The fall armyworm larvae leave frass(which I assume is poop) behind while eating the plant. There is a certain compound in their frass that deceives the corn plant. It induces the plant into "thinking" that some pathogenic fungi or bacteria is attacking it; thus, the plant will turn on their defend mechanism for these micros. The important fact is that corn plant could only switch on one defend mechanism at a time. So if it is defending micros, it can't defend against the larvae. I think this is quite an amazing attack that evolved over time. Time and time, nature develop some amazing stuffs to show the world what she is all about!