Fight horn flies the systemic way
By Dr. Jim White

With a feed-through approach to horn fly control, you can keep the parasites off your cows year round

Willis Bruce spent his career studying horn flies. Now, you might think, “Hey, you mean Bruce Willis. And he’s an actor, not an entomologist.” No, indeed, I refer to the USDA entomologist Willis N. Bruce who designed an ingenious horn fly trap, and spent his career working on horn fly control.

Dr. Bruce was particularly interested in horn flies because horn flies are one of the most widespread and costly external parasites of cattle in North America. Adult flies bite the animal, causing irritation and often drawing blood. As calves spend less time grazing, weight gains will be reduced by an average of 20 pounds per year, but losses may go much higher. The parasite has a range of effects on cattle herds:

• cows produce less milk and may fall out of breeding condition;
• horn flies have been implicated in the spread of summer mastitis, which tends to be highest in fly season, but responds to effective fly-control measures;
• some eye problems are linked to horn flies.

Current estimates are that horn flies cost the American cattle industry a couple million dollars everyday. Under the current herd owner’s economic realities, any money saved is money earned, or at least less of a loss. So what can you do to prevent economic damage from this pest? Pay attention to prevention.


Recognize the pest
Horn flies are small, about half the length of a house fly. Adults are about 3⁄16 inch in length. They are dark gray bloodsuckers that stay on cattle almost continuously. The picture to the left shows the little monsters on the side of an animal. Both males and females are blood feeders. They spend most of their time on the shoulders and backs of cattle. During extremely hot weather or when it rains, the flies may move to the more protected underside of the animal. When disturbed, horn flies will fly up in a swarm, but they will return to animals almost immediately; they are not strong fliers. Females leave occasionally to lay their eggs in fresh manure piles. If horn flies are not on manure, they are on the animal, as shown in the life cycle illustration below.

The close association between horn flies and host helps with control. The flies leave animals only to lay eggs or to change hosts. Thus, many methods will expose flies to control practices—ear tags, topical products, feed additives, etc. With some chemistry (read pyrethroid), insect resistance can be an issue. The classic sign of resistance is if fly tags are applied, but after 2 weeks, there has been no decline in fly population. Resistance is not a problem when using IGR, or insect growth regulator (chemical name S-methoprene). And there is no known resistance to IGR-methoprene-Altosid. That compound has been very effective in controlling biting adult insects that have specific larval environmental requirements such as horn flies and mosquitoes.

The appropriate application of an IGR feed-through product for horn fly control is long-term and preventive, not reactive. IGR is ingested as part of the animal’s feed. The IGR is then excreted in manure. Given that horn flies must lay their eggs in fresh manure, the IGR is there to keep the horn fly pupae from developing into breeding, biting adult horn flies.

MFA Fescue Equalizer is a convenient, ready-to-use mineral formulated to balance range or pasture feeding programs for beef cattle. The inclusion of IGR and CTC allows for both horn fly control and medicated feed claims.


When to feed
Begin use in the spring before horn flies appear on cattle. The flies emerge in the spring when 14-day mean temperature is 65 degrees or greater. Continue feeding through the warmer months until cold weather stops horn fly activity. The success of the fly control program next year depends on finishing completely through the fall season. So try continuing Fescue Equalizer Altosid for thirty days after a killing frost. This will ensure that the adult population is minimized and egg populations are as low as possible. If IGR feeding is ended too early in the season, there is a late-season increase in the adult population to lay eggs. The eggs overwinter and are ready to establish fly populations next spring. That can be avoided by extending the feeding period for a month after the frost. The payback is better fly control early next season.

Follow the Guidelines Horn fly control with IGR feed-through is the easiest, most effective and economical way of controlling horn flies for cattle producers. In order to achieve optimum fly control, Fescue Equalizer Altosid should be used in conjunction with other good management and sanitation practices. There are a number of guidelines available for integrated pest management. The work at www.ipmguidelines.org, a guide from Cornell University is a good example. The Insecticide Resistance Action Committee and the Regional Research Committee for livestock Pest Management provide these recommendations: 1) Treat for horn flies when fly numbers exceed 200 per animal. 2) Fly control on growing and lactating animals is more likely to provide an economic return; separate animals if possible. 3) Use periodic treatments with insecticides that have other modes of action to break fly exposure to a single product group. 4) Remove ear tags in fall to reduce horn fly exposure to low concentrations of pyrethroids. 5) Use a late season application to reduce the number of horn flies that will enter the over-wintering stage.

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