Fly control methods for spring grazing cattle

horn flies on livestock

An important component of planning for spring grazing is fly control. What products to use, steps to monitor fly populations and timing of treatments are elements contributing to the control of pests that cost the North American industry nearly a billion dollars every year.

Flies of economic importance on western Canadian range are horn flies (Haematobia irritans); face flies (Musca autumnalis); and stable flies (Stomoxys calcitrans). Warble or gad flies (Hypoderma bovis and Hypoderma lineatum), once a huge economic concern to the cattle industry, were practically eliminated with the advent of systemic avermectins and pour-on organophosphates.

Pesticide resistance has severely limited the range of effective fly control products so it is wise to consult with your veterinarian to learn what products are most effective in your geographical area, the different classes of control products that are available and the all-important issue of when they should be used.

Economic impact

The horn fly is considered one of the most important blood-feeding pests of pastured cattle. Between 200 and 250 flies per animal is sufficient to trigger economic losses. The irritation and blood loss they cause reduce milk production and change grazing patterns and reduce weight gains on calves by 20 to 27 pounds. Studies in both Canada and the U.S. report reduced weight gains in yearlings on grass as high as 18 per cent. Horn flies have been implicated in the spread of mastitis in brood cows and they are also important vectors for anaplasmosis and bovine leucosis.

Face flies feed periodically through the day. As the name implies, they persist around the face consuming secretions from eyes and nostrils. Data does not exist on the number of face flies needed per animal to cause economic loss. While annoying, heavy infestations do not seem to cause reduced weight gains in pastured animals. However, face flies often play a role in transmitting pink eye from one animal to another during outbreaks.

Fly behaviour

Understanding basic habits of the three main fly types helps formulate control options.

Horn fly

Horn flies are normally found on the backs, sides, and polls of cattle but shift to the belly during the heat of the day. They feed on blood with both male and female flies helping themselves to some 20 to 30 blood meals per day.

After mating, females deposit eggs in fresh manure. Eggs hatch within a week and mature in the soil beneath the manure pat. When adult flies emerge they are capable of flying several miles in search of a host.

Their entire life cycle is completed in 10 to 20 days depending on weather. Under ideal conditions, horn flies produce as many as 35 generations in a single summer.

Monitoring fly numbers on the cattle is a tedious but important part of any control plan. They should be monitored weekly in fly season and treatment in the form of back rubbers, dust bags, insecticidal ear tags, pour-ons, oral larvicides, and sprays brought out when numbers reach 200 flies per animal.

Insecticide ear tags were once the hallmark of horn fly control until resistance ended their usefulness in many areas. Rotating insecticide classes every year with ear tags and seasonally for other application methods has extended the usefulness of these products. Two tags per animal may be required for maximum benefit.

Oral larvicides find their way into the manure to prevent fly larvae from developing into adults. To be effective they need to be consumed daily to keep up with local fly populations as well as the migrating flies from untreated herds across the fence.

Recent U.S. research has shown patch burning of pastures is tremendously effective at reducing horn fly populations. Where it is feasible, patch burning could be considered as a part of an overall integrated pest management strategy.

Face fly

Face flies are non-biting and feed on animal secretions, nectar and dung liquids. Adult female flies typically cluster around an animal’s eyes, mouth and muzzle, causing extreme annoyance. When they find wounds on an animal’s face they will also become blood feeders. Face flies are present throughout the summer with populations peaking in late July and August and are most numerous along waterways, and irrigated pastures.

Face fly control can be difficult because of where flies feed and the significant time they spend away from their hosts. The best method of reducing face fly numbers is using a treatment that forces animals to come in daily contact with an insecticide through dust bags, oilers, sprays or insecticide-impregnated ear tags. Both cows and calves must be treated for effective control.

Stable flies

In some areas, stable flies are significant pasture pests. They are blood feeders found mainly on the front legs of cattle. Their bite is painful and the cattle react by stomping their legs, bunching up in the corners of pastures, or standing in water to avoid these pests.

Female stable flies deposit eggs in spoiled or fermenting organic matter mixed with animal manure, soil and moisture. Winter hay-feeding sites using hay rings are prime sites for larval development throughout the summer when enough moisture is present. The most common breeding sites are in feedlots or dairy lots, usually around feed bunks, along the edges of feeding aprons, under fences and along stacks of hay, alfalfa and straw.

The economic threshold of five flies per leg is easily exceeded on many pastures.

Adult stable fly control on pastured cattle can be difficult based on the amount of time the fly is away from its host and the fact that stable flies can roam at least 10 miles or more. Cleaning up wasted feed at winter feeding sites is one positive step to reducing fly populations.

Bottom line: we’ll never eliminate flies or fly problems, but if you plan ahead you can reduce the negative impact they have on your cattle during the pasture season.

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).

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