Flies are more than just a major nuisance, they can also be detrimental to your horse’s overall health. In addition to many types of fly prevention, you may want to consider feed-through fly control.

Even though it’s springtime and dog days of summer still seem light years away, it's never too early to get a jump-start on the war against summer flies. There’s even a new weapon to add to your arsenal, and it works best starting now.

Before you have to drag out the sprays, smelly traps and fly masks, consider using some feed supplements that “pass through” your horse to help control flies the moment the weather warms up. These feed-through insecticides are made by several companies that also sell basic fly sprays, and can be a good way to keep the fly population down around your barn.

Dress this stuff on your horse’s grain and it will do its job in his manure, killing adult flies and larvae on contact. The active ingredient is usually a chemical called tetrachlorvinphos, which destroys an insect’s nervous system.

Tetrachlorvinphos inhibits cholinesterase, which is essential for transmission of nerve impulses. It’s an organophosphate pesticide that is chemically related to the nerve gasses developed during World War II. There is some debate over whether it is absorbed by the horse or passes through quickly enough to not get into the horse’s bloodstream. Signs of absorption in a horse would include abdominal pain, lethargy, sweating, tearing, and excessive salivation.

This product is designed to pass through the digestive system too quickly to be absorbed. Regardless, it is a neurotoxin and, three years ago, the EPA published a notice advising horse owners to consult veterinarians if using these feed-through pesticides on old or debilitated horses, or pregnant or nursing animals.

Farnam’s Equitrol keeps larvae from hatching via its main ingredient, Rabon (tetrachlorvinphos). It takes one ounce of Farnam’s Equitrol daily to do the trick (you can buy 3.75 pounds for $17, up to 20 pounds for $65). Crystalx also offers Rabon in a block supplement called Rolyx Pro, which adds additional protein supplementation. The company claims the block availability offers more consistent intake, and this would save you the time of feeding it.

Something New
A new component in feed-through options is IGR, or Insect Growth Regulator. It doesn’t affect adult flies, but prevents fly larvae from developing into biting adults. The active ingredient is not an insecticide, but rather an agent to keep larvae from reaching adulthood.

The 2.12 percent cyramazine in Pfizer’s Solitude IGR is not an organophosphate. The cyramazine interferes with a fly’s production of chitin, a compound that makes up an insect’s exoskeleton, or hard outer shell. Flies lay their eggs in manure, but because of this product, the larvae is unable to produce chitin. That means it can’t properly form a protective outer shell as it grows, so the immature fly dies without ever reaching adulthood. Rather than being outright toxic to the fly, this product breaks its life cycle.

“Solitude IGR has been proven to help significantly reduce the number of flies that plague horses without creating toxicity issues for humans, horses, other animals, plants, or water supplies,” said Pfizer’s senior veterinarian, Jay Donecker, VMD.

Farnam’s Equitrol II uses Dimilin as its main ingredient, and it works the same way. To administer Solitude IGR (formerly Serene Feed-Through Fly Control), mix a half-ounce in a horse’s daily feed ration. It costs $32 for 2 pounds (64 doses); or 6 pounds for $87; 20 pounds for $250. Equitrol II contains 0.24 percent diflubenzuron (Dimilin). Four pounds of it will cost you $17.50; 20 pounds runs $66.

Farnam also makes a product called Simplifly, made with a Fly Growth Regulator called LarvaStop that it claims will prevent 97-100 percent of flies from developing into biting, breeding adults. The company also claims flies haven’t thus far developed any resistance to its exclusive LarvaStop. Simplifly costs $22 for 3.75 pounds or you can buy 10 pounds for $50 and 20 pounds for $86.

These insect growth regulators are said to be specific to insects and won’t adversely affect horses because chitin isn’t a substance found in mammals. IGRs have been used to control flies in poultry for two decades, and researchers have found no clinical signs of toxicity in horses eating the product. Cyramazine, for instance, breaks down into a monomer of melamine—a compound that’s been used as a slow-release nitrogen fertilizer.

To stop flies from growing up, it takes about two weeks of top-dressing your horse’s feed. If you wait until you’re already fly-infested, it will take four to six weeks (a fly’s lifespan) to really see the adult populations die off.

That’s because fly growth regulators don’t have any effect on existing adult flies. Considering that flies can migrate to your place from up to 80 miles away, you won’t want to throw out all your sprays, masks and sheets.

Natural Options
There are also all-natural feed-through options that have been used for decades to repel insects from your horse. Several companies make garlic-based products for this purpose, and they have the added benefit of boosting your horse’s immunity.

The do-it-yourself option is to mix roughly one clove of crushed garlic daily in your horse’s food (about an eighth of a teaspoon), combined with a quarter cup of apple cider vinegar and brewer’s yeast (found in health food stores).

A company called Cheval International makes Inside Out, which combines all three ingredients into a powder. The object is for biting insects to be repelled by the taste of your horse. At the same time, Cheval claims this product won’t stink up your horse or barn. A horse can accidentally eat an entire bucket with no adverse effects, while you may even see allergies and coughs disappear. Inside Out costs $28 for four pounds, up to $300 for 63 pounds.

Springtime Inc., sells Bug Off Garlic with pure garlic powder, which it claims provides a 24-hour shield for horses against flies, ticks, mosquitoes, gnats, etc. One top racehorse trainer, the late Bill Worthington, had claimed that garlic picked up horse’s appetites, kept coughs away, reduced flies and aided endurance. Springtime also claims that Bug Off promotes friendly bacteria in the digestive tract, is a natural source of MSM, and has antibacterial, antifungal, antiviral and anti-parasitic properties.

“The dietary sulfur comes through the skin, repelling insects,” said a Springtime representative. “Your horse may give off a little hint of garlic, but it’s not like you’ll have a virtual garlic clove running around.”

AHC Products, Inc. markets a product called BugLyte that contains diatomaceous earth, brewers yeast, thiamine monohydrate, garlic, niacin, and grape seed extract. You can get 1.5 pounds for $10, and you’ll feed one ounce daily for 21 days, then a half-ounce for maintenance. Bug Off Garlic costs $40 for five pounds, up to $179 for 27.5 pounds. You’ll feed about an ounce and a half daily.

Whatever method of feed-through fly control you go for, it never hurts to stay true to the old standbys for reducing flies. These consist of removing manure and cleaning up leftover feed daily. Also consider dragging out manure piles in your pasture so they dry out. Finally, make sure your stalls and barn have proper drainage, and keep manure storage dry.