Minimizing fly numbers
Methods that can help reduce fly population on a farm include use of premise sprays, fly traps, parasitic wasps and feed-through larvicides.
Some flies, however, especially horse flies, deer flies and stable flies, can fly long distances and come to your farm from neighboring regions.
Tia Nelson, a veterinarian in Helena, Montana, says some horse owners utilize parasitic wasps – the harmless tiny wasps (sometimes called fly predators) that lay eggs in fresh manure.
The wasp larvae feed on fly larvae and can help control flies that breed in manure. These wasps should be released early in the fly season.
They only work on flies that lay eggs in manure, such as houseflies, horn flies and stable flies. But stable flies are aggressive biters that feed on the horse’s legs and lower body.
They have a very painful bite, so the horse spends a lot of time stomping, kicking, licking his legs and swishing his tail.
Nancy Hinkle, entomologist at the University of Georgia, says horsemen should try to control flies and mosquitoes at the beginning of the warm season – whenever it occurs in their area, before the insect population becomes large.
“Become familiar with your stable fly season and try to get ahead of the curve by reducing early populations so there are not so many to reproduce. Cleaning up organic debris (old bedding and manure, decaying plant material) they use as breeding sites is very effective.
Old hay or bedding should be completely removed or scattered so it can dry out. These flies must have moist, decaying material in which to lay their eggs.
Don’t pile up organic material; a pile keeps moisture in and makes ideal habitat for fly larvae. Some people pile up lawn clippings, which may supply enough stable flies to torment all the horses in the neighborhood,” says Hinkle.
“Several farms I work for use a feed-through fly killer,” says Nelson. This product is fed in the grain and goes on through the horse.
Some of these contain a larvicide that kills fly larvae that hatch in manure. These products (Rabon, Equitrol, Vita-Plus with Equitrol) contain tetrachlorvinphos.
Other products, such as Simplifly, Solitude IGR and Equitrol II, contain an insect growth regulator (such as diflubenzuron, dimilin or cyromazine) that hinders growth and maturation of immature fly larvae, and they die.
Many horse owners think this method is safer than overhead fly sprays in barns. You don’t have to worry about contamination of feed or irritation of the horses’ eyes.
Nelson doesn’t like the feed-through products, however, because they only work in the small area around a stable or pasture and have no effect on flies that come in from neighboring areas.
Another drawback to this method is that stable flies also breed in other material, not just manure. People often become lax in clean-up of old bedding and other organic material, thinking they have the problem under control.
Fly sprays and wipe-ons
There are dozens of sprays, wipe-ons and spot-ons for use on horses, but almost all of them contain pyrethroids (such as permethrins) or pyrethrins as their active ingredients.
“These are about the only options for effective products safe to use on horses,” says Hinkle.
“Fortunately, these are fast-acting, so you could apply them to the horse just before you plan to work with the animal. Most products should be applied to the legs or belly, since that’s where stable flies will be biting.”
Dennis French of the University of Illinois says one of the things he’s found that works well for horse flies is Ultraspot, made by Absorbine.
“This liquid spot-on is a 45 percent permethrin that you apply in six different spots. You put one milliliter at the poll and another at the tail head, and on a spot at the point of each hock and behind each knee.
This seems to give protection for about two weeks,” explains French. Spot-on products tend to last longer than most of the sprays and wipe-ons and seem to work better for horses allergic to some of the sprays.
If biting midges (also called punkies or no-see-ums) are a problem, making horses itchy from an allergic sensitivity reaction to the bites (“sweet itch”), these tiny flies can often be thwarted with diligent application of insecticide.
“Midges can make animals very miserable,” says Hinkle. “They can bite anywhere on the body, but often bite along the midline of the belly and create a crusty, itchy area.
They are easiest to kill if you get enough insecticide on the animal and it stays on. Since they tend to feed on the belly, it is essential to apply it all along the belly, and reapply it if the horse has walked through tall grass, or stands in a pond or sweats,” she says.
Hinkle says stable flies are hard to kill. “These flies don’t spend much time on the animal, so they don’t pick up enough insecticide to kill them.
They zoom in, feed quickly and fly away. Many of them survive to come back again a few days later and to reproduce,” she explains.
The horse’s lower legs don’t retain insecticides very long. Even though a spray or wipe-on tends to bond to the hair after it dries, and is not easily rubbed off, it can still be washed off.
“Every time it rains, or the horse walks through wet grass or water, stands in a pond to protect himself from flies, or sweat runs down the legs, it washes off the insecticide,” says Hinkle.
If the horse has been out in the rain or sweating a lot, you may need to reapply a product sooner than the label recommends.
It’s difficult to keep enough on the legs to do much good, and the products that are effective need to be reapplied frequently.
Always follow directions when using any fly repellent or insecticide, to make sure you’ll get the optimum benefit from the product without putting your horse’s health (or your own) at risk.
“For instance, the label for Equi-Spot, by Farnam, says it is for external use on horses only, and to use it in a well-ventilated area and to not handle or use it if you are pregnant,” explains Nelson.
Some horse owners try different methods, such as tying cattle ear tags (formulated for horn fly control) on the horse’s halter or braiding a fly tag into the mane.
“But this is a systemic type of control, which may not be good for your horse,” says Nelson. Some of the cattle fly tags contain organophosphates, a more toxic type of chemical than the pyrethrins or permethrins.
Fly traps
Some flies are difficult to control with premise insecticides or manure management because they come in from other areas.
Horse flies and deer flies usually emerge with the first hot days of summer, after their larvae have developed in mud or water in marshy areas.
“We usually can’t control the source of these flies, since they may come from miles away. Since they attack a horse quickly and leave, most topical insecticides are not very effective against them,” says Hinkle.
“There are some fly traps that help, however. The University of Missouri has a website that shows how to construct a trap for horse flies.”
There’s also a commercially available trap that works well for horse flies, deer flies and other types of biting flies.
The Epps Biting Fly Trap, invented by a cattleman in Oklahoma and marketed by Horseline Products, uses a dark-colored panel to simulate the silhouette of an animal, and light-colored panels above and below it.
Since horse flies and deer flies tend to fly over, under and around the legs of an animal before biting, they strike the light-colored panels, fall into soapy water in trays under the trap and drown.
The soap breaks the surface tension of the water and the flies can’t float – they immediately sink and drown.
“Some of my colleagues worked with that trap and say it is very effective,” says Hinkle. “These types of traps may not completely control flies, but they cut down on the number, and the fewer flies pestering your horses, the better.” 
PHOTO
Above:Insecticides can often be washed too easily off the legs, leaving horses, like this one, to rub her face on the leg to dislodge flies. Photo by Heather Smith Thomas.
Middle:The Epps Biting Fly Trap uses dark-colored and light-colored panels to simulate shadows and light with soapy water trays to control flies. Photo courtesy Horseline Products.
