Calls continue to come in of feeding damage found even in corn larger than 5 leaf stage. Some of the larger plants (6-7 leaf) are being mined into at the base of the plants and smaller plants are being cut off. In most of the cases, the culprit is black cutworm. Though it is odd to see plants larger than 5 leaf still experiencing feeding, I am not surprised given the year we have had. Corn went in early and clearly got ahead of the cutworm larvae in terms of size. The larger the plant, the harder it is for the cutworm to chew completely through the plant and cut it off so instead they are mining into the stalk.
To identify black cutworm, use a handlens and look at the spots/warts or tubercles on the body of the larvae. On each body segment there are a pair of tubercles, one always slightly smaller than the other.
A couple of issues with trying to manage this damage now.
1) Size of larvae: From what I am seeing, the larvae so far in all of these cases are over 1 inch in size. Foliar insecticides will not work well on these larger larvae. Also, the larger the larvae, the more likely they are almost finished feeding. So essentially the damage that they are going to do has already been done.
2) Weather: It is going to be difficult enough to get out and spray at night during this week’s rain events. But the other key point to spraying them is to leave the soil undisturbed for 5 days so that the larvae that were not directly in contact with the spray, will come in contact with the residual product left on the soil when they do come out to feed. Unfortunately if it rains during that time (especially the kind of downpours we’ve been having) this residual is washed away or diluted, reducing its effectiveness.
So before making the call to spray, check the size of the larvae to make sure it is worth the effort.