Armyworms Showing up in Grassy Fields Moving to Soybeans

Angus Catchot, Extension Entomologist
By Angus Catchot, Extension Entomologist June 6, 2014 11:50

Over the last few days I have started getting calls about fall armyworms showing up in soybeans.  We see some of this every year, and it is primarily related to larvae moving off of a grass host after a Roundup application kills the grass but this year it is much earlier than most.

Nearly every caterpillar pest species will consume 90 percent of the leaf tissue they will eat in their entire lifetime in the last 3 days.  This is why most soybean thresholds have a “qualifier” of only counting larva greater than ½ inch size.  The problem we see with fall armyworms moving off of grasses into soybeans when you kill the grass is, more often than not, the larvae are already large when they move into the soybeans so the foliage loss can happen very quickly.  It is not uncommon under heavy pressure for a fall armyworms to completely defoliate a field of small soybeans in matter of couple of days.  If you have a grassy field, I would strongly encourage you to scout the grasses for the presence of armyworms prior to your Roundup application, and if fall armyworms are present add a pyrethroid in with the Roundup. (keep in mind there are a lot of bollworms around and when beneficials are killed off they tend to get worse, so only do this if necessary).

Typically grasses with fall armyworm feeding will have white tips where small larvae have been etching the leaves.  The good news is these armyworms are the “rice or grass strain” or may even be mixed with true armyworms and are controlled very easily with mid to low rates of pyrethroids.  Scout these grasses closely and you can save yourself a headache down the road like the picture below from Scott Stewart, University of Tennessee.

Fall Armyworms in Soybeans, by Scott Stewart, Click to Enlarge

Fall Armyworms in Soybeans, by Scott Stewart, Click to Enlarge

 

 

 

 

 

 

 

 

 

 

 

 

 

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Angus Catchot, Extension Entomologist
By Angus Catchot, Extension Entomologist June 6, 2014 11:50
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