Keep Flies from Bugging Your Herd

Published on: 11:02AM Jul 12, 2010

By Dan Goehl, DVM

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Fly control is one of the summer chores we all dread. Not only is it frustrating and time-consuming, but it also can easily dip into our profits. When dealing with flies on cattle, there are essentially two species involved: the horn fly that is often seen on the backs of cattle and the face fly that, as its name implies, is seen on the animal's face. There are a couple of things that need to be understood before trying to control either fly.  

The horn fly is a biting fly that spends a large majority of its life on the backs of cattle. It lays larvae or eggs that hatch in manure piles in the pasture. Horn flies are tolerated in low numbers with little significant effect on economics, but studies show that the economic threshold for treating is 200 flies per animal. Horn flies are known to develop resistance to pyrethrins, one of two major classes of drugs that make up most common pesticides. There is no known resistance to the other common pesticide, organophosphates, among either fly. The horn fly primarily causes harm by irritation and blood loss. It is also a common vector for ananplasmosis and has been incriminated in other diseases such as Bovine Leukosis virus.  

The face fly spends only 10% of its life on the cow. Its main cause of economic loss is irritation and being a vector in the spread of pinkeye. It shows no resistance to pyrethrins but is somewhat difficult to kill due to the limited amount of time spent on the cow.

Let’s address a few of the common ways to help control these pests with the use of chemicals. Which one you implement will vary depending on your individual operation. The three I have chosen are fly tags, pour-on pesticides and growth inhibitors in mineral. If you have another you prefer or are interested in, contact your local health professional.

Fly tags generally use pyrethrin or organophosphate as the primary drug. Fly tags can be very beneficial to your operation and have the advantage of being labo- free once they have been put in place. We want to be sure to avoid creating resistance with fly tags and two methods of achieving this are timing of placement/removal and rotating products. The timing mistakes most commonly encountered with fly tags are putting them in too early and not removing them in the fall. As the tag becomes less effective, the fly population starts to become resistant. Products should be rotated between chemical classes of pyrethrin and organophosphate annually. There is a great difference among fly tags depending on the class of the active drug.  

Pour-on products tend to last from two ro four weeks. The disadvantage is the need to reapply often and the labor involved, but they tend to be very good products to use at peak fly burden. Similar to tags, there are many classes of drugs used. 

Growth inhibitors are undemanding on labor. The biggest drawback is that they only work on those flies that lay larvae in the manure, such as the horn fly.

One of the big issues with fly control is that horn flies develop resistance to some of the products that we use. Pyrethroids are a class of drug that should 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 larva go through a life cycle that lasts approximately seven more days and emerge as adult flies. As fall approaches, if we can eliminate the pyrethroid-resistant flies that have developed over the summer, we 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 tags lose efficiency over 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 containing 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.

One final method of fly control I have found useful in confinement 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.

It is important when implementing control next spring to have a thorough understanding of what drug and class of drug was used the prior season. Be sure to use a different class of drug, not just a different brand or color of tag. Many companies produce various fly tags, and just because the tag appears different it will not help prevent resistance unless you rotate chemical classes each year. Look at the fly tag box and view the active ingredient to determine what class of drugs you are using. 

By rotating chemicals, increasing to a new generation of chemical, removing tags properly and applying chemicals at appropriate times you can help assure that your cows will be free of the burden of flies for summers to come.  

Dan Goehl, DVM, and his wife own and operate Canton Veterinary Clinic in Canton, Mo., where Dan works primarily with stocker and cow-calf beef operations. Dan is also partner in Professional Beef Services, LLC, which offers herd consultation and helps in data management and marketing of beef cattle.

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