Living things and even systems of living things are made to cope with stress. As a matter of fact, a moderate to low level of stress may actually be required to maintain health. For example, the stresses exerted on bones by walking and running cause more calcium to be deposited in bones and increases bone density. In contrast, bed-ridden individuals’ bones lose calcium and over time, bone density decreases. However, when stress exceeds these moderate to low levels, it becomes harmful.

Affects of Stress
So, what does stress have to do with horn fly control? Horn flies are just one of the stresses pastured cattle bear. At very low horn fly numbers, cattle can cope with the stress without significant loss of production. However, some experts estimate as few as 25 horn flies on one side of a cow can result in economic losses. Although blood loss is not significant enough to reduce the animal’s blood volume at this low number of flies, production losses can occur.

The major cause of production losses at these levels (25 flies per side) appears to be irritation to the animal, causing the animal to “fight” flies. Consequently, cattle spend less time grazing, resting, and ruminating. So, a major determinant of extent of production losses an individual animal will experience at a given population level is the tolerance of the animal. This is analogous to the person who can’t stand the loud ticking of a clock while another person in the same room doesn’t realize a clock is present.

Another factor affecting the ability of cattle to tolerate horn flies is the experience of the cattle. For example, it appears that cattle in the southern U.S., where the horn fly season is longer, are better able to ignore low numbers of horn flies. These cattle don’t “fight” the flies until numbers reach a higher level.

Cattle are best able to handle the stress of horn flies when other health needs are met. One of the biggest compensating factors is the quality of pasture forage. Pastures with high densities of very digestible forages require less effort from an animal to obtain the nutrients required for maintenance, growth, and/or production. Any extra energy can be used to cope with flies.

It is more difficult for cattle to obtain the required nutrients on pasture when forage quality is poor. Drought periods also impact forage nutrient availability. The buffer of extra energy isn’t available and production losses will occur at lower numbers of flies per animal. This same effect is true for parasite burdens, diseases, or other factors that rob the energy supply available to the animal for maintenance, growth, and/or production.

Economic Threshold
Another concept important to producers when considering horn fly control is the action threshold or economic threshold. This is usually defined as the number of flies on one side of a cow which create production losses sufficient to justify treatment. This number is determined by studies which attempt to control all factors affecting weight gain except for varying the number of horn flies per animal.

In this process, what is hoped for is a nice, clear relationship between the number of flies on an animal and reduced weight gains when compared with a group of animals kept under the same conditions that have few to no horn flies bothering them. After this relationship is established, scientists look for the point where production loss (measured in dollars) is equal to the cost of horn fly control.

Taking Counts
It is less disturbing to cattle and easier for producers to count flies on one side of an animal. When counting horn flies, remember to adjust the counts on windy, sunny, and cool days. On very windy days most of the flies will be on the downwind side. On sunny days, the flies will be on the shady side, and on cool days they will be on the sunny side, withers, or back.

To arrive at a practical estimate, count the number of flies on at least 10 animals, 1/2 from the upwind or sunny side and 1/2 from the downwind or cool side. Then, use an average of the counts. Bulls tend to have heavier fly burdens, so only count the number of flies on cows to get a more realistic picture of the herd’s average burden.

The action threshold producers should use will vary. For average production situations, approximately 50 to 75 horn flies per side will be an appropriate threshold. When forage is poor, the overall condition of the animals is poor, or several factors are stressing the cattle, reduce the action level toward 25 flies per side. On the other hand, if forage is excellent, the weather is mild, and the animals are in excellent condition, increase the action threshold toward 100 flies per side.

Counting the Cost
Remember, the other element of the action threshold is cost of control. These costs partially offset the benefit of improved production. For more expensive control measures, increase the action threshold toward 100 flies per side and reduce it for less expensive measures. Costs to consider are the actual cost of the control materials, the cost of labor to apply it, and the cost in lost energy due to any stress the animal might experience from the application.

Stressful Control Methods
Some control methods are inherently more stressful and therefore, extract a higher toll from the animal’s energy reserves. An example of a stressful control measure is spraying or applying pour-ons every four to six weeks. A less stressful method would be applying ear tags once per season at the same time cattle are worked for other reasons.

Ear tags pose a drawback for horn fly control due to development of resistance to the pyrethroid and organophosphate insecticides used in ear tags. Resistance management practices, such as rotation with other methods of control and removing the tags after the effective time period listed on the label, can increase the useful life of this control technique.

Passive Control Methods
Least stressful to cattle are the “passive” control measures. Passive control measures include backrubbers, dust bags, and insect growth regulators (IGRs) or conventional insecticides added to minerals. The passive control measures are essentially non-stressful because they are applied by placing them in a situation where the cattle are forced to use them in the course of getting to water or mineral sources. An additional benefit of mineral supplements with an insect growth regulator is the provision of nutrition and horn fly control in a single product.

Conclusion
Controlling horn fly populations can ease the stress encountered by grazing beef cattle. Utilizing a passive control/prevention measure, like an insect growth regulator, is advantageous because it does not create additional stress, and lower stress can help create positive production responses.