Watch for Fall Armyworms in Late Planted Soybeans

Over the last 10 days I have started getting calls about fall armyworms showing up in late planted 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.  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.  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” 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 25 Sweeps, by Tyler Horn