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Controlling Horn Flies

Use these tips to reduce insecticide-resistant horn flies.

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.


Horn Fly insecticides

Click here for larger version of this chart

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. Resistance management should be included in any fly-control program regardless of the product used or the application method. 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 beef cattle. 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
Insecticide ear tags are typically administered to cattle at the time we work cattle that is closest to fly season; however, 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, usually before flies are present.


Remove 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 sublethal doses of insecticides is another reason horn flies develop resistance.


Use periodic applications with sprays, dusts and back rubbers
Ear tags constantly expose flies to insecticide. With periodic treatments we can treat, reduce the population to an acceptable level and stop. Periodic treatments also allow us to rotate to an insecticide of another chemical classification, which is a huge part of resistance management.


Use IGR and oral larvacides
Insect growth regulators (usually included in cattle minerals) are excellent tools for horn-fly control. The IGR is eaten by cattle in the mineral, passes through the animal, and is present in the manure pat, where it inhibits normal egg development. Because flies only lay eggs in manure pats, feed through insecticides are very effective control measures.


Use late-season treatments to reduce overwinter populations
Horn flies overwinter as pupa in the soil and emerge the following spring. Late season treatments limit the number of egg-laying adults, and therefore overwintering pupa. Select a product in the fall with limited resistance that will eliminate flies before they lay eggs, reducing numbers of resistant flies that reproduce, and making traditional fly-control methods more effective the following spring.


Rotate between organophosphate and pyrethroid products when using insecticide ear tags
The rotation schedule uses organophosphate tags for two consecutive years, rotates to a pyrethroid tag for one year, then back to organophosphate tags. Now that a macrocyclic lactone fly tag is available (XP 820), we can implement a three-way rotation of fly tags.


We have a long fly season in the South. Season-long horn-fly control will likely require application of two or more products from different chemical classes to keep insecticide resistance management in your fly control plans.

 

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Editor’s Note: Brian Beer is a Clemson Cooperative Extension area livestock agent in Lancaster county. This article was reprinted with permission from CU in the Pasture, the Clemson Livestock & Forages Team newsletter at .

www.clemson.edu/extension/livestock/
newsletters/ivestock_newsletter_05_2015.pdf


 

 

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