When livestock animals (and especially cattle) are distracted by discomfort, they may spend more time fighting flies than grazing—slinging their heads over their shoulders to try to dislodge them, kicking flies off their bellies and constantly using their tails as flyswatters.
Bites of horseflies, deerflies and stable flies are very painful for livestock, and these flies go from animal to animal and can spread blood-borne diseases. Horn flies, by contrast, live their whole adult life on the host animal, and large numbers can suck enough blood to reduce weight gains.
Cattle Conundrums
During fly season, some cattle may have thousands of these little flies covering neck, shoulders and back, with smaller numbers on the rest of the body or along the midline of the belly. After these flies feed and mate, the females move to the rear of the animal and fly to the ground as the animal defecates, immediately laying eggs in the fresh manure.
Horn fly eggs are generally laid in clumps on grass and other vegetation covered by the cowpat. After a time, which varies depending on temperature, the eggs hatch and maggots develop and grow. They then pupate in or below the pat, and the emerging adults later seek cattle hosts.
Horn flies live on the hosts—only flying up temporarily in a cloud of flies if the animal brushes them off—until they have sucked enough blood to mate and prepare to lay eggs. As summer moves into fall, horn fly larvae in manure mature only to the pupal stage and overwinter, waiting to become adult flies the following spring.
Although most cattle can tolerate up to 200 horn flies without economic losses, larger numbers of these flies decrease livestock weight gain and milk production. Studies have shown that a calf with more than 200 flies during summer may weigh 15 to 50 pounds less at weaning than calves with fewer flies.
Most stockmen try to reduce the number of biting flies that torment their cattle. The typical weapon against flies during the past century has been insecticides; more than 100 years of research has seen numerous strategies, starting with fly-killing chemicals as sprays. In the 1950s, chemicals in back rubbers and dust bags (where cattle could self-treat by rubbing on these applicators) came into common use.
In the 1970s, the invention of ear tags impregnated with long-lasting insecticide made it easy to give season-long protection against horn flies by installing these tags in early summer. But with nearly every cattle producer using fly tags, flies quickly developed resistance, and people had to switch to a different type of insecticide tag for their livestock. Researchers had to come up with different chemicals. 
Many of the most popular tags no longer work very well. Problems with chemical control include the ease with which flies develop resistance, and the fact that insecticides are toxic to other creatures and not just flies. They may adversely affect beneficial insects and other life forms in the ecosystem.
In recent years, some stockmen have been finding ways to reduce fly populations without resorting to chemicals. Even though nontoxic methods won’t eliminate livestock flies completely, they can help keep numbers down to more tolerable levels.
Different kinds of flies have different life cycles and behavior. Something that might work to reduce one kind may not affect another, so a combination of tactics is usually most effective.
Stable flies spend very little time on livestock animals—just long enough to grab a blood meal—and lay eggs in manure and rotting organic matter like old hay and bedding. Horn flies spend all their time on the host animal and only breed in fresh cattle manure. Horseflies and deerflies breed in swampy areas and can fly long distances, so it’s almost impossible to control them at their breeding sites.
Keep Clean Quarters
Stockmen can control stable flies and horn flies to a large degree by reducing access to breeding sites. To reduce breeding sites for stable flies, manure and wasted hay should be spread thinly for quick drying or be composted.
Properly composted material gets hot enough during fermentation to kill fly eggs and larvae. Piles of old hay and bedding are a huge incubator for flies. Texas A&M University researchers found that the area around just one big bale feeder produces more than a million stable flies.
In the spring, move feeders and spread wasted hay so it’ll dry, or put it all into a big pile—and cover it with black plastic—to start heating and composting. Otherwise, it may stay wet through summer, providing an ongoing breeding site for stable flies.
Stable fly numbers can be reduced by a combination of diligent clean-up—not letting manure or old hay/bedding build up to create breeding sites—and biologic control by dung beetles and parasitic wasps. These tiny wasps lay eggs in manure so their larvae can feed on fly larvae. A few of these wasps are always present, but some people buy additional ones from companies that sell them to release in barnyard areas where stable flies breed. This helps augment the natural wasp population.
Dung beetles are another beneficial insect native to North America and generally present in the environment—unless people use pesticides or deworming drugs such as ivermectin that end up in manure, which tend to kill dung beetle populations. Adults hinder fly breeding by disrupting manure pats. They consume liquid in the manure and lay eggs in it.
Hatching beetle larvae thrive on manure for food. Some species of dung beetles remove and bury balls of manure in which they’ve deposited their eggs.
An active population of dung beetles can bury or destroy 95 percent of horn fly eggs and larvae and about 90 percent of other cattle parasites that are passed in or depend on manure. They can’t get back up to ground surface after dung beetles bury the manure.
Birds are attracted to manure containing dung beetles and tear the pats apart to eat the beetles—which helps disrupt fly larvae development. One manure pat without beetles can generate 60 to 80 adult horn flies. A herd of cattle can provide a nursery for millions of horn flies.
On the Move
Rotational grazing can be another means of habitat control, if you can move cattle frequently enough—and far away enough—to leave the fresh manure behind so there are no cattle to fly to when they hatch into adults. If the cattle are moved far enough and don’t come back to that pasture for several weeks, this can help break the fly life cycle because livestock are no longer available to the hatching flies.
Horn flies develop from egg to adult within 10 to 20 days (depending on weather and temperature), and the adults live for about three weeks on a host animal, feeding 20 to 30 times a day. If they don’t find a host, they die.
If the cattle don’t come back to the pasture where the flies hatched out (while giving that pasture adequate time to regrow), the fly life cycle is broken. Moving cows every few days can make a big difference, but if they aren’t moved far enough, the flies can still find the livestock.
Other methods of biologic control include birds that eat flies. Birds that can catch insects in midair—such as tree swallows, barn swallows and eastern kingbirds—eat many mosquitoes and flies. Some people put up bird houses to attract tree swallows. These swallows feed from dawn to dusk in areas full of flying insects.
They eat all kinds of flying insects, including some of the flies that bother cattle.
Chickens that roam in barnyards and pastures also eat flies, and their scratching around in cattle manure disrupts the life cycle of several types of flies. Some types of ducks are also great fly-eaters and hang around cattle to eat flies. Cattle egrets also eat insects off the backs of cattle.
Fly Traps
Biting flies, including some that come from other areas, can sometimes be trapped before they attack your animals. Several sticky traps exist for use in barnyards, as well as electronic bug zappers that attract flying insects. One effective method for incoming flies is a trap invented by a cattleman in Oklahoma, now made and marketed by a company in Tennessee.
Horseflies, deerflies, stable flies, black flies and mosquitoes are attracted to large, dark objects—the shape and silhouette of an animal. This trap is a frame with a dark portion and transparent panels that simulate air space above an animal and under its belly, where flies circle before landing. When flies hit the transparent sheets, they bounce into trays of water and drown.
If a person adds a few drops of dishwashing soap, the soap breaks surface tension of the water so the insects can’t float. They immediately sink and drown. Dish soap isn’t harmful to the environment, and some organic farmers use a soap product that is safe enough to drink.
Research at several universities showed this trap kills about 1 pound of biting flies per day. A three-year project at Cornell University, University of Florida and New York Pest Management compared 15 flytrap products and found this trap most effective. The trap was also tested three years on New York dairy farms, looking at nonchemical approaches versus pesticides.
The trap catches many flies in the evening when the dark portion is still warm after air cools off. The fly thinks this is an animal. It works best in an open area where flies see it from a distance. You need to scoop out dead flies every other day or so with an aquarium net, add more water and soap if needed, and change water every two weeks.
If it’s in an area where animals might rub on the trap and damage it, you can put an electric fence around it. A portable version can be moved from pasture to pasture for rotational grazing. It has an aluminum frame (lightweight and easy to move) and sandbags—to be filled and placed on the legs to help secure it in strong wind.
Around the Horn
Horn flies tend to stay on one animal instead of flying from animal to animal and need a different kind of trap. A walk-through fly trap was made by USDA entomologist Willis Bruce before World War II, but the advent of insecticides—pour-on products and ear tags—quickly gained more attention and use. There was new interest in his trap in the late 1980s, however, after horn fly resistance to insecticides became an issue. Instructions for building a simple walk-through trap are available from University of Missouri Extension.
Cattle enter the 10-foot trap through either end—as when going and coming from water or along a travel route from pen to pen. As they walk through the enclosure, they contact a series of canvas or carpet strips that dislodge most of the horn flies on their backs and sides. The dislodged flies are then attracted to light and travel toward the screened sides of the trap and can’t escape.
Zigzag screening forces them to crawl from a large opening through a smaller one. As they go through this cone effect, they’re trapped between the exterior screen on one side and the zigzag screen on the other. With the small end of the “cone” facing them, they don’t find their way back out.
If cattle are reluctant to enter the trap, it may be necessary to remove most or all canvas strips, and then gradually replace them after the cattle overcome their fear. Feed can be used as bait to attract cattle into the trap, or it can be placed where cattle must go through it to get to water.
After livestock become accustomed to it, they often go through it several times a day because they realize it brushes the flies off and provides relief.
Field studies in central Missouri in 1986 showed that the trap reduced about 50 percent of horn flies during the season. This level of control was less than that from insecticide ear tags and some other treatments but maintained horn flies below the damaging level, which is about 200 flies per animal.
Other Ways to Reduce Fly Load
Some farmers put apple cider vinegar during summer in the cows’ water supply to repel flies. This makes the skin slightly more acidic (changes the pH) and flies are less attracted.
Genetic selection for fly resistance is another strategy. Some cattle are naturally more attractive to horn flies and always have higher numbers. If a person culls those animals and selects fly resistant cattle to keep in the herd, this can make a difference.
Hair coat is another factor. Livestock with short, sleek, oily hair are less attractive to flies. There are many genetic traits that people select for when breeding cattle; fly resistance is rarely on the list, but some stockmen pay attention to this trait when making breeding decisions.
This article originally appeared in the May/June 2023 issue of Hobby Farms magazine.
