhibiting its nerve impulses. Available as pass-through larvicides, topicals, and premise sprays, organophosphates are gradually disappearing from the marketplace. Says Butler, "Most of the topical organophosphates are gone, with maybe only one or two formulations still out there. They've been heavily under fire because of their mode of action and possible effect on hormones. They have the same resistance problem, and the residual effect doesn't last very long."

Most of today's organophosphates are formulated as premise sprays, occasionally as a topical, and in one larvicide product. Says Moon, "Although organophosphates are cheap, they tend to have a more offensive odor. As nerve poisons, the risk for the applicator, the people around the premises, and the animals is slightly greater, although the formulations are such that if used as instructed, they're safe."

DEET (N, N-diethyl-m-toluamide) is an old, long-used insect repellent invented for human use. Developed by the USDA back in 1946 and registered with the EPA in 1957, it has only been available in formulations approved for horses for a few years. According to a document at http://edis.ifas.ufl.
edu/IN181, Protecting Florida Horses From Mosquitoes, posted by the University of Florida, Institute of Food and Agricultural Sciences, "Because (DEET) is a repellent only and has no insecticidal effect, it will have less impact on the overall mosquito population. This may be the best choice for a horse that is sensitive to pyrethroids."

Explains Moon, "DEET affects flies, ticks, and mosquitoes to greater or lesser degrees, depending on the kind of fly and tick, although permethrin seems more effective against a wider range of flies, mosquitoes, and ticks." It's unclear how DEET works, but the mode of action is via vapors that interfere with the insect's chemical sensors.

Ticks will drop off the host when they contact the repellent, while flies and mosquitoes are unable to complete landing maneuvers when they fly in to feed on the host.

DEET evaporates from the surface of the skin as well as being absorbed into the skin. "The safety record of DEET is really outstanding," Moon states. "The number of cases of human poisonings, at least, is miniscule compared to all the other household products people are exposed to. If you want three hours of protection for a horse when it's out exercising, DEET is probably a better choice."

Au Natural

Repellents made from plant extracts are especially appealing to those who abhor chemical products and lotions. The big question: Are they safe and effective? Cautions Hinkle, "Just because a product is 'herbal' or 'natural' does not mean it is safe: Strychnine is a botanical product!"

In thumbing through equine catalogs, one finds many varied herbal repellents containing ingredients such as citronella, cedar oil, lemon grass (geraniol), pennyroyal, and so on. Hinkle notes that some of these commercial herbal products have been demonstrated to provide short-term repellency. "Typically their effects wear off within a few hours, so the product must be reapplied at frequent intervals," she says. "As with any product, some animals become allergic or sensitized following exposure, so they should be observed for adverse effects."

The best of the herbal repellents might be citronella and geraniol. "Citronella is usually good for up to four hours, seldom longer than that," reports Butler. "There is a geraniol-based product for horses from the University of Florida that we're trying to get in the marketplace that may be effective against mosquitoes and biting flies for 24 hours or more."

Some of the other herbals, however, are less horse-friendly. "Cedar oil has not been shown to offer any repellency, but is known to be neurotoxic in mammals," Hinkle states. "Pennyroyal has both abortifacient (abortion-causing) and hepatotoxic (liver-damaging) effects, and I know of no research indicating that it has any insect repellent properties. In fact, many essential oils (any of a class of volatile oils that give plants their characteristic odors) are powerful convulsants, triggering epileptic-type episodes in mammals. Owners who want to test these compounds on their horses should be prepared to have the animal euthanized to prevent its suffering from irreversible brain, liver, and kidney damage."

Folklore also suggests that certain food products can produce a repellency action. "Some people assume that because a product has an offensive aroma, it must be repellent to insects," says Hinkle. "Insects, however, do not smell the same way we humans do, so typically are unaware or unfazed by odors that we find obnoxious. Feeding your horse garlic or onions will not repel insects, nor will spraying him with garlic or onion juice. Feeding a horse B vitamins or brewer's yeast has no effect on insects, either."

As for the hand lotions made for humans, including Avon's Skin-So-Soft, again, results are dubious. For one thing, horse skin is different from and much more sensitive than the skin of humans and other animals. "Horse's skin is quite different than that of other animals," explains Butler. "Horse's skin can really react violently sometimes and is very difficult to heal."

Something as innocuous as baby oil, for example, can pose big risks for some horses. "I know a formulation that included baby oil that caused severe edema on the horse, with pieces of skin falling off," Butler says. "Although this problem generally occurs only during periods of high temperatures and high humidity, it's a problem in Florida."

While Skin-So-Soft usually doesn't produce adverse reactions in horses, the product might not perform as well as some hope. Butler, who studied the product, says, "We looked at five different ingredients in Skin-So-Soft. Three were repellents and two were attractants. How well it works depends on what the target insect is, as it attracts certain insects and repels others. Also, some of these materials come off quicker, so if some of those repellents come off quickest it will turn into an attractant." Butler notes that Skin-So-Soft can repel various mosquitoes and flies from 10 minutes to three hours.

Surprisingly, there is one simple human product that does work well in the horse--petroleum jelly, which is seen in such popular products as Vaseline. Says Williams, "Petroleum jelly is as effective as anything for smearing on the inside of the ears during black fly season. It discourages black flies from feeding by preventing them from landing in the ears. A light smearing applied every couple of days during the black fly season keeps the surface greasy and unsuitable for landing."

Playing it Safe

When it comes to pest control products, the safest choices lie with EPA-approved chemical formulations developed for horse use, such as DEET, pyrethrins/pyrethroids, and organophosphates, or the EPA GRAS ("generally regarded as safe") products like citronella and geraniol. Says Williams, "I only recommend EPA-registered insecticides and repellents because they have been shown to be effective. Plus, EPA-registered materials have been shown to be very safe to use because the companies have to go through stringent safety research to prove these products safe. Shy away from anything that does not have an EPA registration; it may not work effectively."

Make sure you apply products correctly and responsibly. "For best results, always read the product label (it may be different from the last time the product was purchased) and follow its instructions scrupulously for safety and to maximize efficacy," states Hinkle. "It is important not to use products registered for other uses (such as for other animals, humans, etc.) on horses unless equines are specifically mentioned on the label. Pesticides formulated for environmental use frequently contain additives that are irritating to equine skin, or that are dermally absorbed and may prove toxic. In particular, products registered for cattle should not be used on horses because some of them can be deadly to equines (ruminant physiology is quite different from that of monogastric mammals)."

Nor should pesticides be applied to feed, water, or areas where horses might consume them (portions of fence chewed by a cribbing animal, for instance), Hinkle warns, or in a manner which allows them to contact eyes, nose, mouth, or urogenital areas. Mucous membranes are irritated by pesticide additives, which can also make the animal sick.

Bottom line: "Users should always read and follow the label directions," Hinkle emphasizes. "The label is not just advice! The horse's health should not be put at risk in an effort to protect it from insects and ticks."



Wenholz, Sushil Dulai. Halt the Assault. The Horse, April 2003, page 49.

See the Insect Control section online at


In the electronic bug-be-gone arena, one can find ultrasonic bug repellents, bug zappers, and mosquito premise traps. Should you consider such devices?

Ultrasonic bug repellents purport to work by emitting a high-pitched tone that is inaudible to humans, but annoying and distracting to bugs. Says Nancy C. Hinkle, PhD, Medical-Veterinary Entomologist of the University of Georgia, "Ultrasonic devices do not affect mosquitoes, ticks, or any other blood-feeding arthropods; there is no evidence that bloodsucking arthropods can perceive ultrasonic emissions."

Ultraviolet light bug zappers lure flying nighttime insects into a small, mesh-enclosed electrical field where they're vaporized. Unfortunately, most zappers attract and kill beneficial insects. Additionally, they pose potential health problems. A study conducted by researchers at Kansas State University discovered that electrocuted bugs shower the surrounding area with airborne bacteria and viruses, posing potential harm to humans.

As for their protection value for horses, it's limited, says Ralph Williams, PhD, professor of entomology at Purdue University. "For mosquito control, they're not effective at all in either reducing mosquito biting activity or mosquito populations."

Bug zappers will work moderately well if they are placed in areas where there isn't much insect activity to begin with, such as indoors.

Mosquito premise traps are the latest entry into the high-tech pest control market. "These traps burn propane, generating hot, moist carbon dioxide, mimicking mammalian exhalations," Hinkle explains. "Biting flies such as mosquitoes, black flies, and biting midges are attracted and sucked into a trap by an intake fan. The traps employ no toxicant; pests desiccate (dry up) or die of starvation. While the traps have been shown to catch large numbers of biting flies (number of flies in trap's area), there has been no research to investigate whether their success in luring in pests actually increases the ambient numbers of blood-sucking flies. Recommendations have been made that purchasers give these traps to a neighbor down the block to avoid attracting large numbers of biting flies to their own property!"

Notes Roger Moon, PhD (entomology), professor of veterinary entomology at the University of Minnesota, "These traps are interesting: They're attractive to buyers! But no one to my knowledge has shown that putting these things up around a horse premise materially reduces the numbers of insects biting the horses. Mosquitoes and other insects are out there in extremely large numbers; we only see a small fraction of them coming to us, and removing some with a trap doesn't cause a shortage. They don't trap enough to put a dent in the population that's biting. I think horse owners would be better advised to house the horse indoors if they're concerned about it being bitten by mosquitoes, and when they go out to use a topical repellent."

Adds Ed Schmidtmann, PhD, a research entomologist with the USDA's Arthropod-Borne Animal Diseases Research Laboratory, "I have used these traps in the field to catch mosquitoes and little biting flies, but have not evaluated them for their effect in control. This is a very interesting new area that needs research. It's possible that if enough of these traps are employed in the right places, they might well be very effective. They do capture mosquitoes, but they also attract mosquitoes, so whether they really keep mosquitoes from feeding on horses effectively in a barnyard situation needs to be evaluated scientifically."--Marcia King


While the various kinds of insecticides and repellents can provide varying degrees of protection, you can maximize their effectiveness by employing a few tried-and-true fly management commendations.

Practice good sanitation. "If you have good sanitation, you can use insecticides carefully and selectively and keep the problem under control," says Schmidtmann,. "If you don't have good sanitation, insecticides won't bail you out."

Reduce breeding areas of mosquitoes. Eliminate standing water in containers, tires, tree holes, etc. Keep water buckets and troughs clean. Clean up wet feed, soiled bedding, and dung, which provide breeding habitats for the filth flies (stable flies, house flies, etc). "Render these materials unsuitable for fly larvae and maggots by composting," suggests Moon. "Either spread thinly, cover it with plastic and let it cook in the sun, or compost it. Do this regularly, at least once a week. Simply throwing soiled materials out the backside of the barn and letting them stack up isn't sufficient, nor is moving them a quarter-mile away. As long as soiled material goes from one pile to another, all you're doing is transporting the source, not preventing it from contributing to the world fly population."

Avoid resistance. "Resistance, with loss of efficacy, is a risk whenever any pesticidal product is repeatedly used against a pest population," warns Hinkle. "The best way to preserve insecticide effectiveness is to use them only when necessary, employing other suppression tactics in an integrated strategy against pests."--Marcia King

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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