Fly control is one of the summer chores we all dread. It is frustrating, time-consuming and dips into our profits. When dealing with flies on cattle, there are essentially two species involved: horn flies located on the backs of cattle and face flies.
The horn fly is a biting fly that lays larvae or eggs that hatch in manure piles in the pasture. Horn flies are tolerated in low numbers, but studies show the economic threshold for treatment is 200 flies per animal. Horn flies can develop resistance to pyrethrins, one of the two major classes of drugs in common pesticides. The horn fly primarily causes harm by irritation and blood loss. It is a common vector for anaplasmosis and bovine leukosis.
The face fly spends only 10% of its life on the cow, causing irritation and the spread of pinkeye. It shows no resistance to pyrethrins but is difficult to kill due to the limited amount of time it spends on the cow.
There is no known resistance to the other common pesticide, organophosphate, among either fly.
Control measures. Here are a few common ways to control these pests with the use of chemicals:
- Fly tags generally use pyrethrin or organophosphate as the primary drug. Once in place, tags are virtually labor-free. To avoid creating resistance, make sure not to place tags too early, remove tags at the end of the season and rotate between chemical classes each year. There is a great difference among fly tags depending on the class of the active drug. Be sure to change chemical classes, not just brands. Look at the active ingredient on the tag package to determine which class of drugs you are using.
- Pour-on products last two to four weeks. The disadvantage is that they need to be reapplied often, but they tend to give good control at peak fly burden. Similar to tags, there are many classes of drugs used, so look at the active ingredients listed on the label.
- Growth inhibitors are undemanding on labor. The biggest drawback is that they only work on the flies that lay larvae in manure, like the horn fly.
- One final method of fly control I have found useful in confinement facilities is the use of predator wasps. These very small insects can be released periodically and help to control nearly all species of flies in a limited area.
Stop the cycle. One of the big issues with fly control is that horn flies develop resistance to some of the products we use. Pyrethroids are a class of drug that should therefore be carefully monitored.
There are some things that can be done to help alleviate this problem. The horn fly lays eggs in manure, where they pupate in five days; then the larvae go through a life cycle that lasts about seven more days and emerge as adult flies. As fall approaches, if we can eliminate the pyrethroid-resistant flies that have developed during the summer, we can stop the next generation of resistant larvae from hatching in the coming spring.
Prevention of pyrethroid resistance involves avoiding long-term, low-level exposure to the active
insecticide and rotation of chemical classes of external parasite products. Ear tags need to be removed with fall cattle work because they lose efficiency in time. After a summer of weather and wear, the tags have sublethal drug levels and allow resistance to occur at a rapid pace. Applying a pour-on with a different class of drug than what is used in the tag will help to remove the flies that have developed resistance throughout the summer.
DAN GOEHL, DVM, and his wife own Canton Veterinary Clinic in Canton, Mo., working with stocker and cow–calf beef operations. He is also a partner in the management and marketing of beef cattle. You can e-mail him at BeefToday@FarmJournal.com.