Although prevention of these parasites is routine in many operations, success can be varied due to timing and frequency of intervention. Most research suggests horn fly prevention is not necessary until cows have more than 200 flies per cow, while stable flies can start to cause production losses at five flies per leg.
In most instances in the Midwest, these counts are not reached until midsummer or later.
Fly tags are highly effective when used during fly season. However, many fly tags have a payout of fewer than 45 days. Thus, if fly tags are applied at the beginning of the grazing season, they are often “wore out” before peak fly season occurs.
Similarly, products such as pour-ons also have limitations on how long they are effective. If you typically do not handle cattle after they go to pasture, rubbers and dust bags may be your best means of fly and tick control. However, be sure to read the label of all products used, as many pour-ons do not kill ticks – rather, the product claims are to aid in the control of these parasites.
Although about a third of the reduced production losses are due to flies feeding on blood from the host, the bulk of body condition and milk production losses are the result of cattle congregating to combat flies instead of grazing. Because both horn and stable flies feed on the blood of their host, it is possible blood-borne diseases such as anaplasmosis could be transmitted when these flies are not controlled.
Even though they are not biting insects, face flies can contribute to decreased production. These parasites feed on secretions from tear mucus and saliva – and thus, pinkeye is typically the largest risk associated with face flies.
In addition to use of fly tags, dust bags and pour-on products to control fly population, clipping pastures can reduce eye irritation from pollen or seeds. Although several pinkeye vaccines are available, research has shown efficacy is limited.
Evaluate your fly control program regularly during the fly season and adjust as necessary to maintain good control of fly populations. Ultimately, be sure to read labels and use products that control parasites specific to your operation. 
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Erika Lundy
- Extension Beef Program Specialist
- Iowa Beef Center - Iowa State University
- Email Erika Lundy
