Horse manure provides the breeding grounds for flies—that’s where they lay their eggs. Even well-maintained barns that regularly clean paddocks can’t eliminate all manure piles all the time. Feeding a daily fly control in a “medicated” pellet prevents larval development in the manure before it can mature into flies.

The effectiveness of this insect control method depends on several factors including:

  • The horse receives the correct dose,
  • The pellets are fed every day,
  • All horses on the farm receive it. Otherwise flies will flourish in untreated manure.

Manure is not the only place where flies breed, especially stable flies. They also thrive in decomposing organic debris anywhere on the farm, including soiled stall bedding or damp, fermenting hay beneath pallets. Further, flies that migrate in from other areas are unaffected by this medication at least until they lay eggs in treated manure.

Several choices of feed-through larvicides are available:

  •  Cyromazine: Solitude IGR (Zoetis, used to be called Serene)
  • Diflubenzuron: Equitrol II Feed-Thru Fly Control; Simplify with Larva-Stop Feed-Thru Fly Control (Farnam)
  • Tetrachlorvinphos: Equitrol Feed-Thru Fly Control
  • Cyromazine and diflubenzuron contain a growth regulator that hinders the maturation of immature fly larvae so they die. Tetrachlorvinphos kills larvae that hatch in the manure. However, it is an organophosphate, a drug that is toxic to the equine liver, so it is no longer recommended.

There are reports that resistance is developing to drugs like cyromazine, thereby rendering this treatment method less effective. Another point of consideration is how the feed-through larvicide might affect other invertebrates that breed in manure. Some are beneficial, such as dung beetles. The larvicide also has the potential to adversely impact parasitic wasps. The watershed should be protected as larvicides can be harmful to aquatic life if not used properly. Discuss with your veterinarian if any of the feed-through larvicides are appropriate for use with your horses. 

By Nancy S. Loving, DVM