Rotating the type of insecticide used to control horn flies on cattle is the most important step you can take to control the horn fly resistance.

Resistance occurs when flies become genetically resistant to a particular chemical class. Susceptible flies are killed, leaving the resistant flies to breed-unless a different chemical class is used. There are two popular types of insecticides used on cattle, pyrethroids and organophosphates. Each class of insecticide has several chemical names, so it can be difficult to tell by the packaging, ask dealers or distributors to be sure.

In the South and Southeast, entomologists generally advise using an organophosphate for at least two years before returning to a pyrethroid. Entomologists in the Midwest and Western Plains advise annual rotation.