When fly tags are applied at the beginning of the season, they emit large amounts of insecticide, but as the season progress, the amount of insecticide emitted decreases until by the end of the season the amount of insecticide emission is low.

Although only a few horn flies remain, when they are exposed to that low level of insecticide that is not adequate to kill them, they can develop enzyme systems that breakdown the insecticide, especially the pyrethroids, says Larry Hawkins, DVM. “If the flies that have this ability to breakdown the insecticide are selected as the late year population of horn flies by killing the flies without the enzymes, resistance begins,” he explains. “Then, next year, because horn flies burrow down into the soil to survive the winter and emerge the following spring, the population has the ability to breakdown insecticide and 6 to 8 weeks into the season there is a resistant population of horn flies.”

The low level exposure can be reduced by removing last season’s tags during fall processing, after the fly season is winding down or over, eliminating one of the resistance development concerns.