By Tim John, DVM

When it comes to protecting your horse from pests, what’s the difference between an insecticide and a repellent … and why should you care?

The difference, as the names suggest, is that one kills and one repels. Does this really matter when you’re trying to combat the swarms we usually encounter?

The answer is maybe.

It depends on what type of bugs you are trying to stop and how the bugs interact with your horse. The two most important bugs we may want to repel (instead of just killing) are:

1. midges (no-see-ums) and
2. mosquitoes.

Midges, in susceptible horses, may lead to problems with “sweet itch” – a hypersensitivity reaction to the bite itself. Mosquitoes can transmit West Nile virus and other nasty neurological problems.

In both of these cases, it is much better that these pests do not come near us if possible, so you should reach for an insect repellent.

Most repellents act as vapor barriers. That is, they slowly (or too quickly) evaporate to form a shield that stops the bugs from coming near. DEET is the “classic” repellent, but its use in horses is not recommended.

Interestingly, most pyrethrins (neurotoxins that attack the nervous systems of insects, commonly used in sprays) are good insecticides but poor repellents. EXTOXNET, a university-supported toxicology network, states that permethrin (a common chemical used in insecticides and repellents) has only a “slight repellent effect against insects.”

So what happens most of the time with these products is that the bugs are actually landing on the horse, contacting the spray on the horse before the active ingredients in the sprays achieve their maximal effect … and not doing a very good job of protecting your horse from direct contact with the midges and mosquitoes.

So the next time you’re reaching for insect spray, ask yourself: “What am I trying to accomplish? Are these sprays the right ones for the job?”