Below is an article dealing with horn fly control:
The approach to ward off flies is multi-prong and challenging. However, doing nothing can severely hinder productivity, profits, and herd welfare. I’ve asked Dr. Nancy Hinkle, Extension Veterinary Entomologist at UGA, to give her tips on horn fly control for the Southeast.
“Since much of the Southeast has such a long horn fly season, we recommend that producers use a pour-on, spray, or dust on their cattle in the spring. If you’re getting up your animals to vaccinate and treat otherwise, this works okay. Once fly numbers start to take off, likely in early June, it is probably time to install ear tags. If organophosphate tags have been used more than the past three years in a row, it’s likely time to switch to another mode of action such as a pyrethroid tag. Similarly, if you’ve used a pyrethroid continuously for the past three years, it’s probably time to switch to something else. If neither pyrethroids nor organophosphates are providing the level of control you’re seeking, consider using the abamectin tag (XP-820).”
“Some producers are still getting good control using feed-through products containing insect growth regulators (IGRs) such as methoprene and diflubenzuron. Remember, these products prevent fly maggots from developing in the manure, but they do not kill adult flies. So, reductions in fly numbers on cattle will be apparent only a week or two later. And if your herd shares a fence line with a herd that is covered in horn flies, nothing will stop those flies from flying across the fence to infest your animals. If ear tags begin to fail late in the season, consider treating with a spray, dust, or pour-on.” Fly tag removal is also essential to help reduce insecticide resistance.
“Of course, fly control can always be supplemented using self-treatment devices such as back-rubbers or dust bags.” Dr. Hinkle suggests using pest control handbooks to understand which mode of action each product uses to determine rotation strategies. Your local University Extension is a good resource to find information on pest management information in your area.
Lastly, although pour-on avermectin dewormers do provide some fly control, excessive use may cause internal parasites to build resistance to those products. You definitely want to avoid both internal and external parasite resistance. Work with your local veterinarian and Extension resources to develop a multi-year game plan targeted for your area and herd.
This article is from the UGA beef extension blog. It can be found at https://site.extension.uga.edu/beef/.