We have a long fly season in the South. Season-long horn fly control will require two or more methods, applying two or more products from different chemical classes. Keep insecticide resistance management in your fly control plans.

Cattlemen are faced with fly control decisions every year. Horn flies are the most important in terms of economic loss to cattle producers, accounting for about $1 billion annually in the United States.

Control of horn flies became very convenient with the introduction of insecticide ear tags. The early ear tags contained pyrethroid insecticides and were very effective…for a while. After only a few years flies began to develop resistance to pyrethroid insecticides. Why? The short generation length and high reproductive rate of horn flies played a large role in resistance development. Another important contributing factor was the widespread use, and often misuse, of pyrethroid tags.

Many of the insecticides used for horn fly control have documented cases of resistance, especially pyrethroid and organophosphate insecticides.

Here are some guidelines that help with insecticide resistant horn flies:

Only Treat When Levels Exceed 200 Flies per Animal. Research has determined that 200 horn flies per animal is the economic treatment threshold for horn flies on beef cattle. Economic threshold is the number of pests that must be present before treatment is justified. By waiting until populations exceed threshold we assure that treatments are necessary and the fly population is exposed to fewer insecticide treatments.

Delay Early Spring Treatments, Wait for 200 Flies per Animal. Insecticide ear tags are the major culprit breaking this rule. Insecticide ear tags were (and still are) administered to cattle at the time we normally work cattle that is closest to fly season. Many fall calving herds are processed in early spring. It is best to delay insecticide ear tag application until economic threshold is met. Insect Growth Regulators (IGR) are the exception to this rule. Use of IGR products should begin early in the fly season.

Removal of Insecticide Ear Tags in the Fall. Insecticide ear tags contain a four or five month effective supply of insecticide. After then the tags will still contain some insecticide, but the amount is not a lethal dose. Exposure to sub-lethal doses of insecticides is another reason horn flies develop resistance.

Use Periodic Applications With Sprays, Dusts and Back Rubbers. With ear tags the flies are constantly exposed to insecticide. With periodic treatments we can treat, reduce the population to an acceptable level, and stop. This limits the time flies are exposed to a particular insecticide. Periodic treatments also allow us to rotate to an insecticide of another chemical classification, which is a huge part of resistance management (more on that later).

Read entire article at Southeast Cattle Advisor.