Controlling horn flies in the herd
Of all biting flies, horn flies cause the most problem for cattle because adult flies spend almost all their time on the host. During fly season, an animal may have thousands covering neck, shoulders and back, with smaller numbers on the rest of the body or along the midline of the belly.
Studies have shown a calf with more than 200 flies during summer may weigh 15 to 50 pounds less at weaning than calves with fewer flies.
Roger Moon, professor emeritus of entomology at the University of Minnesota in St. Paul, said it is important to know which flies you are trying to control, and utilize the appropriate tactics. “The flies that bother cattle in pastures are mainly horn flies, face flies and sometimes stable flies. The stable flies may be coming from a nearby barnyard; they are generally not coming from the pastures, unless there is debris from old round bales or from hay/straw residue that was fed in winter, creating breeding sites for stable flies,” he said. By contrast, horn flies and face flies are breeding wherever there are fresh cattle dung pats.
Horn flies have a short life cycle — from egg to adult in 10 to 20 days. Adults live about three weeks and suck blood so they can produce eggs, which they lay in fresh cow manure. “In terms of misery to the animal, horn flies in large numbers may be worse than stable flies. In larger operations where cattle are spread over big range pastures, stable flies are not as big a problem as horn flies. The cattle are far away from winter feed pastures where stable flies breed. In hot, dry summer, horn flies will keep going, whereas stable flies will slow down,” he said.
Horn flies only develop in fresh cow manure. They don’t breed in feed-yard debris. “Pasture operations often have horn fly problems but feed-yard operations don’t. Many people with horn fly issues have been relying on insecticide-impregnated ear tags, which are probably the easiest method of insecticide control. Tags can be put into the ears at the start of fly season, and the animal does the rest, rubbing the insecticide over shoulders and body as she slings her head at the flies,” Moon said.
The first insecticide ear tags became available in the 1970s. Pyrethroid tags came into use in early 1980 but by the mid 1980s horn flies began to develop resistance to pyrethroid insecticides. Next came organophosphate tags, and flies were slower to develop resistance. But in many areas the fly populations are no longer susceptible to these chemicals.
With nearly everyone using fly tags, the flies quickly developed resistance, and people had to switch to a different type of tag. “A Texas A&M website shows pictures of 15 or more kinds of ear tags available for beef cattle. They are mainly pyrethroids and organophosphates, but two more categories have come on the market and ivermectin is one of them. There is also a relatively new compound called tolfenprol, a new class of insecticide registered for horn fly control,” Moon said.
The new tag, containing tolfenprol, works for about 10 weeks. “We’ve switched among the different kinds of pyrethroid insecticides, and cross resistance is probable, which means we’re really not rotating unless we change to organophosphate tags. Now, however, we can have a three-way rotation, and if you count the ivermectin we have a possible four-way. If pyrethroids have lost efficacy on a certain ranch, a switch can be made to organophosphates and then to the tolfenprol. This will work, for a while.”
A LOT OF WORK
There is not much else a person can do to control flies in range operations because cattle are spread over such large areas that you can’t depend on dust bags, oilers, back rubbers, pour-ons or sprays. It’s a lot of work to round cattle up and pour them. Most people put ear tags in just before they turn the cattle out (which is often too early, before the fly populations are building up), and leave them in too long — which hastens fly resistance as the tags lose their efficacy. If you have a less extensive operation where you can see the cattle more often, it pays to monitor them more closely, observe their behavior, and do some fly counts before installing ear tags.
For horn flies, it pays to forgo control until fly numbers start hurting calf growth (in a cow-calf operation). “It’s not that the horn flies are bothering the calves so much as they bother the cows, and milk output is dropping. If people can knock down horn flies for a six-week period, they can get their money back. But aiming for full season control may be too costly. The horn flies don’t become really abundant until July or August, depending on how far north or south you are,” Moon said.
For several decades the most common way to control horn flies was with pesticides such as pour-ons, rubbers/dusters (self-applicators) and insecticide ear tags. Horn flies develop resistance to these chemicals after a few generations, however, and chemicals must be rotated yearly or every other year. Another drawback is that there have been indications that certain pyrethroids may have negative effects on semen quality of bulls. Many stockmen are now trying to use fewer chemical pesticides to control parasites.
One alternative for horn fly control is to use natural methods such as parasitic wasps (that work best in small areas like a feedyard or barnyard) and dung beetles. The tiny wasps can be purchased in immature form (to be scattered around the feedyard) from insectaries. The adult wasps lay their eggs in fly larva in manure and the young wasps eat the fly larvae.
Dung beetles live in manure. The adults feed on liquid portions, and lay eggs in manure. Hatching larvae consume more manure. Dung beetles help control all parasites that depend on manure for part of their life cycle. Some species of beetles bury manure, which helps fertilize the soil.
Some types of dewormers and pesticides destroy dung beetles, however, killing their larval stage. Use of ivermectin products can decimate beetle populations because fresh manure of treated cattle is toxic to beetle larvae. Some stockmen with small herds control flies with Muscovy ducks. This breed is not a water duck; it eats insects. The ducks can range freely in pens and pastures and are prolific breeders. They follow cattle around, searching through manure for fly larvae and scattering the piles. It takes about four ducks per cow to adequately control the fly population. The ducks also eat adult flies and pick flies off cattle when they are lying down.
Some farmers use 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, reducing fly load on the animals. Some rotation grazing programs help minimize horn fly development, if cattle are moved often enough, and far enough, to new pastures to get away from the manure where flies are breeding. If cattle are moved far enough, and don’t come back to that pasture for several weeks, this can help break the life cycle because cows are no longer available to the hatching flies. Moving cows every day or every few days can make a big difference. If they are not moved far enough away, however, the flies can still find the cows.
A walk-through fly trap was constructed by U.S. Department of Agriculture entomologist Willis Bruce before World War II, then gained new interest in the late 1980s after horn fly resistance to insecticides became an issue. Detailed 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 it 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 attracted to light and travel toward the screened sides of the trap, and cannot escape. Zigzag screening forces them to crawl from a large opening through a smaller one. As they go through this cone effect, they are 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.
A combination of several strategies (fly trap, dung beetles, etc.) can reduce horn flies to less damaging levels. Though there are some new methods (pour-on products, insecticide ear tags, special fly traps, and a “gun” designed to shoot a ping-pong ball size ‘bullet’ of insecticide onto a cow from a safe distance), some of the more traditional methods are still useful. On some farms, the old style back rubbers, oilers and dust bags can help fill some gaps in a fly control program, especially if cattle have to walk through a narrow gate to water and rub on the oiler or dust bags.
Some breeds of cattle are more resistant to flies than others, and individual cattle within breeds also show differences. A few people are selecting breeding stock for horn fly resistance, among other traits. “In any herd, often the most productive and efficient cows tend to have resistance to parasites and flies,” Moon said. ❖
— Smith Thomas is a cattle rancher, horseman, freelance writer and book author, ranching with her husband near Salmon, Idaho. She can be reached at firstname.lastname@example.org.
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