Armyworms on the March!

Have received reports of significant populations of fall armyworms (FAW) feeding on foliage in corn and sorghum and consuming pasture grasses throughout the RGV. Egg and larval survival are favored by cooler temperatures and wet conditions so our recent conditions have been perfect. Like all caterpillars, they are eating machines and will actively feed for several weeks, reaching a length of 1^1/2 inches before pupating in the soil.  While small larvae do not cause much damage and may be hard to detect, more mature larvae can consume significant amounts of foliage and cause considerable defoliation.  As their name suggests, they feed in large groups, moving like an army and consuming foliage as they go.  They are most active at night, or on cloudy days thus sampling will be most successful early morning, at dusk, or during the cloudy periods we have had plenty of lately.  Infestations often begin in grassy pastures, weedy roadsides and along fence rows and move into small grains, corn and sorghum.  

 

In young corn and sorghum, the smaller the plant, the less defoliation can be tolerated, but sorghum in the RGV is largely past seedling/whorl stage damage.  As a general recommendation - insecticide applications may be considered if feeding reduces leaf area by 30% or more and you are finding 1-2 larvae per plant. Foliar feeding at later stages has been pretty impressive (see photo) but unfortunately, we do not have established thresholds for these later stages of corn and sorghum.

A major concern is if/when fall armyworms move into sorghum heads where they may cause significant damage. Thus, sorghum should be scouted for in a timely manner from flowering through soft dough.  

In general, 1-2 fall armyworm and/or corn earworm per sorghum head may warrant control. For a more detailed management decision calculator, please click here. For more information on control options for armyworms in sorghum, please click here (page 37).  Note that pyrethroids are not recommended for corn earworms and FAW larger than 1/4 inch (2nd instar). 
 
In pasture thresholds very with crop stage and value but 2-3 armyworms per square foot may justify control. For more information on FAW and insect management in pasture, please click here.