As I’m sure many of you have noticed, it’s shaping up to be another bad fly year.  Horn flies are common on beef cattle here in Montana.  Annual control costs and cattle production losses can exceed $780 million annually in the U.S.

Horn flies spend the majority of their time on animals and will move from the back to the sides of the belly during the heat of the day. Female horn flies deposit their eggs in fresh manure and can deposit up to 500 eggs during her life. Horn fly larvae hatch and develop in manure. Several generations of flies occur during the summer, with adult horn fly populations typically peaking during late summer. When temperatures decline in the fall, horn fly pupae hibernate in the soil.

Horn flies prefer larger animals (cows, steers, heifers, and bulls), and tend not to bother calves until late summer. One horn fly can bite 20 to 30 times per day, which can result in thousands of bites per day in large populations. Cattle that are infested with over 200 flies will begin to bunch together in an attempt to escape the flies. Large populations of horn flies cause changes in grazing behavior and can reduce feed intake, which ultimately decreases production. Production losses include reduced feed intake, reduced feed efficiency, decreased milk production, and decreased weight gain.

Multiple methods are available for controlling horn fly populations. Insecticides are the primary method for horn fly control as other methods may be ineffective. Methods available for cattle on pasture include dust bags, oilers, ear tags, sprays, pour-ons, boluses, and feed additives. 

Dust bags, back rubbers, and oilers are most effective when cattle must pass under them on their way to water, feed, or mineral. Dust bags can also be placed in loafing areas where they can be used free-choice. Ear tags contain insecticides that allow for small amounts to be released over time by traveling through the hair coat when the animal is moving or grooming. When using animal sprays to control horn flies, complete coverage and penetration to the skin is essential.  Sprays are easily applied but require multiple treatments throughout the summer because the control of horn flies only lasts about a month. Pour-ons are also easily used and provide effective treatment for several weeks when used properly. However, pour-on control may vary with weather and other factors. Feed additives and boluses may be incorporated in mineral blocks, loose mineral or tubs. The insecticides included as a feed additive pass through to the manure and kills the fly larvae. Feed additives are most effective when consumed in consistent and sufficient amounts all season long.

Typically, it’s recommended to provide fly control after reaching the economic threshold of 200 horn flies per animal. Ear tags and oilers or backrubbers seem to be the most common methods to reduce fly numbers. One thing to keep in mind is that ear tags with the same active ingredient should not be used more than one year in a row. Switching active ingredients reduces the likelihood of horn flies developing resistance to the insecticide being used. Additionally, waiting until after June 1 to provide fly control, especially in the form of ear tags, can help prolong their effectiveness throughout the summer. Applying fly tags prior to June 1 may limit how late in the season they are effective against flies, always read the label to determine the length of use for the product.

There are also several products available that utilize garlic as a method for fly control due to the sulfur-based compounds in the garlic. However, there has been little research to support garlic as a potential fly control. One study out of Canada was conducted and the results were mixed with garlic working one year and not the second (Durunna and Lardner, 2021). It is also important to not that garlic may repel flies, but it does not kill the flies or stop the larva in the manure from maturing.

Another consideration when determining fly control methods is to consider the cows next door. Fly control is difficult to maintain throughout the summer and can be even more difficult if animals have contact with other livestock where flies are not being managed.

There are several methods to consider when thinking about fly control. It’s best to determine which method is most economical for the ranch and the most effective for the cows.