What Goes Around, Comes Around

Mar 16, 2021

A carefully planned insecticide rotation program can help guard against horn fly resistance. Last year alone, horn flies cost the U.S. beef and dairy industry an estimated $1 billion in lost profits. If that wasn’t bad enough, studies show that mini horn fly populations have developed a resistance to commonly used organophosphate and pyrethroid insecticides.
It pays to know your enemy
The horn fly (Haematobia irritans irritans) is the most detrimental external parasite found on pasture cattle. Horn flies are obligate parasites of cattle meaning they require several blood meals each day in order to reproduce.
Horn flies only breed in fresh cow manure, and the manure pat must remain intact until the adult flies emerge which is why they are primarily a pest to pastored cattle. The adult flies emerge from the manure in as little as 7 to 10 days depending on the ambient temperatures. In southern states, there may be up to 10 generations in a single season. This is one of the reasons horn flies develop resistance to insecticides so quickly.
“Each horn fly consumes up to 45 blood meals a day, and it has been proven that 100 horn flights per animal can cause significant economic damage to cattle,” notes Dr. Mike Fletcher, vice president of research and development at Y-TEX Corporation in Cody, Wyo. “For beef cattle, losses result primarily from reduced milk production in lactating cows, leading to lower calf weaning weight by as much as 40 pounds per calf. Horn flies also have been shown to reduce weight gains in yearling stocker cattle.”
Stay one step ahead of horn fly resistance by rotating insecticide ear tags each year
Studies shows the best way to avoid horn fly resistance is to implement an Integrated Pest Management program, incorporating chemical, cultural, and biological components. In terms of chemical control, insecticide air tags are the most cost-effective option, but it is vital to rotate classes of insecticide to maintain efficient long-term horn fly control.
“By rotating from one class of insecticides to another every year, you will significantly reduce the risk of horn fly resistance,” says Dr. Fletcher. “Organophosphates, synthetic pyrethroids, and macrocyclic lactones are all effective in controlling horn flies and other pests, but continued use of one insecticide class over two or more seasons can allow horn flies to develop resistance to that chemistry.”
Take these additional measures to keep horn flies in check
Along with applying insecticide air tags when horn flies first appear in the spring, the following measures will improve your horn fly control program:
  • In late summer, supplement your insecticide tag program with a pour-on or dust bag treatment to extend fly control until the first killing frost.
  • Drag or harrow your pastures after the first killing frost to break up manure pats and kill overwintering horn fly pupae.
  • Preserve beneficial insects and predators, such as dung beetles, as they can be your biggest allies against the horn fly population.
“Horn fly control more than pays for itself,” says Dr. Fletcher. “Studies in the U.S. and Canada show that during the grazing season, yearling cattle protected from horn flies gain up to 50 pounds more than heavily infested animals. Other research shows a 10- to 15-pound advantage to calf weaning weights when a good horn fly control system is in place. Bottom line, the return on investment is significant.”

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