Insecticides that had never been used on livestock were selected. This procedure reduced the chance of resistance already being present in the horn fly populations. The phosphate insecticides are less toxic, do not have as long a residual, do not migrate over the hair coat as much as the pyrethroids and, consequently, are less effective. These factors, combined with good management practices, should delay or inhibit the development of resistance. Our studies indicate that horn fly numbers of under 200 per cow do not reduce calf weaning weights.
Canadian studies indicate the same thing for yearling steers.
The phosphate insecticide ear tags generally keep horn fly populations below 200, at least until late in the season. Removing the tags in the fall after a frost. Rotating the insecticides in the tags yearly. These tags can be rotated with phosphate tags. However, if they are used more than two successive years, horn flies will develop resistance to them.