Fly-free cattle graze contently on a well-managed pasture. (Photo by Robert Waggener)

The economic injury level of face flies, a common pest of pastured cattle, is only 10 insects per animal.

However, when face flies (Musca autumnalis) are carrying the bacterium that causes pinkeye, as is the case this season in several areas across the country, the economic threshold may be as low as one fly per animal.

“Face flies are very annoying to cattle. In fact, the irritation is quite remarkable,” says Dr. Wes Watson, professor of entomology at North Carolina State University. “But when those flies are carrying Moraxella bovis, you potentially have a real serious problem. The damage to your cattle herd can be quite significant.”

The highly contagious bacterium M. bovis can lead to outbreaks of infectious bovine keratoconjunctivitis—or pinkeye, as it’s commonly called—in pastured cattle, which can damage eyes and, if not treated in time, lead to blindness.

And market cattle showing signs of pinkeye lose value—some reports say as much as $75 for a 700-lb. steer.

“I’ve seen as many as 25 or 30 cattle in a herd of 150 showing signs of infection: tears forming below the eyes, liquid running down cheeks and stains on the cheeks,” Watson says. “When an outbreak occurs, it’s quite startling to see in your herds.”

Fortunately, producers can take proactive measures to keep face fly-populations down—and pinkeye in check during years when M. bovis comes knocking.

“What works best is an integrated approach,” Watson says.

Face flies require intact dung pats to support their offspring. Because of this, some producers find that well-managed pastures, including the use of short-duration, high-intensity grazing, can help reduce fly populations by trampling fresh pats, where face flies lay their eggs, and then moving cattle to clean pastures.

Watson encourages producers to monitor face-fly populations, and when numbers begin rising the use of insecticide feed-throughs in combination with other control measures can be effective.

“An on-animal spray can knock back adult populations, which will give your feed-throughs the time they need to start working,” he says.

Approximately 100 face flies congregate on a calf, 10 times above the economic threshold. Face flies not only annoy cattle, but they spread damaging diseases, including pinkeye. (Photo by Wes Watson)
Approximately 100 face flies congregate on a calf, 10 times above the economic threshold. Face flies not only annoy cattle, but they spread damaging diseases, including pinkeye. (Photo by Wes Watson)

Watson and colleagues recently conducted a study in North Carolina and found that feed-throughs definitely help control face flies.

“The feed-throughs negatively impacted the population pretty significantly, but it’s important to follow label recommendations to ensure that you are getting a high enough dose to have an impact on a population,” he says.

Treatments such as pour-ons, back rubbers, dust bags, oilers and ear tags can be used to supplement feed-throughs, adds Watson, who stresses: “Make sure the label says that the product aids in face-fly control.”

Watson says that face flies annoy the dickens out of cattle, affecting performance, and even greater economic losses can occur if that fly population is carrying the M. bovis pathogen.

“The flies are quite capable of transmitting this agent very quickly,” he emphasizes. “They are kind of opportunistic feeders, moving from animal to animal.”

Face flies have sponge-like mouthparts, which allow them to mop up fluids around the eyes of cattle.

But their mouthparts also sport spine-like structures, and it’s these spines that damage the animal’s conjunctiva, the mucous membrane that lines the inside of the eyelids and tear ducts. This, in turn, allows passage of M. bovis.

“If flies are present, keep a close watch on your cattle, and if you believe pinkeye might be establishing, call your veterinarian,” he says. “Excessive tearing of the eyes is a clear sign of an infection starting.”

Watson says that having a dual-action plan that deals with both fly populations and the diseases they carry will help keep your cattle healthy.

“Have an action plan in place, and be prepared to address a situation before it becomes a problem,” he says. “A pinkeye outbreak can be a very serious and costly loss for your herd, and that is why it’s important to reemphasize the point of being prepared.”