Horn Fly Control – Dealing with Insecticide Resistance

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.

Resistance management should be included in any fly control program regardless of the product used or the application method. 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).

Use IGR and Oral Larvacides. Insect Growth Regulators are excellent tools for horn fly control. IGR is usually included in cattle minerals. They are one of the best tools for resistance management because flies are not able to build resistance to these compounds. Most inhibit fly eggs from developing properly by using hormones or hormone mimics. The IGR is eaten by cattle in the mineral, passes through the animal, and is present in the manure pat. Because horn flies only lay eggs in manure pats, feed through insecticides are very effective control measures.

Use Late Season Treatments to Reduce Over-winter Populations. Horn flies over-winter as pupa in the soil and emerge the following spring. By using a late season treatment we can limit the number of adults laying eggs that will become over-wintering pupa. More important is to select a late season treatment that will help eliminate resistant flies before they lay eggs and produce offspring that will over-winter. Insecticide resistance is heritable and is passed to the next generation of horn flies. By using a product with little or no fly resistance in late season we can limit the number of adults emerging next spring that have built in resistance at the beginning of fly season, making our traditional methods of horn fly control more effective.

Rotate Between Chemical Classes When Selecting Products. Insecticide Ear tags are still very useful fly control tools. Rotating between organophosphate and pyrethroid products is recommended. The suggested rotation schedule is to use organophosphate tags for two consecutive years, rotate to a pyrethroid tag for one year, then back to organophosphate tags. Chemical rotation is a good idea with any method of fly control.

Keep in mind that rotation between chemical classes is necessary. That is more than changing brand names. The table below lists common insecticides and insecticide combinations by chemical class. Check the active ingredient of the products you are considering and determine its chemical class.

Some classes of insecticide are more prone to developing resistance than others. Flies will develop resistance to pyrethroid insecticides very easily and quickly. Organophosphate insecticides have documented cases of resistance, but aren’t as prone to resistance as pyrethroids. Macrocyclic lactone and insect growth regulators have very few or no documented cases of fly resistance. Be sure to consider these characteristics when developing your resistance management strategy.

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


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One Comment

  1. Joe Calcote April 1, 2015 at 1:13 pm #

    This is a very informative and useful article.
    Question: I will be using pyrethroid ear tags this spring since I have been using Prolate/Lintox HD (organophosphate) spray-on for the last two years. When I change out the pyrethroid tags for late season, is this the time to go back to organophosphate or do I continue with pyrethroid until next spring? Thank you.

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