As we move out of May and into June, farmers need to scout their fields for insect pests.

The primary concern at the moment, says University of Illinois Extension field crops entomologist Nick Seiter, is the black cutworm. The larvae are now large enough to do more than the simple pinhole damage you might find in the corn plant’s early leaves and can cut the corn plant off.

“It’s a good time to scout any fields where you had poor winter annual weed control and in particular winter annual broadleaf weeds. A good time to check those and see if you don’t have some cutworm damage. But in recent years, even when we’ve had fairly high moth flights, it hasn’t translated into a lot of injury from that pest.” 

Because black cutworm larvae are mostly nocturnal, it is usually easier to scout for their damage first. When damage (particularly cutting) is observed, the Illinois Agronomy Handbook recommends scraping away the top layer of soil or residue in the area surrounding the cut plant to uncover the larvae. Cutworm larvae have a characteristic “sheen” that can help to distinguish them from other caterpillars. Consider a broadcast insecticide application when 2 to 5% of plants have been cut and cutworms are still present in the field.

The other field crops insect pest to be aware of right now is the western corn rootworm. Egg hatch has reached 50% in central Illinois and will get to that point around the first full week of June in the northern part of the state. He says root feeding by the western corn rootworm, then, is expected until the end of June.

“Now, if it’s one of these like planted fields where you don’t have corner roots out there yet, those larvae are going to hatch and they’re going to die. So when we see planting delayed, you typically see less of a threat from corn rootworm feeding in those fields. But that’ll be kind of the next thing to keep an eye out for as this root feeding develops. In about a month from now, we’ll be able to know what we’ve got. We’ll be able to go out and evaluate how well our controls have performed.” 

The western corn rootworm has been adapting to the corn hybrids used to control it. So, it’s important to understand what’s happening in a field this season to adjust what control measures will be most successful in future years. The western corn rootworm has historically been the most economically damaging insect pest of field corn.