Flies and Bugs

Photo: Christy M. West

Your horses face external parasites year-round; be prepared to fight!

Part of your management program for the year should include controlling external parasites and protecting horses from the ones that are difficult to control. Most parasites are seasonal, with populations dramatically increasing when temperature and moisture conditions are conducive to their survival and reproduction. In Northern regions this seasonality is very pronounced; warm weather insects are completely absent for a few months. By contrast, some Southern areas experience pests year-round. Every horse owner must become familiar with local parasites and their life cycles in order to find ways to combat them.


The primary external parasites in cold weather are lice and mites, but both are relatively uncommon in horses. Roger Moon, PhD, professor of entomology at the University of Minnesota, has only seen one case of lice in horses. "The ones I hear about most are from horse rescue operations, where animals have been overcrowded and undernourished and break with lice during winter," he says.

There are two kinds of lice--biting/chewing lice and sucking lice. Both types irritate the host animal, causing itchiness and rubbing, often to the point of losing hair.

If you find a horse with lice, Moon suggests treating all horses that have been in contact with the infested one. If you leave one untreated, the others can be infested.

"Chewing lice are more prevalent than sucking lice," Moon notes. "The products that control chewing lice also control sucking lice, but not vice versa."

He recommends applying an insecticide to the horse rather than using injectable or feed products. There are several dusts and pour-ons available, and these are safe if you follow label instructions. Don't use something off-label intended for cattle or you might severely irritate the horse's skin.

Mites (tiny arachnids closely related to insects) can affect horses in winter, but they are less common than lice. Proper diagnosis is important.

"If animals are itchy and you can't see lice, work with your veterinarian to diagnose the problem," says Moon, who notes skin scrapings might be helpful for detecting some lice. Treat horses with an acaricide (which kills mites, but not insects); this treatment might be diagnostic (if it works, it can be assumed the problem was mites--but you might never be sure).


With the first warm days mosquitoes and flies begin to emerge. Lane Foil, PhD, professor of entomology at the Louisiana State University Agricultural Center, says the first types of flies to emerge in his region are black flies--small biting flies that get into the horse's ears and create intense irritation. He says, "For many species (of black flies), the larval stages live in fast-running streams. In Louisiana we have mainly slow-running water, and we usually just have one generation of a certain type of black fly, in early spring."

These flies appear later--into May and June--in Northern regions.

Since it is impossible to control these flies by eliminating breeding sites, the best strategy is to find ways to protect horses. Insecticides and repellents are not always very effective against these flies, so fly masks and nets are often helpful to keep the flies out of horses' ears.

"You can put baby oil on the insides of the ears to keep the flies from biting, if the horse is not ear-shy," says Foil. "(In our region), the black flies we have won't enter a barn, so you can stable horses during the day when these flies are active." Give horses free access to shelter to escape the flies.

Stable flies also emerge in early spring. Feeding round hay bales can contribute to the problem; there's usually buildup of rotting hay around the bale or the feeder that makes an ideal breeding site.

"The fly larvae eat bacteria in fermenting hay piles," says Foil. "The greatest numbers of flies are found at different times of spring and summer in different regions, depending on when you feed hay. Once these piles are done fermenting and have dried out, they won't support fly larvae."

Horse owners in Northern regions feed hay until early April or into May, so the debris piles won't ferment until they thaw out. "Then the fly population gets going and they all come out at once," says Foil. The best control is to keep organic material cleaned up before fly populations build, and move the hay feeding area regularly so any wasted hay is spread thinly and won't be deep enough to ferment.

If you do end up with piles, spread them so they are no deeper than an inch. Simply piling the material elsewhere just moves the source of flies, and most stable flies can easily fly a mile to find your horses.

Fly predators are another way to manage fly populations. See article #11954 at TheHorse.com for more information.

Ticks emerge at different times in different regions. In some Western locales, spring and early summer are the only time Rocky Mountain ticks seek hosts. The American dog tick, lone star tick, and smaller black-legged tick (the deer tick, which spreads Lyme disease) appear at different times in different regions. In some Northern areas, spring and fall are both a concern.

"Ticks on horses are generally three-host ticks, feeding on different species of animals as they go through their life cycles," says Moon. The key to protecting horses is to keep them out of the woods, mow pasture vegetation short (to deter the rodents that act as first hosts), and check horses regularly to remove any attached ticks.


Summer can start early in the South and last longer than in the North. Biting midges (Culicoides) are a summer problem in some regions. "The primary problem they cause in horses is sweet itch, a genetic hypersensitivity to saliva inserted into the bite," says Foil. Affected animals become irritated and itchy. In different regions there can be different species of midges causing the problem, but most are highly active at sundown and feed through the night until dawn. Some midges spread the virus that causes vesicular stomatitis.

Some mosquitoes are night feeders also, and they might be just a summer problem in Northern regions, or they could be problematic nearly year-round in the deep South. There are hundreds of species of mosquitoes, and some breed in water that collects in containers--old cans, tires, tarps, or even a stock tank that's not periodically dumped and rinsed, for example. Others breed in any standing water along a wetland, or where irrigation water collects. Mosquito control is never complete, so it is imperative to vaccinate horses annually for mosquito-borne viruses, including Eastern equine encephalomyelitis and West Nile virus ahead of your main mosquito season.

Horseflies (tabanids) and deerflies are everywhere, and there might be many different species in an area. "We can't generalize about these flies because they all have different feeding sites and behavior," says Foil. "Horsefly season in Louisiana starts in March and ends in November. During that time about 50 different species may be present, but not all at the same time."

By contrast, horseflies in Northern climates might not appear until the first hot days in June, and they might have peaked and disappeared by August.

Stable fly control is ongoing through summer when you're eliminating breeding sites. Sawdust or wood shavings (instead of straw) used as bedding create less-ideal breeding habitats, but you must keep stalls clean and dry. Proper composting of bedding also helps. "Hot, active compost will kill any fly maggots, eggs, or pupae in material that's added to the pile, and will also convert it into something that flies will never breed in again," says Moon.

Some people rely on feed-through larvicides; these might help, but their efficacy can be limited because stable flies breed in other organic material, not just manure. Owners using feed-through larvicides can become lax in their other efforts (such as cleaning up old hay/bedding), thinking they have the problem under control.


In Northern regions some fly populations drop as nights get cooler. But in the South, some of the flies that slowed their breeding during the hot, dry summer days begin to reappear in the fall. Nancy Hinkle, PhD, an entomologist at University of Georgia, says in her area stable flies start in March and are active through early summer before receding, only to increase again during September through November. In Florida, the stable fly season is October through April.

"Late summer and early fall are often the peak seasons for our biting midges," says Hinkle. "There is a species that bites humans and animals and will eat you up about sunset if you are near the marshes. There are also a few that are active through winter, but their numbers are lower."

Take-Home Message

Horse owners need to be aware of external parasites year-round. Mites, lice, flies, and mosquitoes can cause irritation and carry dangerous diseases. Be aware of the external parasite species that are in your area and when they are prevalent so you can control them. Measures to control include repellents for your horses, insecticides for the barn and areas where your horses live, and larvacides for killing parasite stages in manure, water, or other areas.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.

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