The threshold level for economical fly control begins at around 200 flies/animal, says Eldon Cole, University of Missouri Extension livestock specialist. He points out research indicates blood-sucking horn flies alone can cost 25 lbs. of gain in stocker cattle.
“Counting flies isn't easy so most of the estimates are made using the assumption that if there's an area of flies on the animal the size of the palm of your hand that's roughly 50 flies,” Cole explains. “I've assisted with a field trial that involved using binoculars and actually counting flies early in the season when they weren't too numerous and it is easy to get 200 flies/animal.”
In addition to horn flies, horse flies, stable flies and face flies can create problems for cattle as the summer continues. Cole says each creates a unique problem for animals and is difficult to control.
For instance, Cole points out, “The routine control measures for horn flies will only have limited success with the other fly species .” Sprays, insecticidal ear tags, dust bags, back rubbers, pour-ons and oral larvacides are the controls used on horn flies.
As you apply fly control, keep in mind how choice and application can affect other parasite resistance. For example, research by North Carolina State University’s Matt Poore and Mark Alley point to injudicious use of some parasite controls contributing to anthelmintic resistance .
“One of the biggest culprits to increasing resistance we see in this part of the world is producers using low doses of ivermectin to control horn flies in the summer,” Poore says. It’s cheap. It’s effective. But Poore explains it’s also an engraved invitation to increased resistance to the anthelmintic by gastrointestinal pests.
Other risky practices include dosing based on weights taken with the eyeball rather than a scale and dosing based on average weights rather than individual cattle weights.
Cole offers these considerations for choosing and applying effective fly control:
Resistance  to pesticides can develop, so rotating each year or so between the pyrethroids and the organophosphates will help slow down the resistance buildup.
Remove old fly tags at the end of their useful life. Leaving them in aids in fly resistance buildup.
Many fly tags  are still effective, but frequently are put in too early in the season. Rotate the active ingredient used in the tag from year-to-year.
Back-rubbers or dust bags are highly effective and economical, but they require regular management to make sure cattle use them.
If you're fortunate enough to have a supply of wire and burlap bags you can make your own rubber at a significant savings. Old flannel material will work in place of burlap.
Feed-throughs offer convenience, but cost more per day or month. They are most effective when consumed by all cattle in a fairly large area. The additive interrupts the life cycle of the horn fly in the manure pat.
Combining horn fly control tactics may be helpful to give the cattle maximum relief. Remember you'll never have totally fly-free cattle.
Face flies are generally not resistant to pesticides and insecticidal ear tags and other control methods for horn flies are effective against them.
- Treatment costs per head per day can vary from a couple of cents up to 8-10¢ depending on method and expected length of effectiveness.