Warmer weather is a sure sign that flies – including the irritating horn fly – are on the way. Don't let them steal valuable weight gains this season.
There are several different species of flies that aggravate cattle during the warm summer months but, hands down, the horn fly is the most costly ectoparasite of all.
The economic impact of losses it causes in North America's beef production systems approaches $1 billion a year. In Florida alone, losses from this fly – which is about half the size of a house fly – are estimated to annually total $36 million.
Horn flies are blood suckers, so when they feed on cattle they bite – some 20 to 40 times per day. Cattle without good horn fly control will expend considerable energy fighting these parasites. Often they'll bunch, stand in water, or seek shade to find relief and when they do, they fail to graze normally.
Studies in the U.S. and Canada show that during the grazing season, yearling cattle free from horn flies gain from 15 to 50 -pounds more than heavily infested animals. Nebraska studies and others show a 10- to 15-pounds. advantage in calf weaning weights where cows have good fly control.
An effective horn fly control program keeps this pest to less than 100 flies per side of animal – the number at which the value of the damage caused is equal to the cost of control.
Control is necessary
Without some type of fly control program, University of Kentucky Extension entomologist Lee Townsend points out that every animal in a herd may have several hundred horn flies by mid- to late summer, with bulls usually carrying the heaviest infestations.
Control strategies for horn flies include mechanical and biological methods, but chemical ones are the most widely used. These include insecticide impregnated ear tags, dust bags, concentrated pour-ons, animal sprays, and oral larvacides available in minerals and feed supplements.
Horn flies are found on the shoulders, backs, and sides of cattle, and only leave to lay eggs in fresh manure. This close association with the host animal makes them easier targets, than other flies, to control.
But through insecticide ear tag use and the continued use of same mode-of-action insecticides, they have built up resistance to some classes of chemicals, including the early pyrethroids. To help avoid horn fly insecticide resistance, Townsend offers these management practice tips:
* rotate insecticides in ear tags with different modes of action;
* withhold tagging until horn fly numbers reach 100 per side of animal;
* only treat cattle in the growth mode;
* use alternative insecticides and application methods late in the season to reduce the percentage of overwintering flies with resistance; and
* remove insecticide ear tags in the fall as soon as horn fly numbers begin to decline.