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Controlling flies – Flies pose economic implications

by Wyoming Livestock Roundup

Controlling horn flies is a given in every beef operation. In fact, with $1 billion in losses in the U.S. each year, it is an area many producers could improve upon.

According to University of Nebraska Entomologist Dave Boxler, cows experience blood loss and annoyance when horn fly populations are high.

Impact of horn flies

This blood-feeding fly also impacts milk production in nursing cows, changes grazing patterns in pasture cattle and horses, causes bunching of animals and decreases weaning weights and yearling weights.

In fact, University of Nebraska studies indicate calf weaning weights could be 10 to 20 pounds heavier when horn flies are controlled on the mother cows. In yearlings, weights can be up to 18 percent higher when some type of horn fly control is managed.

In addition, Boxler said horn flies have been implicated in the spread of summer mastitis.


The economic injury level (EIL) of the horn fly is 200 flies per animal. They typically feed on the shoulders, back, sides and belly of the animal and can take up to 35 blood meals each day.

“They only leave the animal when the female deposits eggs in fresh cow manure,” Boxler said. “Because of this, horn fly population can reach very high levels – in the thousands during the summer. Horn flies numbers can exceed 5,000 on bulls.”

The best time to evaluate horn fly levels is between 8 and 11 a.m., when they can be found on the top and sides of the animal. During the heat of the day, the flies will move to the belly region.

Controlling the pest

Boxler said if producers have a horn fly problem, there are several methods of control from animal sprays, mist blower sprays, pour-ons and insect growth regulator (IGR) feed-throughs to dust bags, oilers and rubs to ear tags.

The best method of control depends on the management program the producer has in place.

“Pour-ons can cause stress to the animal and offset the benefits,” Boxler said.

If an IGR feed-through is used, the animal must consume a certain amount each day for it to be effective. The IGR can be fed through mineral, mineral blocks or tubs. The IGR kills fly larvae in the manure.

“Proximity to untreated cattle and under-consumption can prevent it from working,” he said.

Dust bags, back rubbers and oilers can be effective ways to control horn flies, but Boxler said they can be 25 to 50 percent less effective when they are used in a free-choice use system.

Other options

Insecticide ear tags can also be effective if two tags are applied to the cattle after June 1. To keep up resistance, Boxler said it is essential for producers to rotate insecticide classes every year to prevent the horn fly from building up resistance to the fly tag.

In an example, he showed using an XP 820 tag in year one, an organophosphate like Corathan in year two and a synthetic pyrethoid like Python in year three.

At the end of the fly season, Boxler said it is important to remove all insecticide ear tags.

The University of Nebraska also conducted a study evaluating the impact of a permethrin pour-on applied at the recommended rate on July 21 and Aug. 21. The first application provided 20 days of fly control and the second 24 days of fly control.

“Basically, the pour-on provided three weeks of fly control at a cost of about $2.16 per animal,” he said.

A relatively new product available to ranchers is a python strip that can be applied behind an existing ear tag. In a 2014 study, Boxler said these strips were applied in June and provided fly control for 15 weeks below economic injury levels.

“No strips or tags were lost, and no adverse affects were observed,” Boxler said. “It reduced the horn fly population 88 percent and cost $3.04 a head for the treatment.”

Face flies prevalent from wet conditions

Face flies don’t surface every year, but during a wet year like this one, they are appearing in numbers. Although they can be found most anyplace, face flies will be more abundant in waterways, canyon floors with trees or shaded vegetation, irrigated pastures and in areas with abundant rainfall.

These fly numbers peak in late July to early August.

According to University of Nebraska Entomologist Dave Boxler, the face fly resembles a house fly, except it is somewhat larger and darker.

“They were introduced to North America in the 1950s on cattle coming from Europe,” he said. “They feed on animal secretions, nectar and dung liquids.”

The female face fly will feed around the eyes, mouth and muzzle causing the animal annoyance. They also feed on wounds and mechanical injuries, he said.

The female face fly can also cause damage to the host’s eye tissue, promoting diseases like pinkeye.

“Face fly control is challenging because these flies feed around the face and spend a significant amount of time off the animal,” Boxler said.

The most effective forms of control are back rubbers/oilers, dust bags, oral larvicides and IGR feedthroughs and insecticide ear tags.

“Ear tags will need to be applied to both the cow and the calf,” Boxler said. “The best control of face flies is daily control.”


Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to

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