The economic threshold, or time to control horn flies, is when cattle average approximately 200 flies per animal, or 100 per side, Baldwin notes.
"Benefits of horn fly control will be in the growth and weight," Baldwin says, adding, "Cows will wean heavier calves, while stocker, or yearling cattle, will gain more weight faster. Research indicates an approximate 1.5 pound weight increase for every week of horn fly control."
LSU AgCenter experts recommend several ways to control horn flies. These methods actually are different means of insecticide delivery, Baldwin points out. They include ear tags, pour-ons and sprays, plus self-treatment devices, such as back rubbers and dust bags.
Each of these methods of delivery has advantages and disadvantages. Sprays, ear tags and pour-ons require cattle to be rounded up and placed in a catch pen. Ear tags and pour-ons also require a squeeze chute.
"Of these methods, the application of ear tags is the most labor intensive and most stressful to the animals," Baldwin says. "But ear tag application is much easier with cattle that already have an ear hole from previous ear tag use. On the other hand, in general, ear tags also are the most expensive treatment."
The advantage of insecticide ear tags is that they can provide three to four months of highly effective fly control, and this reduces the overall cost of labor and insecticide. The residual effectiveness of pour-ons and sprays is generally much shorter, with pour-ons usually providing longer control than sprays.
But Baldwin says the advantages of an ear tag quickly disappear if the fly population is resistant to the ear tag being used.
"Insecticide resistance can make horn fly control very unpredictable," the LSU AgCenter entomologist explains. "Although resistance can develop with any means of insecticide delivery, ear tags are the most notorious because they provide constant insecticide exposure for most of the horn fly season."
Ear tags contain insecticides from two major chemical classes - pyrethroids and organophosphates.
"Pyrethroid resistance has been a bigger problem in Louisiana, but horn flies can develop resistance to an organophosphate tag just as well," Baldwin says, adding, "Research has demonstrated that continuous, annual use of an ear tag in either chemical class can result in resistance after three to four years."
Producers who experienced control failures with ear tags last year should not use the same ear tag or any other tag in that same chemical class for at least two years, Baldwin advises, saying research has shown that it takes about two years for a horn fly population to breed out resistance or to revert back to a susceptible population that can be controlled again with the same ear tag.
"To maintain effective horn fly control with ear tags, producers should follow a three-year rotation," the entomologist says.
Pyrethroid ear tags should only be used once every three years. Organophosphate ear tags can be used the other two years, but it is preferred that ear tags not be used at all in one of the three years, according to Baldwin.
"In that year, producers should rely on other means of insecticide delivery, such as sprays, pour-ons or self-treatment devices," he says, adding, "Also, do not use more than one ear tag treatment in a season, regardless if it is one or two tags per animal. If an ear tag does not provide season-long control (and most of them do not), the producer should use some other means of delivery to finish out the fly season."
Pyrethroid ear tags recommended by the LSU AgCenter include Cutter Gold, Python and Saber Extra.
"These pyrethroids are more potent than the original pyrethroid ear tags sold in the early 1980s, but they can still lose their effectiveness if a resistance management program is not followed," Baldwin says.
Recommended organophosphate ear tags include Patriot, Cutter1, Commando, Warrior, Cutter Blue, Terminator, Optimizer and Rotator.
For additional information about horn fly control, contact an Extension agent in your parish LSU AgCenter office.
Contact: Jack Baldwin at (225) 578-2180 or firstname.lastname@example.org