Stop Flies Before They Start

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In part two of our series on fly control, we tackle the pros and cons of parasites and feed-through methods for ridding your barn of flies. Be sure to check the slideshow.

Houseflies and stable fly populations can generally be reduced by a combination of diligent clean-up—not letting manure and old hay/bedding build up to create breeding sites—and through the use of parasitic wasps or feed-through products that end up in the manure to kill fly larvae. Several types of flies lay their eggs in fresh manure and rotting organic matter (read:?bedding) so tackling the problem where it starts makes good barn management sense.

Keep It Clean

Texas A&M University did a study several years ago and found that if areas where big bales are fed in feeders aren’t cleaned up, this creates an ideal breeding ground for flies. The researchers estimated that the area around one round feeder could produce more than a million stable flies.

In the spring, before flies emerge, the first thing to do when weather warms up is to move the feeders and spread wasted hay around so it will dry out, recommends Dr. Bill Clymer, a Texas entomologist. Or, remove the waste to a separate area for composting. Otherwise, the material will stay wet through the summer and continually provide breeding sites for stable flies.

Wasp Versus Fly

The second line of defense is to try to keep fly larvae from hatching. This is where parasitic wasps can be beneficial. Wasps can be purchased from several different suppliers (see box for more information) and can help reduce fly larvae by about 90%. Even though these wasps are present in the environment—wherever there are flies—there are not enough of them to control the fly population unless you tip the scales in your favor by putting out more wasps. A female fly lays three times the number of eggs laid by a wasp, according to Clymer.

The wasps work best for fly control if there are just a few breeding sites and you can put out enough wasps to deal with the fly maggots. These tiny nocturnal wasps are almost too small to see, and spend their entire lives on or near manure. Adult wasps are harmless to humans/animals because they do not sting. The females search through manure and lay eggs in the pupae of houseflies, stable flies, horn flies and any other flies that breed in manure. The wasp eggs hatch quicker than the fly eggs and the wasp larva use the dormant fly maggots as food, killing the fly before it can fully develop.

Parasitic wasps give more control in dry climates and during dry years. During a wet season—which provides a more ideal habitat for breeding flies—the wasps may have a tough time dealing with the mushrooming fly population.

Assuming an average climate, the number of wasps needed is generally based on the number of animals in the barn, says Clymer. Wasp suppliers recommend releasing them early, before flies become numerous, and putting out more wasps every 30 days throughout the fly season, spreading them around in corrals and barns—wherever there’s manure. For example, a paddock with one or two horses needs 5,000 wasps each month, whereas a facility with three to five horses would need 10,000 wasps per month, and a larger herd should have about 1,000 to 2,000 wasps per animal, per month. Parasitic wasps will not make up for lack of sanitation, but used in conjunction with manure/bedding/hay cleanup, they can be helpful.

Prevention Through Feed

Another popular line of defense are feed-through products. These products are fed in the horses’ grain, then travel through the horse and ends up in the manure. Some feed-through products contain a larvicide that kills the fly larva that hatch from eggs laid in manure, and some contain an ingredient that hinders growth and maturation of immature fly larvae, and they die.

Feed-through products only work in the small area around a stable or pasture, if all the horses on the property are fed the product, but would have no effect on flies that come in from neighboring areas. This solution also won’t work in a boarding situation where individual horse owners make their own decisions regarding fly control.

There are a number of feed-through products, including Solitude IGR (active ingredient cyromazine, an insect growth regulator), Equitrol II and Simplifly. The recommendations from manufacturers are to start using the product about two to three weeks in advance of when you anticipate the emergence of stable flies and house flies,” says Dr. Dennis French of the University of Illinois.

“Solitude, for example, is an alfalfa-based pellet fed daily in the grain, passing through the horse’s digestive tract and ending up in the manure, where it inhibits the development of fly larvae,” he says. As soon as the fly eggs hatch, the larvae feed on manure and ingest cyromazine, which interrupts the cuticle formation of the developing larvae. They cannot complete their transformation into adult flies and die.

Nathan Voris, DVM at Pfizer Animal Health says that feed-through products work best if you start feeding them a month before fly season begins. Otherwise there will be some untreated fresh manure and fly larvae will complete their development in those piles. “If Solitude IGR is started after adult flies appear, it will take a couple weeks before you see a reduction in the fly population, and four to six weeks before maximal results will be observed,” says Voris.

As for the safety of this fly control method, French explains, “The compounds used in these products have been in use for 25 years in the poultry industry to control flies in chicken and turkey houses, and remain effective in that use—and no adverse effects have been noted in trials conducted on horses,” says French.

Whether using parasites or feed-through products, good fly control practices try and stop the problem before it even becomes one. Through good manure and waste management and methods to stop fly populations before they develop, human and horse populations can live a lot more comfortably during the warmer months.

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