That leaves us with horn flies. Traditionally, and yet today, the main ways to control horn flies are with ear tags, pour-ons, and oilers or dust bags in the pasture.

Insecticidal ear tags came out more than 25 years ago and are still among the top choices for horn-fly control, says Phil Kaufman, University of Florida veterinary entomologist. “When they [ear tags] first came out, they killed everything and they'd last all summer long. Well, it only took horn flies in some parts of the country a couple of years to develop resistance. So we need to be a little smarter about this.”

Which means, the experts agree, only use ear tags, and pour-ons for that matter, when you need them. While that's good advice, the problem is that the life cycle of the fly doesn't coincide very well with the work cycle of a cattleman.

“Say you're in Tennessee and horn flies don't really show up there until April or May, but you worked your cattle back in February,” Kaufman says. “You really need to bring those animals in again and put the insecticide ear tags in April or May when the horn flies become a problem. Not everybody wants to do that.”

So you apply the tags or the pour-on when it's convenient for you. “You're basically putting on this product and it's doing you absolutely no good,” says Bert Stromberg, University of Minnesota professor of parasitology. While there's a cost associated with working your cattle a couple of extra times to apply fly control, there's also a cost associated with using the products incorrectly, not to mention what you're doing to encourage insecticide-resistant flies.

“You might as well keep it in the can and use it for another day,” Stromberg says.

That “other day” will vary by region, of course, but you can gauge when you need to start fly control by watching your cattle. The general threshold is 200 flies/animal. Take a look at 10 or 12 animals and if it looks like they're harboring more than that, get to work.

But just as applying the tag too early can foster resistant flies, leaving the tag in too long has the same effect. You want to pull the tags as soon as possible after fly season or if you notice populations beginning to increase.

“Ear tags release the insecticide at an effective level for quite a while, but then the level drops off fairly quickly,” Kaufman says. “It's when you are in that drop-off period that you are allowing for more of those partially resistant individuals to survive.”

Since cattle on summer pasture often aren't easy to gather and treat, there are a couple of additional options.

“The most primitive, but still effective, is back rubbers and dust bags charged with some sort of topical insecticide, usually pyrethroids, although organophosphates are still used,” Moon says.

Fence off an area where cattle come and go, like a water tank or mineral feeder, and make an entryway. Hang a dust bag or oiler in such a way that the cows are forced to contact it as they walk in and out.

“That ensures pretty good coverage of the herd,” Moon says. “If you just hang an oiler out under a tree, a few of the animals will come play with it every once in a while, but you won't get anywhere near the coverage that is required to get effective horn fly control.”

Another option is feed-through products added to the mineral. These products target horn-fly reproduction by inhibiting the growth of the larvae as it develops in the fresh manure. While these products can be effective in killing the larvae, they don't kill the adult flies. So if your cattle are close to a neighbor who isn't using fly control, you may continue to see adult flies. While it's best to wait until flies become a problem to use topical control like ear tags or pour-ons, start using the feed-through product before flies appear to ensure it starts controlling flies early in the season.

Typically, Moon says, flies pose the greatest problem during the second half of the grazing season. “If you control horn flies well with any kind of insecticide, they sooner than later develop a resistance to the products.”

That process can be slowed by rotating products and using them according to label directions and only when fly populations dictate. “Letting the flies go uncontrolled for a while (early in the grazing season) slows down the development of resistance,” he adds.

Know your enemy and pick your weapon. But remember, fire only when necessary, and then aim to kill.