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Devastating pests: UW range specialist shares knowledge on horn fly management

by Wyoming Livestock Roundup

Riverton – On Feb. 9, during the Fremont County Farm and Ranch Days, University of Wyoming Range Specialist Derek Scasta highlights the importance of horn fly management on beef cattle. 

Horn fly lifecycle

“Wyoming is a pretty unique place to think about fly management on cattle,” notes Scasta. “Horn flies are not native to the U.S. They are blood feeders – requiring bovine hosts.” 

The horn fly is considered a filth fly – an insect reproducing in the dung of cattle. They have a two- to three-week lifespan, he shares. During the winter months, the insect spends the winters below the surface of manure piles.  

“One of the interesting things about these flies is, once they pierce the hide of a cow, they have this chemical in their saliva making the blood not coagulate as easily – so it flows really nice,” he explains. “They are an irritating pest and there are multiple ways producers can disrupt their lifecycles and fix this cattle and fly issue.” 

A female fly stays on the cattle for almost 24 hours per day and only leaves to lay eggs. The eggs will go through these different stages throughout a two-week period and an adult fly will emerge after three weeks – starting the cycle over again, Scasta mentions. 

“As we go through the summer, there is a new generation of flies impacting cattle every couple of weeks and this is a part of the problem – what makes it a challenging thing to manage,” he says. 

Negative impacts 

There are several reasons why the horn fly is such a devastating parasite to the bovine species and industry. 

“In the U.S., negative impacts of horn flies cost around $1 billion worth of losses to the domestic beef cattle industry every year. They are the most economically deconstructive parasite to cattle in the U.S.,” says Scasta. 

When a cow has to fight off flies by stomping their feet, swinging their head, or shaking their hide it, results in a reduction of grazing time. Several other impacts include reduced milk production for cow/calf pairs, lower weaning weights and reduced total gains in feedlot cattle, he shares. 

In addition, horn flies can transmit bovine mastitis and can cause granular dermatitis – a common infectious foot disease. 

Horn fly treatment 

“There is a threshold for when it pays to treat these cattle, and this threshold is what we call the economic threshold – when an injury level causes a value of loss more than what it costs to control and treat it,” he says. “When looking at cattle to be treated – if there are 100 flies per side, it pays to treat.” 

Scasta notes, throughout Wyoming in lower elevations, particularly near Torrington, horn fly infestation is significant. Grazing cattle at higher elevations – over 8,000 feet, might be one scenario where cows are less infested. If producers are unable to graze at higher elevations, a management plan should be developed.  

When developing a horn fly management program, he encourages producers to consider the following: long-term prevention; monitoring; correct pest identification; management when needed; preventing problems; combining tools; and managing for economic thresholds. 

The first thing producers can do is move animals or manipulate the habitat for the host. Harrowing and rotational grazing are strategies to manage the habitat for horn flies. 

The next thing producers can consider is breed selection – parasites discriminate, he notes. 

“Flies discriminate by breed and sex – horn flies are more attracted to bulls than cows. In addition, wool, hair and hide environment are several other factors,” he explains. “There’s this preference in Wyoming, flies prefer darker skinned cattle, and it’s probably because they are a warmer environment for them to land on.” 

Sprays, pour-on, ear tags, back oilers and oral fed products are several fly management practices producers can utilize. 

“To avoid the development of resistance to a chemical, producers want to use three types of chemicals – organophosphates, pyrethroids and macrocystic lactones if they are using ear tags,” Scasta adds. “If producers use ear tags, it’s important to rotate through a different insecticide every year over a three-year period and put a tag in each ear for maximum efficacy and remove the tags at the end of the year. Fly tags will usually work for most of the season but pour-on or spray treatments likely will need to be re-applied regularly.”

“The take home point is, some years producers might not have to treat, it all depends on the environment and the year, so make sure to be monitoring for this costly parasite of cattle.” Scasta concludes. 

Brittany Gunn is the editor of the Wyoming Livestock Roundup. Send comments on this article to 

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