Armyworms are sneaky so watch your fields!

 Fig 1. Corn stand with all leaves
below the ear removed

Doug Johnson, UK Extension Entomlolgist and Will Stallard, UK ANR Agent.

Will Stallard, UK ANR agent in Casey Co., KY, came across an odd set of damage on a corn field. It seems that in about a 7-acre area all of the leaves on the corn plants below the ear level were gone, while leaves at and above the ear level suffered only minor damage. There were no pests present at the time of observation so what would eat all the leaves up to the ear, but not cause much damage above the ear? After several iterations of photos and some consultation about what might have caused this unusual damage pattern, and why, we have finally come to a conclusion.

This damage was likely caused by armyworm. Further, the damage was caused before the leaves above the ear had emerged to any great extent. This means that an entire population of armyworm caterpillars completed their development while feeding on the leaves below the ear; changed into the pupa (non-feeding, resting stage), and then into adult moths as the leaves above the ears were emerging. So, only a few caterpillars in the tail-end of the population were around to feed when the leaves above the ears were available, thus the upper leaves were only slightly damaged.

Fig. 2 Armyworm frass at the juncition
of leaf blade and stalk.

Another photo taken later,though somewhat blurry, shows the only direct evidence of the presence of the armyworm, a collection of their excrement or “frass” at the junction of the leaf blade and stalk.How much effect this damage will have on yield is difficult to say. No doubt it was a treatable population. But one can say that this field was not being watched for problems.

 Damage of this magnitude in the making would have been pretty easy to see and the number of caterpillars that it takes to do this damage would make them easy to find. Truth is these armyworms are not “sneaky”, there was simply no one watching.

Checking fields for the presence of problems remains an important task. It is sometimes difficult and important insect populations do not occur with predictable or even common frequency. It is discouraging to constantly look for pest problems and not find them, but it is the only real way to avoid situations such as this.