Monday, September 20, 2021

Alert: Growers report poor control of fall armyworms in Mississippi

• By Jeff Gore, Angus Catchot, Whitney Crow and Don Cook •

fall armyworm
Fall armyworm — photo courtesy Mississippi State University

We have had numerous calls over the last week or two about poor control of fall armyworm with pyrethroids. They started in the southern part of the state and have rapidly expanded north.

The assumption has been that we are dealing with grass strain fall armyworm because most of the calls in soybean have been in grassy fields where the worms are moving off of the grass after a herbicide spray.

However, we have also been getting the same calls about misses with pyrethroids in hay fields and wildlife food plots. Also, in rice we typically only deal with grass strain fall armyworm. We are hearing the same things from our colleagues, Sebe Brown and Tyler Towles, in Louisiana.

Historically, we have always been able to get excellent control of grass strain fall armyworm with low to mid rates of pyrethroids. That is not happening this year in a lot cases. We are not sure why.

It could be that the grass strain has developed resistance to pyrethroids or we could be dealing with corn strain fall armyworm in areas where we typically don’t have them. The specific reason is irrelevant and we need to decide what to do for fall armyworm when we need to spray.

In soybeans, we have numerous good options, but unfortunately there are not a lot of good cheap options if pyrtheroids are not working. Pyrethroids are still a good option to knock down some of the worms, but add 4 ounces of Intrepid (or generic) to clean up the rest of the worms and provide some residual control.

Another good option in soybean that is still fairly inexpensive would be Diamond at 6 ounces. We typically don’t talk about Diamond much in soybeans, but it is excellent on armyworms and will provide residual control.

Finally, the more expensive products such as Intrepid Edge, Besiege and Prevathon (Altacor) would also provide excellent knockdown and residual control.

The bottom line is DO NOT rely only on a pyrethroid to control fall armyworm in any situation.

Dr. Jeff Gore is a research and Extension entomologist. Dr. Angus Catchot is an Extension entomologist. Dr. Whitney Crow is an Extension entomologist, and Don Cook is a research entomologist. All are with Mississippi State University.

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