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Researchers publish report detailing approaches to mitigate horn fly impacts on cattle

(Shutterstock/Alexandre Moraes)

GILLETTE, Wyo. — A new report recently released by the University of Wyoming details how cattle ranchers can mitigate multi-billion-dollar losses resulting from parasitic horn fly infestations. 

Long-lived and fast-breeding, horn flies were first recognized in the United States in the late 1800s and today are found nationwide. They are birthed from eggs laid in cattle dung piles in warm, humid conditions every 10–20 days, with adults living anywhere from two to four weeks, according to the report. 

Swarms of flies suck blood from cattle during the day and night, which in turn expend energy by stomping their feet, swinging their heads, swishing their tails, and shaking their bodies in an attempt to relieve irritation caused by the flies, the report says. 

The expended energy translates to a reduction in cattle production, per UW, which says cattle suffering from horn fly infestations graze less, which, in turn, leads to reduced milk production and lower calf weights. 

Horn flies can also serve as vectors for disease pathogens such as bovine mastitis, an inflammation of the mammary gland that can cause less and lower-quality milk, per the report. 

“Horn flies are the most damaging external parasite of cattle in the U.S.,” said J. Derek Scasta, UW Extension rangeland management specialist and co-author of the report. 

Every year, horn flies are responsible for losses exceeding $2 billion across the U.S. beef cattle industry, the report states, which notes that pest management strategies could help cattle producers earn as much as $62 more per calf by increasing their weight in herds suffering from horn fly infestations. 

In a comparison published in the report, the authors determined that calf-weaning rates in horn fly–infested herds were down approximately 4–15%, resulting in yearling weights 18% lower than those in protected herds. 

Counting the number of horn flies on a single cow to help estimate the extent of infestation in the entire herd. (UW Extension Office)

To protect herds, a study published by New Mexico State University suggested cattle producers could see lower horn fly infestations by utilizing insecticide ear tags on mother cows combined with a pour-on insecticide treatment. 

In the study, calves were not treated directly but benefited from protective treatments applied to their mothers, per the report. 

Ear tags, according to the report, must be applied in pairs and cost approximately $5 total per head of cattle. The devices require a single application per season and last for about 18 weeks. 

Pour-on treatments, the report estimates, cost around $2 per head and require around six applications per season with each one lasting three weeks, according to the report.   

Additional options range from back rubbers, devices soaked in an insecticide that the cattle can rub against, and oral larvicides or insect growth regulators that are fed to the cattle and kill horn fly larvae in manure. 

Outside insecticide treatments, cattle producers could utilize other approaches such as rotating cattle between pastures or dragging pastures to break up manure, both of which are ways of disrupting the horn fly reproductive cycle. 

Breaking up the manure exposes the flies’ eggs to the air, drying them up and making them susceptible to predators like birds, according to the report. 

Slow-release insect growth regulators in the large pill (bolus) being given by animal technician Keith Shelley will prevent development of horn flies and face flies in the manure. (Agricultural Research Service/U.S. Department of Agriculture)

The report says if cattle producers choose to rotate their cattle out of areas infested with horn flies, thereby sparing their animals from emerging fly populations, the rotation area should be significant. 

Research, per the report, has shown that moving cattle between pastures 8 acres or smaller is insufficient with the flies being more than capable of migrating to the herd at such distances. 

Additionally, when it comes to avoiding horn fly infestations, not all cattle breeds are created equal, according to the report, which says breeds with darker hides often see greater populations of horn flies than their lighter counterparts. 

Bulls are also more susceptible to horn fly infestations than cows, a phenomenon attributed to differences in pheromones and respiration rates between cattle sexes, the report says.

“Wyoming cattle producers should consider the complexity of the situation to determine if and when treatment is appropriate,” Scasta said. 

To determine the severity of a horn fly infestation, there’s really only one approach — counting, according to the report. 

“Counting individual flies might seem like an impossible (or impossibly tedious) task, but regular monitoring is the key to making informed management decisions,” the UW Extension office said in a Dec. 15 release. 

In the report, the authors suggested counting the number of flies on a handful of cattle to extrapolate the level of infestation for the entire herd. 

If flies reach what the authors referred to as the economic threshold, approximately 200 flies per cow, then the value of loss becomes larger than the cost of control, per the UW Extension Office. 

For a full copy of the report, please click here