On occasion we see this odd looking injury in corn in Ontario. It can be deceiving as the culprit that did the damage is usually long gone. Only once the whorl of the corn opens up is the damage noticeable. Stink bug injury is often confused with mechanical injury from the sprayer or even herbicide injury, especially when the stink bugs feed on very young plants. Injury is usually concentrated on the fields edge but in some cases, like the one presented this week, it can occur throughout.
The stink bug has a needle for a mouth part. When it pierces the plant to feed, it not only mechanically shreds the numerous leaves it has peirced through in one shot, but it injects enzymes into the plant to help break down the tissue and keep it flowing. On young plants, they can sucker and stunt the plant. In larger plants, if it peirces the whorl of the plant, each leaf will have a tear or rip through it that starts to show up as the whorl opens. If you were to re-wrap the whorl, you would see that the tears line up. Each tear is also lined with a yellow border, again because of the enzymes. In some cases, the whorl may even buggy whip or the entire plant be malform.