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Insecticide-impregnated ear tags are a popular means to control pasture flies (especially horn fly). Tags are inserted in late spring or early summer, and the fly control program travels with the animal. However, using tags containing the same class of active ingredient for several consecutive seasons can select for populations of the horn fly that are resistant to a whole class of related insecticides. This shows up in the form of a shorter than normal period of fly control, but lab testing would be needed to confirm resistance.
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