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Agricultural News: Managing Fly Populations on Pastured Cattle

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Fly infestations cost US livestock producers over $1 billion annually.   Depending on the fly, the losses are attributed to decreased weight gain, loss in milk productivity, transmission of bacteria causing mastitis and/or pinkeye in cattle, and health treatments. 

It is no secret that flies have become resistant to many of the insecticides we use today.  The main reason:  unintentional misuse of the fly control products.

In this article, we will look at the practices that help lead to large resistant populations and outline proper use of fly control products, as well as some other methods of fly control.

The two most common flies we battle are horn flies and face flies.  Both flies have short life cycles.  Horn flies feed on the animal’s blood and live their life primarily on the animal. Face flies feed on the mucosa around the eyes and nose and can travel up to 4 miles. 

The development of insecticide resistance in flies is similar to how cattle producers make genetic improvements in their herd.   The flies that are susceptible to the insecticide’s mode of action are “culled,” leaving the “genetically superior” individuals to reproduce and make more offspring that are resistant to that particular product’s mode of action.  There is nothing we can do to prevent resistant populations from developing, but there are things we can do to manage them.

It’s important to note that resistance develops to a specific mode of action, not the product. There are numerous fly control products on the market, but only three modes of action:  Pyrethroids (such as permethrin), Organophosphates, and Avermectins.  If resistance is developed to ear tags containing permethrin, then resistance is developed to ALL pyrethroid products, regardless of how it is applied (sprays, rubbers, etc).

If we are using an organophosphate ear tag and a pyrethroid pour on simultaneously, we could potentially produce large fly populations that are resistant to both products.   Because of this, UT recommends rotation your chemical classes / modes of action every 1 to 2 years.  This would mean one year you would use only organophosphate ear tags, pour-ons, oilers, rubbers, sprays, etc.  Then the next year you would only use pyrethroid products (i.e. permethrin).

Below are some specific steps you can take to manage resistance:

  • As mentioned previously, rotate your chemical classes/modes of action every 1-2 years.
  • Use ear tags ONLY when 200 or more flies are present.  This is important!  Treating too early can mean poor control in late summer when flies are the most problematic. 
  • Use the correct number of ear tags.  Many labels require two ear tags per cow.
  • Don’t forget to treat your calves for flies with appropriate products.
  • Remove fly tags when horn fly numbers decrease in the fall.  This limits the amount of time the flies are exposed to the product and allows susceptible flies to develop (we need more susceptible flies in the gene pool than resistant ones). 
  • Use dust bags, sprays, oilers, rubbers, etc. in addition to ear tags.  Relying on just one application method is not as effective. 
  • Use feed-through IGR (insecticide growth regulator) products.  These help kill the larvae in the manure.
  • Work with neighboring farmers to use the same chemical classes plus an IGR product.  This will help control resistance that could build in fly species that travel, such as face flies. 
  • Consider using traps to help lessen the flies on your cattle.   You can make one using a 50 or 55 gallon drum and a few other materials (see photo for an example). The flies enter the barrel through PVC pipe and can’t get back out.  These will not eliminate 100% of the flies but they will help lower the amount that are pestering your cattle.  Several video tutorials can be found online.
  • Consider culling animals that consistently have a large amounts of flies on them in comparison to the rest of your herd.  Some animals have more hair and/or thicker hide, which makes them more resistant to certain flies.  These traits may be passed on to offspring. 
  • Consider crossbreeding.  Brahman-type cattle are known to be more resistant to flies than our British and Continental cattle breeds, like Angus or Charolais.  Just like other traits, fly resistance can vary greatly within a breed (UT Publication W 821).

If you have questions or need help evaluating your current fly control program, please contact Megan Harris at 931-722-3229 or [email protected]

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