Flies cost the cattle industry more than $500 million annually, causing decreased weight gains, decreased milk production and transmission of diseases such as pinkeye and anaplasmosis.

Flies that attack beef cattle are classified as biting or non-biting. Non-biting flies include face flies and house flies. Horn flies, stable flies, horse flies and many species of gnats are classified as biting flies. Non-biting flies feed on liquid substances around the eyes, nose and mouth and blood from wounds. They cause eye inflammation and transmit eye disease organisms. Biting flies feed by piercing the animal's skin and sucking blood. Two of the more common flies affecting cattle are face and horn flies.

Face flies feed on secretions around the eyes, nose and mouth; just as their name implies. These flies carry and spread bacteria that cause pinkeye. Reportedly, there is a reduced average weight gain of 17 pounds in calves with one eye affected and 30 pounds to 65 pounds when both eyes are infected. Face flies spend little time on the animal, making control of face flies less effective because the pest is exposed to only a small amount of insecticide.

Horn flies have the greatest economic impact of any fly on cattle. The horn fly feeds about 20 times to 30 times a day, primarily on the back and shoulders of cattle. The horn fly feeds by inserting its mouthpart into the host's skin and sucking blood. A horn fly spends its entire life cycle on cattle, only leaving to lay eggs in fresh manure.

There are a variety of products on the market to control flies—insecticide ear tags, liquids to pour-on, sprays, back rubbers, dust bags and oral larvacides. Effective fly control may require use of a combination of products.

Insecticide ear tags can be highly effective in controlling horn flies and are as effective as any other method to control face flies. Two categories of fly tags are available: pyrethroids and organophosphates. Horn flies can become resistant to pyrethroids, resulting in variable control. Face flies are not resistant to pyrethroid ear tags. Organophosphate tags give good control of horn flies with no evidence of resistance, but poor control of face flies.

Many producers use fly tags for fly control and place them in the ear early in the season. Although this may be the most convenient, it may not be the best timing. Fly tags are designed to provide control for a specific length of time and most fly problems occur later in the grazing season. Timing of placing fly tags is critical as the strength of the insecticide becomes less effective as time passes. The effectiveness of fly tags placed in the cow’s ear a month or two ago has diminished before the current peak fly season. The recommendation is to rotate the type of fly tags (pyrethroids or organophosphates) used each year. At the end of the fly season or as the fly tag begins to fail, removing the fly tag is important to prevent resistance to the insecticide from constant exposure of sub-lethal doses.

Pour-on liquids and sprays can be very effective providing an immediate response in control. These products are short in duration—typically lasting less than 30 days—and require reapplication throughout the fly season. Back rubbers, fly strips and dust bags can be effective if they are strategically located for animal use, for example, near mineral feeders and watering areas. Oral larvacides work by inhibiting larval development to break the fly's life cycle. Use of oral larvacides should be started in the spring. However, if your neighbor is not using a similar control method, flies can be expected to cross the fence and cause problems.

The economic threshold for fly control is when fly populations are greater than 200 flies per animal. Any fly control program that keeps the fly population below the 200 flies per animal level provides the same economic performance regardless of the level of control.

For more information, contact David Hoffman, University of Missouri Extension livestock specialist, at 816-380-8460 or hoffmand@missouri.edu or your local MU Extension Center at extension.missouri.edu.

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