These two groups of tags contain insecticides that attack the nervous system of the fly in different ways. Resistance, indicated by a failure in horn fly control, can develop when tags containing the synthetic pyrethroid permethrin have been used for several consecutive seasons. No resistance to organophosphates, or the new synthetic pyrethroids, has been seen. A relatively new group of combination tags has appeared. These couple an OP and a SP in the same tag.
Current examples pair lambda-cyhalothrin and pirimiphos methyl or cypermethrin and chlorpyrifos.
The assumption is that the OP would control SP-resistant horn flies. Tags need to be removed at the end of the fly season in the fall to reduce the potential for insecticide resistance in both lice and flies. Non-permeable gloves should be worn when tagging animals. Tags should not be handled bare-handed under any circumstances. The tags are manufactured so that the insecticide is rubbed off the surface and on to the animal.