Dr. William Shulaw, Extension Veterinarian, Ohio State University
Did you realize that cattle raisers during the Civil War didn't have to worry about the horn fly, and my grandfather didn't have to worry about the face fly? Both pests were introduced (about 1860 for the horn fly and the 1950s for the face fly) to the US. Today, flies are an economically significant problem for cattle farmers.
The horn fly only lays its eggs in fresh manure, and it is generally recommended not to begin control efforts using tags and sprays until the fly numbers reach an economic threshold - about 100 to 200 flies per animal. Treatment with these methods too early isn't cost effective, and since you never really get all of them (or your cattle are parasitized by your neighbor's flies), early treatment doesn't usually prevent a buildup.
If you use ear tags for horn fly control, treatment too early may actually be counter productive. Many of these tags have an effective lifespan of five months. If they are put in during the early spring, the drug may be nearly gone by the time fly numbers peak (late summer). If the tags remain in the ear after their effective period of use is over, the pesticide (drug) may be below effective killing concentration at that time, and the flies can quickly develop resistance to it. Resistance was observed in a relatively short period of time after ear tags for horn fly control were first introduced. Therefore, it is recommended not to put fly tags in place until the economic threshold of horn flies is reached and to take them out when the manufacturer indicates the tag should be removed.
Unfortunately, spring turnout is often a convenient time for producers to apply ear tags. In fact, it is the observation of many veterinarians and parasitologists that fly tags are usually put in and removed when it is convenient, rather than at the time when they will do the most good. Taking them out when the cattle are brought off pasture or when they are worked in October or November (beef cattle) is a common practice and almost surely has contributed to the resistance problem.
Feed additive larvacides or growth regulators can be used to control horn fly populations. These chemicals work by preventing the development of flies in the manure. They are administered in feed, loose mineral, or block form, and all animals in the group must consume the recommended dosage for effective control. Because horn flies can move from herd-to-herd over a distance of several miles, oral larvacides must be used rather extensively across an area or region in order to be effective. They are not effective in controlling other fly populations that breed in sites other than manure.
Certain systemic dewormers, now commonly used, also provide a significant measure of horn fly control and may be useful in an integrated approach to control of horn flies. However, some controversy exists surrounding some of these products and other products that provide control of fly larvae in the manure. Research has indicated that populations of some beneficial insects that use the manure for some part of their life cycle, such as the dung beetle, may be harmed or reduced. Dung beetles reportedly can aid in the control of horn flies by removing and burying manure before the life cycle of the horn fly is completed.
Field observations and field research indicate that sustainable horn fly control may be best attained by an integrated approach that involves within-season rotation of chemical classes and treatment approaches. Reliance on just one chemical class or one strategy usually results in poor control and may select for resistant horn flies.