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Fighting flies

by Wyoming Livestock Roundup

Experts offer considerations for controlling flies on pastured cattle

A perfect storm of wet and warm weather conditions during peak breeding season has led to an influx in fly populations across the West. Many cattle producers have taken note of the pests, as their cattle spend most of the day bunched up in pasture corners fighting flies. 

However, despite measures taken to control pest populations, many ranchers have become frustrated with the lack of relief they provide to livestock and have turned to experts for advice. 

In a University of Nebraska-Lincoln (UNL) Extension article written in May 2016, UNL Extension Educator Dave Boxler points out producers in Western states need to remember there are three species of fly that can economically impact pastured cattle – horn flies, stable flies and face flies. 

“Insecticide resistance is a major problem for these types of flies because they have been exposed to so much insecticide labeled for application to an animal, so producers aren’t seeing as much relief as they were 20 years ago,” states Scott Schell, University of Wyoming Assistant Extension Entomologist, during an interview with the Wyoming Livestock Roundup

The two experts outline the differences between the three species and offer some control options to help producers mitigate fly damage during summer months.

Horn flies

Schell explains horn flies, which are the smallest in size, are usually the most dominant of the three species in Wyoming. They can be found on the backs, sides, poll area and sometimes on the belly of cattle. 

“With more moisture, cow manure where horn flies breed will stay wetter long enough for them to emerge more successfully than they do in drought,” he says. “When it is dry, manure patches will dry out before maggots complete development, which is why there are more horn flies this summer.” 

Horn flies are blood suckers and can eat up to 30 blood meals per day. 

“Economic losses associated with horn flies are estimated at more than $1 billion annually in the U.S.,” notes Boxler. “Horn fly feeding causes irritation, blood loss, decreased grazing efficacy, reduced weight gain and diminished milk production in mother cows. Additionally, horn flies have been implicated in the spread of summer mastitis.”

Boxler refers to several studies across the U.S. and Canada which have proven horn flies impact weight gain and can negatively affect weaning weights up to 15 percent. More specifically, a study conducted in Nebraska found weaning weights were 10 to 20 pounds higher in calves whose mothers received horn fly control treatments.

According to Boxler, there are multiple insecticide application options available to help manage horn flies including backrubbers, dust bags, insecticidal ear tags and strips, pour-ons, oral larvicides, low pressure sprayers, mist blower sprayers and the Vet Gun.

Schell notes, with the help of Derek Scasta, PhD and UW Rangeland Extension Specialist, he recently published a bulletin on horn fly management in Wyoming, which includes alternative control methods to combat insecticide resistance. 

“Having multiple methods of control will give producers the best relief,” he states.  

Stable flies

Like horn flies, stable flies are also blood suckers, and they mainly feed on the front legs of cattle.

However, according to Boxler, stable fly bites are more painful, and animals will usually react by stomping their legs, bunching up in pasture corners or standing in water to avoid getting bitten.

Boxler and Schell note although stable flies pose a serious threat to feedlot and dairy cattle, they can also have just as serious of an impact on pastured cattle as well.

“We are seeing more stable flies attacking livestock on pasture, especially in areas where hay was fed and there is a lot of waste hay residue,” Schell explains. “This is what stable flies like to breed in.”

“UNL research recorded a reduction in average daily gain of 0.44 pounds per head with animals receiving no insecticide treatment compared to animals receiving a treatment,” says Boxler. 

According to Boxler, animal sprays are the most effective control method for adult flies on grazing cattle.

“Sprays can be applied using a low-pressure sprayer or can be applied with a mist blower sprayer. Weekly applications of these products will be required to achieve reduction in fly numbers,” Boxler says.

Schell points out, for cattle on range, this may not always be feasible or cost effective. Instead, he suggests using indirect methods of application such as dust bags. He also notes, although expensive, producers may consider using oral insecticide products. 

“Sanitation or clean-up of wasted feed at winter feeding sites may reduce localized fly development. If sanitation is not possible, these sites may be treated with a larvicide like Neporex,” Boxler adds. “But, the application of either procedure may not totally reduce the economic impact of stable fly feeding.”

Face flies

Lastly, as the larger and darker of the bunch, face flies more closely resemble house flies and are non-biting flies, feeding mostly on animal secretions and nectar.

Face flies cluster around animal eyes, mouths and muzzles, causing irritation and annoyance. These flies are present all summer – along waterways, areas with a lot of rainfall, canyon floors with trees and irrigated pastures – but populations usually peak in late July and August.

For cattle producers, the biggest concern with face flies is the transmission of pinkeye, a highly contagious inflammation of the cornea and conjunctiva of cattle. 

“Female face fly feeding causes damage to eye tissues, increases susceptibility to eye pathogens and Moraxella bovis (M. bovis), the causal agent of pinkeye or infectious bovine keratoconjuctivitis,” Boxler explains.

“If coupled with the infectious bovine rhinotrachetis virus, M. bovis can cause a much more severe inflammatory condition,” he adds. “Controlling face flies is essential in reducing most pinkeye problems.”

In order to control face flies, Boxler suggests producers utilize daily insecticide application options such as dust bags, oilers, sprays or insecticidal ear tags and strips. He also notes both cows and calves must be treated to achieve maximum control of the pest.

“Pinkeye vaccines are available and should be considered if face flies and pinkeye have been a recurring problem,” Boxler concludes. “Currently, commercial and autogenous pinkeye vaccines are available, but please check with a local veterinarian about the use of these products in your area.”

Hannah Bugas is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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