As few as 40 horn flies are considered an economic threshold for treatment, but untreated numbers can range into the thousands.

One factor that can drastically reduce weight gain for grazing cattle is irritation and feeding caused by horn and face flies. It has been shown that with protection from horn flies, yearling steers gained at least five Kilograms more weight than their counterparts. There was also a 14 percent increase in weaning weight on calves because of an increase in milk production when their dams were treated. Protecting cattle against flies is another economic procedure of total herd management.

Horn flies are the most abundant and are very easy to see feeding with head facing down over the back and shoulders of older livestock. I would advise all producers to pay close attention to their breeding bulls as 3000 to 4000 flies can be seen feeding if untreated. These flies only leave the animal long enough to lay their eggs and you will see them swarm if disturbed. They are also the ones to watch if implementing a fly control program since if effective a 95 percent-plus level of control should be achieved. The economic threshold is considered to be 40 horn flies per beef cow and 20 per dairy cow. Any higher than these levels warrant initiating a fly control program or changing the existing one, as it is not effective. These flies consume five times their weight in blood each feeding and feed up to 38 times daily so damage is considerable. More than 50 face flies are considered significant and are linked to the spread of pinkeye. They are harder to see as only a few feed at one time on eye and nasal secretions. Fortunately most treatments work on both species.

With a short life cycle (14 days) about five generations can be produced in our northern grazing season, so continuous long-term control is a must. Back rubbers and dust bags will provide this control as long as they are strategically placed in walk alleys, by watering areas, or in conjunction with mineral feeders. This greatly enhances usage but don’t forget to move them when rotating pastures. These oilers also do a good job on lice and mosquito control.

Keep a close eye on the family of insecticide used for fly treatments.

Organophosphates and pyrethroids are the main ones used. Pests have been shown to show some resistances to pyrethroids, so products should be rotated if control is becoming ineffective. Bison producers can use these same insecticides and methods in their bison operations. Bison, because of their thick hair coat, are not bothered as much until shedding occurs in the summer.

The insecticidal ear tags revolutionized fly control when they came out in the early 1980’s. They were formulated to release the particular chemical over three months by the bending action of the tag. The insecticide used had a very low mammalian toxicity so are safe, and since applied only externally no slaughter withdrawal is necessary. Well over 10 to 20 varieties have been marketed and again rotation of tags (products) is highly advised between the pyrethroid and organophosphate groups. Insecticide strips if still available can be placed on existing ear tags if producers want to minimize ear damage.

To monitor resistance, the tag methods should give 100 percent control for the first few weeks after application. Compare the weeks of protection, to last years by monitoring your bulls. This will tell you if resistance is developing. The biggest mistake producers make is applying the tags too early in the season. If applied, for instance, in mid April by the time the cattle hit the real fly season in June the tag is half exhausted with no benefit yet achieved. For maximum benefit, the tags should be placed at the latest possible time, ideally just as cattle head out to pasture. One tag per cow calf unit is recommended and I believe it is better placed on the cow, as the flies are attracted to mature stock. In the past a number of producers also used the insecticide tag as the calf’s I. D. These, if put in in February, were all exhausted by fly season and also ink on the tag interferes with release of pesticide. This practice is NOT recommended.

New products on the market include misters which apply a fine spray over groups of cattle. These are often used if cattle can easily be gathered in a calm time and the spray is air released to descend on the cattle. Please note if cattle are treated with a pour on endectocide such as IVOMEC pour-on (MERIAL) the product has labeled control of horn flies for five weeks. So if cattle are treated for lice and worms at turnout, the fly control for at least part of the season becomes an added benefit. Another new product CYLENCE (BAYER) is a pour on specifically for horn flies as well as biting and sucking lice. It has an added benefit of no milk withdrawal (one day slaughter withdrawal) so can be used in dairy cattle as well. Its effectiveness appears to last 45-60 days and cost is proportional to the tags.

Biological control has gained popularity and the Alberta Government has developed a parasitic wasp, which can over winter. In the past parasitic wasps were quite effective by laying their eggs in the fly pupa. Wasps target all species of flies, including house, stable and horse flies. Colonies of wasps (recommended 2-3 colonies per 1000 head in a feedlot) are released just before peak fly season. And earlier problem with biological control was that the next year you had to start the whole process over; now with over-wintering colonies populations can simply be bolstered by new additions. This method is extremely effective in populated cattle areas such as feedlots, feed yards, auction markets and other cattle assembly sites. Often fly bait traps, which remove adult flies, are used in conjunction.

In today’s high-production cattle business, fly control is absolutely necessary. Several very cost effective methods are available to choose from. If used higher weight gains and happier cattle are the result.

Roy Lewis is a practising Large Animal Veterinarian at the Westlock Veterinary Center, north of Edmonton, AB. His main interests are bovine reproduction and herd health.

About the author


Roy Lewis is an Alberta-based veterinarian specializing in large-animal practice. He is also a part-time technical services vet for Merck Animal Health.



Stories from our other publications