Veterinarians and drug companies agree that feed-through fly control for horses–along with proper land and stable management–can keep barn fly populations under control. Fly control management is a serious problem facing horse farms of all sizes.

There are two major categories for feed-through fly control products–those containing organophosphates and those that are organophosphate-free. Products with organophosphates contain the active ingredient tetrachlorvinphos (TCVP). Organophosphate products are formulated to maximally pass through the digestive system in the manure, with minimal absorption. Female house and stable flies then lay their eggs in manure deposited on the ground. These eggs will hatch into larvae, but will be prevented from growing into adult flies because the TCVP affects the larvae’s nervous system. In fact, according to Anne Robertson, public relations director for Farnam Companies that manufactures some feed-through products (Equitrol), “TCVP inhibits cell and muscle cholinesterase activities in flies with consequent disruption of nervous system activity, which results in the insects’ death.”

TCVP is registered with the Environmental Protection Agency (EPA) for use in horses, swine, and beef and dairy cattle. It has been a registered product for more than 25 years as a feed-through oral larvacide for fly control in horses. Organophosphate products have been proven 99% effective against house and stable flies with a daily dosage of the supplement.

The other class of feed-through fly control is organophosphate-free, and can currently only be purchased through a veterinarian. This product works much the same, in that it passes through the digestive tract of a horse, with virtually no absorption. The active ingredient in this organophosphate-free product is the insect growth regulator (IGR) Cyromazine. According to Dennis French, DVM, Dipl. ABVP, of Louisiana State University, “Cyromazine works by inhibiting the synthesis of the exoskeleton (chitin) of the insects. In other words, it does not allow for the formation of the skeleton, and thereby inhibits the development of the eggs to larval and pupal stages