Advisor describes how to fight flies in cattle herds

Four main groups of flies can be problematic for cattle: horn fly, stable fly, face fly and house fly. Depending on the type of facility and animal management system, the importance or impact of each fly group will differ.

Flies can be problematic for beef cattle on rangeland and pasture, according to Alec Gerry, University of California, Riverside, professor of entomology and Cooperative Extension specialist.

All four flies can harm the animals, but the horn, stable and face flies cause the most trouble.

"Horn flies will harm the animal simply by their numbers of flies. They can often build up very large numbers on each animal—in some cases in the thousands of flies," Gerry said.

Horn flies live their lives almost exclusively on the animal and rarely leave it, biting the animal several times a day to take blood. Each bite is painful and studies show that horn flies, at high numbers, can impact cattle growth, he said.

Because the horn fly constantly irritates the animal, it will continuously twitch its skin and flick its tail, which expends energy addressing the flies, rather than eating. This will slow growth, Gerry said.

Stable flies will have the same impact, but at much lower numbers. They are also very painful biters, but they generally bite once, then leave the animal.

The animals' response to stable flies is the same as to horn flies: They will reduce their feeding, water consumption and expend energy that should be directed to growth.

"The impacts to feeding and water consumption are probably the biggest impact on both of those flies, as the cattle attempt to avoid these flies or again express behaviors to address the painful bites," Gerry said.

"There's some debate over whether these flies might increase mastitis in cattle for milking cows," he said, adding he hasn't heard of bites from horn or stable fly species resulting in infected wounds that require treatment with antibiotics.

"Having said that, it wouldn't surprise me to find that at some very low level that there are pathogens, especially fungal pathogens, that may be able to enter the body through the wounds from some of these biting flies," Gerry said, adding that it probably doesn't happen frequently enough to be recognized.

The face fly negatively affects cattle primarily through the transmission of cattle pinkeye. These flies will transfer the pathogen from one animal to another as they feed on eye and nasal secretions.

The disease can also be transferred from cow to cow by direct contact between the animals, and easily move through an entire herd, Gerry said.

"Face flies tend to be more common in warmer weather. We typically see increasing face flies in spring through fall, and then they actually overwinter," he said.

The stable fly, on the other hand, is not very tolerant of hot weather, so it causes relatively few problems in the summer months, except in areas in California, such as coastal regions, where weather stays cool all summer.

Problems with the stable fly begin about the middle of autumn, when temperatures start to cool down enough to allow populations to build. They will peak in late spring through early summer, when temperatures start to warm.

The horn fly is slightly less dependent upon environmental temperatures because the flies spend their entire lives on cattle and experience cattle body temperatures throughout their lifetimes, Gerry said.

On cool or cloudy days, horn flies prefer to sit on the back of the animal and bite along the spine. On hot or bright sunny days, the flies move to the belly of the animal to feed in the shade.

Treatments are available for all three flies.

"Horn flies are relatively easily treated with ear tags. These are insecticide-treated ear tags you would put on the ear of the animal," Gerry said, adding that horn flies have become resistant to some insecticides on ear tags, but there are still some that provide control.

Ear tags don't work for stable flies because they bite mostly on the legs, and ear tags won't move the insecticide down where they are biting, he said.

Depending on the management system, a liquid insecticide application directly to the animals works on stable flies; Gerry recommended focusing on the legs and lower belly.

The problem with liquid applications for beef cattle is it can be challenging to round up the animals and make an application when they are on rangeland or pasture.

Gerry said management practices may be a better solution to reduce stable fly numbers. Stable flies develop in areas where cattle are fed, and when animals are fed repeatedly in the same location and defecate and urinate, the urine and fecal material mixed with the hay make a perfect environment for the stable fly. When stable flies develop at a location, they will oftentimes attack the animals when they come to feed, he said.

Therefore, he said, the best way to control the stable fly is to avoid hay feeding at the same location, to prevent buildup of hay, urine and manure.

This management strategy doesn't work for horn or face flies, because they only lay their eggs in fresh animal feces.

For the horn and face flies, Gerry said, a better option is feed-through insecticides. These materials are fed to the animal, go through the gut and into the animal feces. When fly eggs are deposited, the insecticide kills the developing larvae.

There are also products available as licks or supplements that are mixed with the feed; the animals eat it and pass the insecticide into the fecal material to kill developing larvae.

Gerry said he isn't aware of any resistance to the feed-through insecticides yet.

(Kathy Coatney is a reporter in Bend, Ore. She may be contacted at kacoatney@gmail.com.)