Production losses to the US beef industry have been estimated in the $700 million dollar range from horn flies alone. The flies seem especially bad this year. Perhaps it's the weather this spring or maybe it is the increased resistance of the flies to our common control methods. Whatever the reason, fly control for the cow-calf operation can appear more difficult and confusing each year. However, there are always new options (and some old ones) that give us lots of possibility for control. Let's try to break it down to some manageable pieces.
The flies. The 5 flies that affect cattle the most are horn flies, face flies, heel flies, deer flies and horse flies. Horn flies are the flies we see on the backs, side, and poll areas. They are bloodsucking flies that feed 20 to 40 times per day, and their major economic damage is from blood loss. Face flies feed on secretions of the eyes and nose, and are the major contributor to pinkeye. Heel flies lay the eggs that produce cattle grubs so their damage comes later. However, heel flies cause cattle to be nervous and spend less time grazing when the flies lay their eggs on the lower legs of cattle in late summer. Deer flies and horse flies are biting flies, but they don't feed as regularly on one animal like horn flies. Deer and horse flies are more nuisance flies that disrupt the grazing patterns of cattle and cause them to spend energy to get away from the flies. Deer and horse flies are hard to control in pasture situations. In the summer, the two flies we should work on controlling are horn flies and face flies.
Resistance develops when flies are exposed to insecticides too early, at too low a dose, or for too many years in a row. There are, naturally, a few flies in the population that are hard to kill with a particular type of insecticide. If these flies are exposed to insecticide early in the season, they continue to breed and all the flies by the end of the summer are resistant. If flies are exposed to the same insecticide year after year or at a low level, some of the flies "develop" resistance.
The products and methods. There are a dizzying number of insecticides on the market for fly control. However, they all fall into 5 basic categories -- pyrethroids, organophosphates, combinations, oral larvacides and avermectins. Table 1 lists some of the common names or brands of these categories of insecticides and the usual way they are applied.
Ear tags are the most convenient method for fly control. They give good control of horn and face flies. However, fly tag use has resulted in the most resistance. Ear tags should not be applied until June and must be removed 4 - 5 months later. This put the tags in when it will kill the most flies and creates less resistance. Too often you can see fly tags in cows in the winter because they weren't taken out in the fall. All these winter tags do is create resistance. Flies in Virginia are resistant to most of the "original" tags or first generation pyrethroids. New second generation pyrethroids, organophosphates , and combinations of the two offer some good options for ear tags to use against resistant flies.