Chemical Feed-Through Products Fight Flies

The principle of chemical-based feed-through fly control is that you feed the animal a daily dose of a pesticide that mixes with the fecal material inside the horse and results in manure that prevents the development of flies from eggs, breaking the life cycle. And, yes, it works.

Manure from treated horses shows an 85 to 100% suppression of house- and stable-fly maturation. The reduction is usually noted within two weeks, with peak effects at about six weeks.

However, only flies that lay their eggs in manure will be affected. That means house flies and stable flies. Bots, deer flies, mosquitoes and horse flies won?t be affected, as they don’t lay eggs in manure. Adult flies aren?t harmed either, and they can still lay eggs in rotting vegetation, so You’ll want to use pristine farm management practices.


We find that feed-through products don’t completely eliminate the use of sprays and other bug-control methods.

Back well over a decade ago, the original feed-through fly-control products contained Rabon (tetrachlorvinphos), an organophosphate. However, when questions began to arise about its safety, it quickly disappeared from the market. (See Horse Journal, June 2000, June 2004.)

The two active ingredients in use today ? diflubenzuron and cyromazine ? are still pesticides, but they?re widely considered safe. Rabon had a much narrower safety range in comparison.


Diflubenzuron is the active ingredient in both of Farnam?s products, Equitrol II and SimpliFly. It works by blocking the production of chitin, which is the hard outer skeleton of adult insects. This prevents larvae from maturing into adults.

A short-term feeding trial (31 days) in horses was performed by Farnam as part of the registration requirements of the EPA (Vet Ther. 2007 Spring;8(1):61-76). Four groups of horses were fed 0 (control group), normal dose, 3 times and 5 times the normal dose. Some changes were noted in serum biochemistry but, because they remained within normal range, and the horses were clinically normal, they were not considered to be of concern.

Although we found no long-term studies with horses, we also didn’t find any adverse-reaction reports or complaints filed about these two products. SimpliFly with LarvaStop has been on the market since 2005, and Equitrol II was released just prior to that. They are virtually the same product, so you can let price be your guide.

Diflubenzuron has minimal impact on the environment. Large particles may take weeks to degrade, but small particles, as would be present in the manure of horses fed it, can degrade in as short a period as five days. The product is not known to react with any other drugs or feed ingredients, and it is not believed to be harmful to manure-eating dogs.


SimpliFly and Equitrol II are basically the same product, so you just purchase whichever one is less expensive at the time.

Cyromazine, the active ingredient in Pfizer?s Solitude IGR, works by preventing the molting of larval stages. The precise mechanism of action is unknown. It doesn’t interfere with chitin production per se, like diflubenzuron does, but it does stop the larvae from maturing.

Pfizer reports that acute toxicity studies in horses were done and submitted to the EPA. Dr. John Doneker, now with Pfizer, did those studies and reported no changes in CBC (complete blood count) or serum chemistries were found.

The effective dose for inhibiting larval maturation in manure from treated horses was established by an entomologist and horse owner, Dr. Kathy Palma. Her studies were submitted to the EPA as part of the product-approval process. In the six years since its release, Donecker said, ?There have been minimal problems and complaints associated with Solitude IGR.? Pfizer has a very active approach to consumer concerns.

it’s also interesting to note that cyromazine has been used as a pesticide on vegetables for over 25 years. It only affects the dipteral genus insects, so dung beetles and other insects that molt in horse mature are unaffected.

Natural Ingredients
The major ingredient in the natural-ingredient products is garlic, sometimes with thiamine, vinegar or yeast added.

Garlic is the only ingredient with any formal testing. Garlic oil applied directly to skin is effective against blood-sucking insects at dilutions as low as 0.005%. However, when human volunteers ingested garlic or a placebo prior to being exposed to biting insects, garlic appeared to have no effect.

A concentration of 0.005% on the skin sounds very low, but that is the essential oil and garlic contains only 1% fat, which would be only 142 mg of oil in a half ounce dose of garlic, which is diluted in a 1,000-pound body. Reports about the effectiveness of these products are mixed and difficult to separate from concurrent use of other fly-control efforts, like traps and fly predators.

We caution against using garlic in higher-than-recommended amounts. Garlic can cause Heinz-body anemia, and its effects could combine with those of other plants in this family, like the common weed onion grass.


Feed-through fly control is only one part of effective fly-control strategy. You’ll still need traps, parasites, fly masks/sheets and sprays for full insect control.

The non-chemical products are designed to make the horse less attractive to flies, with an offensive odor and ?bad taste? (to flies). Whether they work or not is debatable since, despite anecdotal evidence to the contrary, we could not find strong research data to prove they?re effective.

On the other hand, feed-through pesticides do work, and the short-term safety profiles are good. The EPA actively monitors its approved products for potential harmful effects on water, soil, plants, animals, humans, etc.

For maximum effectiveness, start feed-through fly control treatment about a month before fly season and continue feeding the product until after a killing frost.

Every horse on the farm should be treated. However, if 70 or 80% of the horses are treated ? or all the adult horses and not the foals ? you should still see a reduction. it’s a volume-of-manure issue.

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