Monday, August 2, 2021

Poor control of fall armyworm seen in Mississippi

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

fall armyworm
Mississippi State University entomologists have received numerous calls the past few weeks about the lack of control for fall armyworms — photo courtesy Mississippi State University

We have had numerous calls over the past 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 corn strain fall armyworm because most of the calls in soybeans have been in grassy fields where the worms are moving off of the grass after an 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 pyrethroids 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.

In rice, we are much more limited on options for control of fall armyworm. Essentially, the only option in rice is to use the highest labeled rate of a pyrethroid and add at least 2 ounces of Dimilin. The good news in rice is that we have seen very little impact of fall armyworm on rice yields. In most cases, any level of control will be enough to prevent yield losses from defoliation.

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

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

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