By Jennifer Dorsett
Field Editor
External parasites aren’t just an annoyance to livestock. The extra energy expended by constant head-tossing, stomping and overall restlessness of animals affected by flies detracts from milk production, weight gain and other markers of livestock performance.
But flies are also vectors for livestock disease, according to A.J. Tarpoff, Kansas State University Extension beef veterinarian.
In a recent webinar, Tarpoff discussed a variety of external parasites and ways ranchers can control them.
“We have to admit when we don’t know something,” Tarpoff said. “And what we don’t understand sometimes is exactly how some of these external parasites can play a role in transmission of diseases.”
Stable flies
Stable flies are found on almost every livestock species.
“If any of you have been around livestock, whether you’re in a stable with horses, whether you’ve been out working your cattle, and you see your horses or your cattle swishing their tails and stomping their feet and, as you get closer, you feel a super painful bite from a little black fly—that’s a stable fly,” Tarpoff said.
Stable flies are found on the legs, sides, back and belly of large animals or on the legs, head and ears of small animals, according to Texas A&M AgriLife Extension Service’s Livestock Veterinary Entomology division.
Tarpoff said unlike other fly species, the insects only come into contact with livestock when they feed. Stable flies rest elsewhere than on the animals when not feeding.
“They only feed during the day, which is really unique about this fly,” he said. “What’s unique about stable flies is during the heat of the day, after they take their bite, they will rest in shaded areas.”
Eggs and larvae are found in decaying organic matter such as decomposing straw on the ground near round hay bales. Larvae are also commonly located in wet straw bedding, algal mats, wet grass clippings, accumulations of manure and partially composted bedding and crop by-products.
“It doesn’t take very many flies per animal to really cause an issue,” Tarpoff said. “We can see different types of avoidance behaviors, like bunching, especially in horses and cattle.”
Horn flies are not an important vector of animal pathogens, according to AgriLife. But they can cause cattle to consume less feed, grow at a slower rate and convert less feed into body mass.
Bunching behavior will lead to increased body temperatures, which can lower milk production in dairy cattle.
Control of adult stable flies is difficult, so it’s best to prevent breeding where possible.
Horn flies
Horn flies are the most damaging insect pest of beef cattle in Texas, according to AgriLife.
Research shows a calf infested with more than 200 horn flies will weigh 15-20 pounds less at weaning, and horn fly infestation in dairy cattle can reduce milk production up to 20 percent.
Horn flies look like house flies and stable flies but rest on an animal between feedings. They feed on an animal’s back, shoulders and sides and often stay on the same animal.
“They only leave that one animal to lay eggs in fresh manure, and then they come back,” Tarpoff said.
He noted horn flies do not necessarily cause disease transfer, because they feed on a single animal. The economic and productivity losses, however, are significant enough to warrant control.
“It can be pretty devastating,” he said. “Those are flies we really want to treat.”
The best horn fly population control technique uses an integrated pest management approach in which cultural, biological and chemical methods are employed.
Face flies
Face flies mainly affect cattle and horses. As the name implies, these insects feed on the secretions around the eyes and nose, and they are a major vector of pinkeye infection in cattle.
Like the horn fly, face flies usually lay eggs in fresh manure, but this pest will fly long distances between feedings.
Unlike horn and stable flies, face flies are not blood feeding insects, but the spongy mouth parts are highly abrasive.
“It can mechanically transmit a little bit of bacteria in some of these tissues, but they’re not a blood feeder,” Tarpoff said. “They’re difficult to control, because they don’t spend much time on our animals.”
Face flies have also been implicated in the transmission of eyeworm in cattle.
House flies
House flies are one of the most common pests. While annoying, they are not a major factor in disease spread in livestock populations.
“We’ve documented that they can transmit different bacteria from one animal to another, but really, this is a nuisance pest,” Tarpoff said.
House flies swarm in large numbers during different periods of the day. Adults feed on available blood, sweat, tears, saliva and other bodily fluids. Instances of secondary wound myiasis have been recorded in which house flies lay eggs in open wounds.
“They will lay their eggs and open kind of perineal wounds,” Tarpoff said. “So even if we do have an infected—particularly vesicular stomatitis premise—some of those lesions may secondarily get a little bit of fly strike, because these house flies will congregate to some of those lesions on the outside.”
House flies tend to rest in sunny areas, whereas stable flies prefer shade.
“If you look on the side of your barn or the side of your house during the heat of the day, and you see massive numbers of flies, believe it or not, those are house flies,” Tarpoff said.
Control and treatment of external parasites
Tarpoff noted there are many product and treatment options available to ranchers and livestock owners to help control external parasites.
Feed-throughs, ear tags, pour-ons, sprays, dusters or oilers, premise treatments and other options may be useful in controlling pests.
“If I had a list, it would be about a mile long and wide about how much we actually have,” Tarpoff said.
Feed-through products are available for a variety of livestock species.
“These are products that are fed to the animal,” he said. “They do not affect the animal. It gets passed into the manure and into the environment where some external parasites will lay their eggs, and that’s how they actually interfere with the general population in an area.”
Feed-throughs are most effective before the vector season begins, Tarpoff added.
Ear tags impregnated with insecticide are another popular control method used effectively in cattle. But producers must remove them at the end of the season to help with resistance, which Tarpoff warned is a pressing issue.
“If we don’t remove them at the end of the season, we can actually incur more resistance issues in the next few years,” Tarpoff said. “We can have season-long duration with these as we have constant exposure, since they’re constantly on the animals.”
Resistance generally occurs because there’s either too low of a dose of product for a long period of time, or the insect has been exposed to the same insecticide classes for multiple years in a row and has evolved some natural defenses against the product.
Tarpoff said rotating insecticides is the best way to prevent resistance as flies become resistant to pyrethrins more quickly than some other chemicals.
“We can do a pyrethrin no more than one set of every three years followed with an organophosphate for two years,” he said. “If you have questions about this, please visit with your local veterinarian.”
Pour-ons and sprays are usually not as susceptible to creating resistance, because they don’t have much residual activity, he noted.
“The residual activity with ear tags is because we have constant exposure for up to five months,” Tarpoff said. “With pour-ons and sprays, they get into the environment, they work, and they go away.”
A variety of self-treatment applicators are available. These require animals to rub against or walk under the applicator, which then dispenses product, but Tarpoff said not all animals will use them when in a group.
“If you do have a group of animals and you’re using some of the self-treatment dusters, oilers or rubs, make sure that there is a little bit of unique fencing around strategic areas and force them to use them on a daily basis,” he said. “We’ll get maximum effectiveness.”
Tarpoff concluded one of the best methods of controlling parasites is environmental management.
“Cleanliness is the key to external parasites,” he said. “Managing manure, making sure we’re disrupting, and we’re not leaving piles of older manure areas that are heavy traveled.”
Undisturbed piles of manure, decaying hay and bedding and improper waste disposal all increase the availability of desirable habitat for flies and other pests.
“Making sure that if we can knock down vegetation on our fence lines and around our perimeter, that’s a pretty good step to take and controlling external parasites,” Tarpoff said. “Any area that we have water holes and things like that. If it looks like it’s going to harbor mosquitoes, it can probably harbor some of these other parasites, as well.”
For more information on the different insects and pests and a library of pesticides available for use in Texas, visit the Texas A&M AgriLife Extension Service’s Livestock Veterinary Entomology division.