Strategies for Fly Control
Biting flies are constant pests during warm weather. Different types of flies have different ways they attack cattle, so a combination of tactics for controlling them is usually most effective. Stable flies breed in rotting organic matter like old hay and bedding. Horn flies breed in fresh cattle manure. Horse flies and deer flies breed in swamps, and black flies breed in flowing water—often miles away from your cattle—so it’s impossible to control them at their breeding sites. Horn flies are generally the most irritating and costly parasite, though stable flies can be a huge problem on some farms. On a bad year, cattle spend more time fighting flies than eating. A cow may be kicking at flies when her calf is trying to nurse, which may discourage the calf.
There are many ways to wage war on flies, including removal of breeding sites, biological strategies like using parasitic wasps and dung beetles, and chemical control with pesticides. Nancy Hinkle, PhD, Professor of Veterinary Entomology, University of Georgia and Extension Veterinary Entomologist for the state of Georgia, says the cattle industry is very dependent on insecticides, unfortunately. “We have come to rely on these but are now faced with new generations of horn flies that have become resistant to these weapons,” she says.
Some stockmen also have serious problems with stable flies, and they are even more difficult to control with insecticides than horn flies. In earlier years, stable flies only occurred around barnyards, but now they are also out in the pastures. Debris left around big bale feeders or bale grazing in winter makes ideal habitat for breeding sites the next spring.
Horn flies probably cause the greatest economic loss. “Across North America, horn flies are still the number one pest bothering cattle, and we need more options for controlling them,” she says.
It often helps to rotate products, using a different chemical every year or so, but this depends on the situation. “There are some regional pockets around the country where horn flies have high resistance to both organophosphates and pyrethroids. Here in the Southeast we’ve been using these products for 50 years, and we have many populations of resistant flies, yet there are still some producers here who can get good control using the very same products that no longer work for their neighbors. There are still populations of horn flies that are susceptible to these products, especially if rotated. Stockmen generally use pyrethroids for two years and then organophosphates for three years and then switch back again. Yet just down the road there may be herds that have a different population of flies and nothing is working to control them,” she explains.
“Horn flies will eventually become resistant to any insecticide that we use consistently (whether in an ear tag, back-rubber, pour-on, etc.); continual use will lead to resistant flies, and we don’t have anything that is really new,” she says.
For stable flies, there is not an effective way to control them with chemicals since they spend very little time on the animal (compared with horn flies) and breeding sites can be extensive. “We want a simple answer, like spraying, but you can’t effectively spray around a bale feeder. The organic material binds up the insecticide and it will never get down to where the maggots are,” she says. Organic debris holds moisture and creates a perfect habitat for fly production all summer.
It only takes a few stable flies to irritate cattle. “Their bites are very painful; even half a dozen stable flies on a cow will make her miserable. The insecticides we use on cattle for horn flies (ear tags, pour-ons, dusters/oilers/back-rubbers, etc.) are not as effective for stable flies because they only land long enough to take a blood meal and fly away again. They bite the lower legs, so if the animal is walking in tall grass any insecticide on the legs is wiped off,” says Hinkle.
“There are a number of insecticides registered for use on stable flies but none of them claim much efficacy. It’s hard to apply these on the legs of cattle where the flies bite, and even if you do, it won’t stay there long enough (you’d have to repeat it often)—and the flies are probably not going to pick up a lethal dose.”
Cows on rangeland are not as bothered by stable flies because there’s not much old organic matter rotting in piles, like there might be in a pasture or feedlot. Horn flies are generally the bigger problem for range cattle.
For acceptable fly control, producers must figure out a strategy that is feasible and workable for their own situation. “It’s frustrating; they call me for advice and I can’t really help them because each situation is different. A neighbor down the road may be having good luck with organophosphate tags, but this may not work for their herd. About all we can do is trial and error.”
Some herds have been managed successfully for several years just using the feedthrough insect growth inhibitor that keep horn fly maggots in the manure from maturing. “These are mainly herds that are isolated from other cattle—maybe a few miles up in the mountains with no neighboring cattle. If there are no flies coming in from other places, this can work,” says Hinkle.
Traditional back rubbers, oilers and dust bags may be helpful in situations where cattle have to go through a gate, or learn how to use them in a small area. Cattle enjoy rubbing because it gives some relief from horn flies. “A person still has to go out there and recharge the oiler or back rubber, and if the flies have already become resistant to that insecticide it may not work,” she says.
There are biological controls like parasitic wasps and naturally-occurring dung beetles. “It’s hard to imagine how bad the horn fly problem might be without those beetles; they are out there every day helping break down manure pats, reducing the number of fly larvae that survive and emerge as adults,” she says.
Some people purchase parasitic wasps to release around barnyards, to lay their eggs in the fly pupae in manure and old bedding/litter. “If you know where those fly pupae will be (the areas to place the wasps), however, why not just clean up that debris and get rid of the problem,” says Hinkle. These wasps won’t have much effect in a big pasture; they are tiny and fragile and there would never be enough of them to make a difference in a large area.
Other natural controls include birds that eat flies or scratch through manure pats looking for maggots. We’d have a lot more flies without these allies. Mother Nature tries to keep things in some kind of balance; the flies provide food for other creatures.
INSECTICIDE EAR TAGS
Dr. Mike Fletcher (an entomologist who has been developing external parasite control products for Y-Tex since 1993) says “fly tags” (insecticide-impregnated ear tags) may not be the cheapest control method but they are less labor-intensive than other methods. “You apply them in the spring and protection stays with the cow all summer. You don’t have to gather cattle again to re-treat them. Any time you have to gather cattle there is more labor cost plus stress, and weight loss on the animals,” he says.
The fly tag business has grown tremendously from its inception in the early 1980’s and several companies market these tags. “The biggest problem is development of resistance; the flies are no longer susceptible to the product. In the early 1980’s all the tags utilized pyrethroids. By 1985 there was widespread resistance in fly populations and those tags were no longer as effective. That’s when organophosphate tags were introduced, and producers could alternate between those two types.”
In 2009 a new tag contained macrocyclic lactones (ivermectin), adding another rotation product. “We still run into issues but it’s usually because the producer doesn’t change often enough. He’s had a tag that worked well and doesn’t want to change, and within 2 or 3 years that kind is no longer effective. You have to change,” Fletcher says.
The producer has three kinds to rotate and today there is also a combination tag with two ingredients. “It can be very effective—but we don’t recommend it for extended use. You wouldn’t want to use it two years in a row; it has to be part of a regular rotation program. Horn flies will become resistant to whatever chemical they are exposed to.” There are always some stronger flies that survive and they are the ones that are left to reproduce, creating a new crop of resistant flies.
Tag manufacturers recommend putting in tags in late spring when fly numbers on cattle start to peak. Remove them in late summer at the end of the label claim period. Leaving tags in too long (after insecticide emission starts to decline) leads to more resistance issues in the remaining flies. “Producers who follow recommendations have better results than the ones who leave tags in. A good practice is to gather cattle late summer/early fall (about the time you might be preg-checking and processing them anyway) to remove the tags and treat the cattle with a pour-on that’s a different chemical than the tag was.” This can provide protection through the rest of the fly season.
“Fly season ends with the first hard frost. A good rule of thumb is to remove tags about 4 to 6 weeks prior to the normally occurring killing frost. If you used a pyrethroid tag you could use an ivermectin pour-on. If you were using any other kind of tag, you could use any permethrin pour-on. It’s economical and will last about 4 weeks.”
Some people put two tags on each cow (one in each ear) and none on the calves. Some put tags in the cows and the calves. “If you are trying to control horn flies, you only need to treat the cows (and bulls) and not the calves. Horn flies don’t bother baby calves until they get larger,” says Fletcher. The calf also gets some benefit from fly control on the dam; the calf comes into body contact with her during nursing.
Face flies are a different story, since they irritate the eyes and physically carry bacteria from animal to animal that can cause pinkeye. “Face flies hammer those young calves and they are also more vulnerable (less disease resistance) to pinkeye than older animals. If you are worried about face flies, we recommend tagging every animal,” he says. This may help decrease incidence of pinkeye in the herd. “For face flies you get better results using pyrethroid tags because of their repellency,” says Fletcher.
“If you are not getting good results with fly tags to control horn flies, stop using fly tags for a couple of years.” Use alternative methods and then come back to a type of tag that contains a chemical you have not been using in pour-ons, dusters, oilers and other alternative methods of control.
SIDEBAR: “SHOOTING” THE FLIES
One method for applying insecticide is the Vet Gun. This works like a paint gun, shooting a ping-pong size ball of insecticide onto cattle from a safe distance away. “I don’t think it would work very well out on range, but with gentle cows in a small pasture you might be able to walk around out there or use a 4-wheeler and dose the cattle!” Hinkle says.
“This is not a new chemical, however. It’s the same (lambda cyhalothrin) as the active ingredient in Saber pour-on. It’s a pyrethroid, so if your flies have already become resistant to pyrethroids it won’t be very effective.”
GENETIC RESISTANCE
Some cattle producers are selecting animals that are genetically less attractive to horn flies. There are some breeds and some individuals within breeds that are resistant to flies. With selective breeding a person can utilize those genetically-resistant animals and select for that trait along with the other desirable traits we want in cattle. Most producers are more interested in other traits, however, in their selection process. Fly resistance may be low on their list of priorities.
“It takes a lot longer to produce a line of cattle than to produce a new soybean! It is doable, but something you have to commit to doing. Today, with more knowledge of genetics, and genetic engineering, we can probably cut down the time it would actually take to produce more fly-resistant cattle,” says Hinkle.
This doesn’t particularly apply to stable flies, however, because they are not that co-evolved with cattle. “These flies would just as likely feed on a horse, a deer or some other animal,” she says.
Want to read more from this month’s issue? click here —>May 2025
By Heather Smith Thomas
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