Bugs and flies are no fun for horses. Not only do they irritate them, but they can also transmit diseases including equine encephalomyelitis (Eastern and Western), equine West Nile virus and equine infectious anemia. Swarms of biting flies can potentially suck more than 20 tablespoons of blood out of a horse in a day. Flies may irritate the eyes when they feed on tears, and ticks can also transmit diseases such as Lyme disease.

There are a few different ways that we can prevent our horses from being overburdened by insects. You can control the number of flies in your facility by using pesticides, insecticides or fly traps, as well as by reducing breeding grounds by removing manure, dirty bedding and standing water from your facility. You could also use fly predators that consume the larvae of several varieties of horse flies and stable flies. You can physically prevent the access of flies to your horse by using fly sheets, masks and leg wraps, or by spraying or dosing your horse with fly repellents that deter bugs, especially mosquitoes and ticks. Most of these fly sprays for horses include natural pyrethrins (derived from chrysanthemums) or synthetic versions designed to be more stable (cypermethrin, permethrin and resmethrin).

Of course, some products also contain DEET, and while these are overall relatively safe and do in fact deter mosquitoes, some horses may have reactions to them. Ticks may be deterred by these, as well as by fibronil, aphacypermethrin or thymol. Mites can be managed with oral doses of ivermectin and moxidectin.

There are also ways to decrease your horse’s fly burden through feeding.

Some ingredients are fed with the intention that they will give off an odour (or taste) that is unpleasant to the flies and decrease the likelihood of the horse getting bitten. Many of these contain ingredients like apple cider vinegar and garlic. Once consumed, a compound within garlic, allicin, which is a sulfur containing compound (allyl methyl sulfide) can pass through the skin and cause an odour. However, there is some concern regarding feeding garlic to horses, such that higher doses have been shown to cause anemia. Lower doses should be safer, but there has never been a safety study in horses to determine if and what a safe dose is. Further, there is no evidence – in horses or humans – that the odour from eating garlic wards off biting insects.

Apple cider vinegar certainly smells unpleasant to some due to the various acids associated with its production (such as acetic acid), and it is also affected by the type of apples used in its production. While apple cider vinegar has a reputation for a few health benefits (although largely unfounded by scientific research), those acids would be likely be neutralized by bicarbonate in the horse’s mouth or further along the digestive tract and wouldn’t necessarily affect the horse’s blood pH or how it tastes. Again, there is no research in humans regarding the efficacy of consuming apple cider vinegar on deterring biting insects; in fact, most research investigates its use as a topical spray (with mixed results).

A pile of white powder.

Diatomaceous earth. (SprocketRocket – commons.wikimedia)

Diatomaceous earth (DE) is another common ingredient in fly control supplements. DE, which is a naturally-occurring, soft siliceous sedimentary rock crumbled into a fine powder. works on contact with flies to destroy their exoskeleton and causes the flies to dry out and die. It is often a component of cattle dusts or other topical repellents, and can be spread over manure piles and waste.

When included in a feed or supplement, the goal is for it to come out in the fecal material and do the same thing to the maggots or larvae. While research on its efficacy for feed-through pest control is lacking, its components would largely be indigestible and should carry on to the feces where it could exert the same effect on a fly’s exoskeleton. As a feed supplement, DE is considered GRAS (generally recognized as safe); however, accidental inhalation of DE can be hard on the lungs.

Some products contain low doses of pesticides that pass through the horse and act as a larvicide or insect growth inhibitor on larvae in the feces. The most common types used in horses works by inhibiting chitin synthesis, the main protein of the exoskeleton. Because horses (and humans, other mammals and birds) don’t have an exoskeleton, these compounds are considered safe.

Commercially available diflubenzuron products include Clarifly (Central life Sciences) and SimplyFly and Equitrol II (both by Farnam). Cyromazine found in Solitude (Pfizer) is another type of insect growth regular that prevents molting and the development of a new exoskeleton. Many other commercial supplements contain these ingredients, and some horse feeds are including them also (eg. Tribute, Purina).

It should be noted that these compounds appear to be relatively effective and safe (although most research has been conducted on other species). These products only impact insects whose larvae grow in equine manure. Organophosphate-based larvicides should not be fed to horses.

Management and control of flies at horse facilities is an important balance between limiting your horse’s risk of disease with the use of safe and effective compounds. Excessive use of any product may have negative consequences for your horse, your wallet, and/or the environment.