Controlling Horn Flies on Pastured Cattle

Controlling Horn Flies on Pastured Cattle

Horn flies on a cow
Understanding the horn fly’s habits, life cycle, impact control methods and products will help design an effective control program. Photo credit Dave Boxler.

With temperatures starting to warm, fly season is not far away, and now is the time to evaluate your 2021 horn fly management plan. Was your fly management program successful last year? If the answer is no, what were possible factors that might have directed your program in the wrong direction. Understanding the horn fly’s habits, life cycle, impact control methods and products will help design an effective control program.  

The horn fly is considered one of the most important blood feeding pests of pastured cattle in the United States. When horn fly numbers are high, cattle experience annoyance and blood loss. This can result in decreased milk production, reduced weight gains, changes in grazing patterns and behavior. Research has demonstrated horn flies can reduce calf weaning weights from 4 to 15 percent, and stocker cattle and replacement heifers up to 18 percent. In addition, horn flies can spread summer mastitis. 

Horn flies are small, approximately 3/16” in length and normally observed on the topline, sides and poll area of cattle. During warm summer afternoons they will often be found on the belly region of cattle where it is cooler. Male and female horn flies take more than 30 blood meals per day. Females will deposit eggs in fresh cattle manure where larvae (immature form) will feed and develop into adult flies. The entire life cycle, from egg to adult can be completed in 10 to 20 days, depending upon the weather. In Nebraska, multiple generations occur during the fly season, which can lead to extremely high fly numbers (Fig. 1) in late August and early September. 

Cow with flies
Figure 1. The horn fly population of this cow exceeds the economic injury level (EIL), which has been shown to negatively impact animal performance. Photo credit Dave Boxler.

The economic injury level (EIL) for horn flies is 200 flies per animal (Fig. 2) and is often reached in late May or early June in Nebraska. Monitoring horn fly numbers on cattle is important in making appropriate management decisions. Cattle should be monitored weekly for horn flies throughout the fly season. These observations are best taken between the hours of 8:00 AM to 11:30 AM when flies are generally located on the shoulders, topline and sides of animals. Observations made later in the day can be less accurate because horn flies have moved to the belly area, making it more difficult to accurately assess population numbers.

Figure 2. The horn fly numbers on the side of this cow are 214. The Economic Injury Level (EIL) is 200 flies per animal. Photo credit Dave Boxler.

Control Methods 

Dust bags contain insecticide dust that filters through the bottom of the bag when cattle contact the bag while passing under it. The best horn fly control is achieved when cattle are forced to pass under the bag(s) on their way to water, feed or mineral. This is accomplished by fencing the water tank and suspending dust bags in the entrance-exit gate. Forced-use dust bags may not be practical if cattle can obtain water from ponds or streams. In this case, dust bags can be placed at locations where cattle loaf during the day to be used free choice. Dust bags in forced use arrangements provide 80-90 percent horn fly control. Studies have shown that horn fly control is 25-50 percent less using free-choice dust bags compared with forced-use dust bags. 

Backrubbers and oilers offer cattle the incentive to satisfy their scratching instinct and are most effective if properly located in pastures. These devices can be made from burlap wrapped chain or wire suspended between two posts and a large variety of backrubbers and oilers are commercially available. Backrubbers and oilers will need to be recharged periodically during the fly season and the insecticide should be diluted with an approved backrubber oil, listed on the insecticide label. Backrubbers and oilers like dust bags, work best in a forced-use arrangement.

Animal sprays come as ready-to-use or are diluted with water before applying. It is important to get complete coverage of each animal with the spray. Animals can be treated in small groups to assure complete coverage. Spray applications can be delivered using a low-pressure sprayer or a mist blower sprayer. Spray applications will provide from 7 to 14 days of control and will need to be re-applied throughout the fly season. 

Pour-ons are ready-to-use formulations applied in measured doses to animals based on body weight. Most function as contact insecticides, causing insect mortality when the chemical is absorbed into the insect by direct contact. However, pour-on products that contain macrocyclic lactones (endectocides) are systemic. Typically, pour-ons can provide 7 to 21 days of horn fly control, so they must be re-applied periodically. Application of endectocides in the late spring and throughout the fly season may impact other pests and beneficial insects such as dung beetles, that use dung as a source of food and for reproduction because endectocides are excreted in dung of treated cattle.

Feed-throughs and Insect Growth Regulators (IGRs) are insecticides that are incorporated into mineral blocks, tubs, or loose mineral. The insecticide is passed out in the manure and kills fly larvae that develop in the manure. For maximum effectiveness, these products should be made available to cattle early in the fly season before fly numbers become very high and require steady consumption. An additional complicating factor when using this type of fly control method is horn fly migration from neighboring herds that can mask the effectiveness of feed-throughs/IGR’s. 

Insecticide ear tags are also termed controlled-release, sustained release, or slow-release devices. One or more insecticides are formulated into a polymer matrix that is slowly released from the matrix as the animal moves or grooms and is deposited on the hair coat of cattle. In Nebraska, ear tags should be applied during the last week of May or the first week of June to achieve maximum control through the fly season. Ear tags applied too early may decline in efficacy while fly numbers are still high and result in economic loss. Follow label recommendations regarding the number of tags per animal. My studies show applying two tags per adult animal will provide the best reduction in horn fly numbers. Only tagging the calf and not the mother cow will not provide the desired level of horn fly control. A list of available insecticide ear tags for 2021 is found in Table 2 (located at the end of the article).

Compressed air application applies an individual capsule of insecticide to an animal using a device like a paint ball gun and can provide horn fly control between 21 and 35 days.

Traps physically captures horn flies. The Bruce fly trap (Figure 3) is a walk-through system where cattle enter through either end, pass through a 10-foot trap area, and contact a series of strips made of canvas or old carpet. These strips dislodge most of the horn flies on the animal’s topline and sides. Since flies are attracted to light the disturbed flies move to the top of the trap where they are captures. The animal exits the trap with fewer flies on it, and the trapped flies cannot escape. Repeated use of the trap can reduce the overall local horn fly population. Like dust bags and oilers, the Bruce trap is best employed in a force-use situation where cattle must pass through it on a regular basis. Field studies conducted in Missouri during the 1980s indicated the trap provided about 50 percent reduction in horn fly numbers. For more information on how to acquire plans to build a Bruce fly Trap, please contact me at or 308-696-6721.

Bruce Fly Trap
Figure 3. The Bruce fly trap.
Cut away view of the Bruce Fly Trap
A cutaway view of the horn fly trap shows the trapping elements are placed along its side. The frame is made from standard-size treated lumber, and the end framing is extended laterally to provide stability. The top is made from plywood. The canvas strips have been removed for clarity.

Resistance Concerns

Insecticides have been placed into numbered Insecticide Mode of Action groups (MOA) based on how they work against insects. Continual use of products from a single MOA group to control horn flies can lead to insecticide resistance and reduced control effectiveness to all products in the group. To improve efficacy, do not apply insecticides within the same MOA group repeatedly even during the same fly season.  A list of delivery methods, labeled insecticides, and their MOA for pasture fly control are found in Table 1.

Horn fly control methods
Insecticide tags available for 2021
Insecticide tags available for 2021