Emergence of House and Stable Flies

Warmer weather means more activity for owners and their horses, but it also signals the beginning of the dreaded fly season. If you support a horse, chances are you’re also supporting entire colonies of flies, and you’re convinced that they are telling all their fly friends to head over to your place and your horse.

You can win the war on flies, but only if you work diligently to eradicate them through good stable and pasture management and an integrated system of fly control. You can choose from a wide range of fly control products, and you may need to use several methods. Some, such as fly masks, simply keep flies away from a horse’s sensitive areas. Others kill the flies or the larvae with everything from insecticides to bait and natural predators.

Flies not only annoy our horses and us, they carry disease.

“We don’t want to eradicate the flies just because they are a nuisance,” confirms Dr. Jay Donecker, senior veterinarian with the equine group at Pfizer Animal Health. “In California, for example, horses are having a problem with pigeon fever, and researchers have found that it’s spread by the housefly. Studies are looking into whether you can reduce the incidence of pigeon fever by lessening the fly population.”

The Center for Equine Health (CEH) at the University of California at Davis reports that flying insects can cause a variety of skin problems in horses. Midges, also called “no-see-ums” or Culicoides, cause dermatitis, leading to lesions on the mane, rump, and belly and sweet itch. A variety of flies can also cause dermatitis, and black flies may produce edema and hives in horses. The CEH says that in extreme cases of fly infestation, pregnant animals may abort, and even death can occur.

If you’ve gone through fly season before, you are probably familiar with many of the products on the market. An overview of some of the most common ones—ranging from fly sprays and mist systems to fly sheets, natural predators, and traps—can be found in the chart on page 78.

Another weapon in the arsenal is the feed-through fly control product. These formulas are designed to work by passing through the horse’s system and depositing a fly-control ingredient directly in the horse’s manure. When flies lay their eggs in the manure, the product prevents the larvae from maturing, thus killing the insect. Because feed-through products work on fly larvae, they should be started a few weeks before the beginning of fly season. That gives the product time to kill the larvae before flies become a problem.

Ingredients in feed-through products differ depending on the manufacturer. Some include insecticides, while others use natural products like garlic or diatomaceous earth.

Keep Flying Pests Grounded
An integrated program is best. Attack your fly problem on multiple levels. Clean stalls, pens and runs regularly; dispose of manure promptly. Situate the manure pile well away from living spaces; cover where practical. Eliminate areas of standing water. Use fine-mesh screens to keep flies away from stalled horses Install horse-safe fans to discourage flies and insects from landing.

Integrated Programs
Even if you choose a feed-through product or rely on other methods, experts advise you to use an integrated system that can get rid of flies in a variety of stages.

“People find they have to use something else too,” says Doug Ross, Ph.D, product development manager at Farnam. “No feed-through product will affect adult flies. Plus, flies are very strong fliers and can travel from other barns.”

Donecker agrees, noting that owners still have to do the simple things, such as keeping stalls and pastures clean and free from manure. When he had a private veterinary practice and visited farms regularly, Donecker says he was concerned with the effect such things as overhead spray systems would have on people since they put pesticides directly into the air. He suggests that with a feed-through product, a mist system would not have to be activated as often.

An integrated fly control system might include a feed-through product, misters in the barn, fly sprays, fly masks, and fly sheets. Or if you prefer more natural methods, you might choose fly predators (tiny wasps that lay eggs in developing fly pupae, killing them), baits and traps. (For an in-depth look at natural fly control methods, see “Fight Nature With Nature,” Perfect Horse, May 2005.)

U.C. Davis’ Center for Equine Health also offers the following recommendations: Stable horses at sunrise and sunset, which are insects’ peak feeding hours; use ultrafine screens in barn windows; keep horses away from standing water; put overhead or stall fans in the barn to interfere with the flight of flies; and cover manure piles or containers and get rid of the manure as often as possible.

“Manure on site is one of the biggest contributors to fly population,” says Farnam’s Ross.

How Feed-Throughs Work
Farnam’s SimpliFly and Pfizer’s Solitude IGR products are formulations that contain insect growth regulators. They keep the fly from developing into an adult. As such, the active ingredients (diflubenzuron in SimpliFly and cyromazine in Solitude IGR), specifically target insects. Plus, the products are able to use small amounts of the ingredients.

“Conventional insecticides have a direct toxic effect on the nervous system,” Ross says. “That can also affect vertebrates such as you and me, horses, dogs, etc. Insect growth regulators disrupt the processes that are unique to insects. Because of that specificity, they’re safer.”

SimpliFly uses an ingredient that Ross says is also used in flea control products. “When the larvae molt, the chemical prevents it from making a new external skeleton or shell. So they never get to the adult stage.” SimpliFly is fed by weight, usually 1 ounce daily for a 1,000-pound horse.

Cyromazine, the ingredient in Solitude IGR, was initially used to regulate flies around chickens. Kathy Palma, an entomologist with Triad Specialty/Piedmont Pharmaceuticals and also a barrel racer, knew of cyromazine through her work and tried it on her horses.

“The active ingredient looks like salt,” Palma says, “so I took a pinch to see what it would do. Four or five weeks later, there weren’t that many flies.”

Palma and a scientist friend began running tests on the manure, which confirmed her results. While Solitude IGR is a new product, Palma says she has been using cyromazine on her horses for five years without any complications.

Donecker says that because cyromazine is water soluble, it’s not easily absorbed in the body and passes through a horse’s intestinal tract and into the manure. “Sunlight breaks it down,” he says. “It breaks down into nitrogenous compounds, some of which have been used for fertilizer.”

Donecker says that Solitude IGR is fed either daily (½ ounce) or every other day (1 ounce).

The Environmental Protection Agency regulates the ingredients found in both SimpliFly and Solitude IGR. Products such as these must pass tests for both effectiveness and safety. As with any fly control product, you should check the label for indications for young, pregnant, aged, or infirm horses, as well as consult with your veterinarian.

Some previous feed-through fly control products contained organophosphates, which targeted an insect’s nervous system. However, a study by a team of researchers at U.C. Davis led by Dr. John E. Madigan, showed that horses absorbed organophosphates, which could affect their nervous systems as well.

“Feeding horses organophosphates, which are absorbed, seems to be a poor way to control flies,” Madigan said. “It makes much more sense to use a metabolic interruptor, which interrupts the exoskeleton development. It’s a much more specific, targeted, and logical way to go.”

Natural Alternatives
Some people, however, prefer natural alternatives. Dr. Warren Porter, a professor of zoology and environmental toxicology at the University of Wisconsin-Madison, is a proponent of natural methods of pest control. He maintains that EPA tests are flawed and cited studies showing that some pesticides can have long-term genetic and carcinogenic affects.

“You are putting a molecular bull in a china shop,” Porter says. “The EPA doesn’t ask if something is safe. It weighs the costs versus the benefits.”

Porter recommends organic alternatives for fly control.

“Chickens are good fly predators,” he says. “Baby powder—very fine powders—will suffocate fleas and bed bugs.”

Diatomaceous earth is a natural substance that can be found in some feed-through products that are marketed to control flies. The FDA regulates it as a food additive for use as an “inert carrier” or “anti-caking” agent and requires that it not exceed 2% by weight of the total ration. Some anecdotal reports indicate that diatomaceous earth works to control flies.

“Diatomaceous earth contains diatoms, which puncture the exoskeleton, causing the insect to die,” says Porter. “This is on the electron microscope level.”

Diatomaceous earth is also used in swimming pool filtration systems, which has a much higher crystalline silica concentration than what is in food-grade diatomaceous earth. Therefore, you should make sure to use a product that is listed as food grade, conforming to FDA regulations.

Other feed-through products contain garlic, also thought to be a fly deterrent, though Farnam’s Ross maintains that garlic doesn’t work. A few years ago when market research showed garlic products to have the second-largest market share of feed-throughs, Farnam investigated it.

“Horses like garlic, so we developed a product that had as much garlic in it as a horse would eat,” Ross said. “We gave it to 30-40 horses for a month, and then stopped feeding it to half of the horses and counted the flies. It did absolutely nothing.”

Everyone’s situation differs. Certainly, some geographic areas have more problems with flies than others. But if you practice good horse management, especially concerning manure control, and choose from among the many fly control products available, you can turn your facility into a no-fly zone.