Horn Fly Control - Impact

Little fly. Big problem.


Know Your Enemy

Horn flies - also fittingly known as Haematobia irritans - are the most pervasive and most costly external parasites of cattle in North America. They're small, black flies, approximately four millimeters long. Adult females lay their eggs in fresh manure, where infestation levels can increase rapidly - up to 4,000 flies per animal in untreated herds. At their peak, horn flies remain on cattle throughout the day and night.


Horn flies pose a unique threat to beef and dairy cattle. Yet, producers often mistake the horn fly for other types of flies, such as the face fly. Unlike horn flies, adult face flies do not bite cattle. Instead, they feed on mucous and watery secretions from the eyes, nostrils and mouths of cattle. Horn flies, on the other hand, can generally be found on the backs of cattle, often clustering on the animals' midlines and spreading down their sides. When at rest, horn flies settle around the base of cattle horns.


Know the Signs

The adult horn fly is a biting insect that takes 20 to 30 blood meals a day. The resulting stress interrupts the grazing patterns of cattle, causing them to go off feed and expend energy in an attempt to dislodge the flies.


According to university research, calves from badly infested herds gain weight more slowly than normal calves and are lighter at weaning by 10 to 25 lbs. If severe infestations are left untreated, cows can go out of condition during the critical breeding period.


Today's treatment strategies and their major drawbacks.

Current control strategies for horn flies include mechanical, biological and chemical methods. Walk-through flytraps or dragging of pastures to disrupt manure pats can reduce the horn fly population in many environments. Predatory arthropods, such as beetles and mites, can also reduce horn fly populations in some habitats.


In recent years, chemical control has been the most widely used horn fly control strategy. Techniques for application to cattle include rubbing stations, liquid feed, dust bags, sprays, dips, ear tags, pour-on formulations, and oral larvicides in mineral granules, mineral blocks or controlled-release boluses.


Self-application devices, such as rubbing stations and dust bags, must be serviced frequently to ensure that insecticide is present and that the devices are working properly. Dusts and sprays remain effective only with periodic retreatment. The major disadvantage of ear tags is that resistant populations of horn flies developed shortly after the tags were introduced.


Ear tags became available in the late 1970s. By the mid-1980s, resistance was widespread throughout the U.S. and, by 1991, throughout Canada, as well. Resistance developed rapidly, due to widespread tagging using a single chemical family, the pyrethroids, against entire adult populations of horn flies. Continuous exposure to a single class of insecticides rapidly selects for flies with an innate tolerance to that insecticide. Eventually, this leads to the failure of the insecticide to provide satisfactory control. Because of the horn fly's cross-resistance to the active ingredients in traditional insecticides, such failure can occur in as little as a single year.


The economic impact is undeniable.

It is estimated that horn flies cost North American cattle producers over $1 billion each year*. Approximately $60 million is spent on control efforts alone. Financial losses can also be attributed to decreases in weight gains, feed efficiency and milk yields, caused by loss of blood and the excessive energy expenditure needed to dislodge flies. The implications are that an animal's total energy balance is altered when it is exposed to horn fly infestations and that this alteration results in decreased productivity.


The economic threshold for horn fly infestation is defined as "the number of horn flies per animal at which the value of the damage caused is equal to the cost of control." Based upon studies evaluating production losses, the generally accepted economic threshold for infestations of horn flies is 200 flies per animal. When adult horn fly counts reach this level, it is generally considered economically beneficial to begin a control program.