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Reversing Horn Fly Resistance

Cattle producers discovered a long time ago that resistance to organophosphate and pyrethroid makes horn flies difficult to manage.

A free test kit can reveal the level of horn fly resistance on a cattle operation.

“We can easily demonstrate that controlling ectoparasites like horn flies offers us a 5 to 1, return on investment,” says Lane Foil, professor of veterinary insects at Louisiana State University.

Horn flies are one of the most costly external parasites. Unfortunately, producers discovered a long time ago that resistance to organophosphate and pyrethroid, the common insecticides used in ear tags, makes them difficult to manage. In fact, Foil says that when such tags are used sequentially — the same insecticide in consecutive years — or rotationally, horn fly resistance develops within three years.

“If you're strictly using insecticide tags for control, my recommendation is to use pyrethroid tags every third year,” says Foil.

However, James Hawkins of Merial Veterinary Professional Services explains, “Using pour-on parasite control at turnout in mid-summer, combined with ear tags, is proven by a decade of trials to dramatically increase horn fly kill and to reverse the trend of fly population resistance to common pesticides.”

Specifically, Hawkins explains the combined use of insecticide ear tags with pour-on eprinomectin can extend fly control up to 12 weeks, while extending susceptibility of the flies to the tags (delaying resistance).

Of course, no one can determine horn fly resistance visually. That's why Hawkins says Merial is introducing the Resistance ID (RID) test kit this summer. It allows producers to gauge the level of resistance among horn flies on their operation specifically or within a region. The test is free, and if resistance is present, the Merial field staff person who performs the test will recommend an effective management program.

Besides managing horn fly resistance with the strategic use of eprinomectin, Hawkins adds, “You receive double benefit to the bottom line. You solve the problem that you can see — the flies. But, you also solve the problem you can't see — internal parasites that reduce gains, cow milk production, body condition and heifer development.”

For more information on the RID test and treatment protocols, contact a Merial sales representative, or call 1-888-848-6632.