The true armyworm is a pest of corn and wheat that farmers need to be watching for right now.
The true armyworm is a pest that rides the winds from the south. It begins a migration northward into states like Illinois in late April. The moths lay their eggs in dense grassy vegetation and the offspring, the larvae, are beginning to feed right now says University of Illinois Field Crops Entomologist Nick Seiter.
“We’re getting to the point, starting back in the third week of May or so, that the larvae are large enough to cause noticeable damage in corn and in wheat in particular.”
The damage is unique in that the larvae feed from the edge of the leaf and tend to work inward. It, says Seiter, gives the margin of the leaves a raggedy appearance.
“Now, if this damage persists, it can ultimately clip a corn plant down to where there is barely any leaf material left and can, in fact, stunt that plant or even kill it. In wheat, occasionally, as the wheat plant starts to dry down and they run out of tissue, they’ll actually begin to clip the heads or destroy the flag leaves. That’s where they can cause some damage to wheat in particular.”
Seiter says to watch for the true armyworm in wheat fields, fields with grass cover crops, and those corn fields that have a lot of grassy weeds in them. As the grass weeds die off from the herbicide applications the true armyworm, he says, can become an issue.
“So, in corn, you’ll want to consider an insecticide application if you are seeing armyworm damage on 25 percent of plants and you still have small armyworm larvae out there. Note that is this armyworm damage, they are feeding on the margins on in. This is not like the regular whorl feeding you would see, for instance, with the fall armyworm or the shot-hole feeding you would see from black cutworm.”
In wheat the economic threshold to apply an insecticide rescue treatment is six true armyworms per row foot. Wheat can withstand a great deal of defoliation without losing yield. However, heavy damage to the flag leaf or clipping of the heads is a key call to action indicator.
By-the-way the true armyworm, sometimes just called the armyworm, is one in series of armyworm insect pests that can be problematic in field crops.
“We call this one the true armyworm, or just armyworm. It tends to be the first one we see for the season in Illinois. Later on in the season we may begin to get flights of fall armyworm or yellow-striped armyworm. One of the real key in those insects is that they have a broader host range. They’ll cause economic damage in a broader range of plants and we saw a lot of that, in particular, in 2021.”
The fall armyworm can become a problem in July and August in the state of Illinois. Seiter says it is always imperative for farmers to scout field crops for insect pests and to apply rescue treatments based on economic thresholds when necessary.