You may have heard the saying “don’t s**t where you eat,” but fall armyworms certainly haven’t. These moth caterpillars are a major threat to corn crops, and eat them voraciously. As they feast, they leave what plant stress biologist Dawn Luthe calls “copious amounts of frass”—the solid excrement of insects—on the plants’ leaves. And according to Luthe’s research, the poo doesn’t pile up just because of carelessness or poor manners on the caterpillars’ part.
While they can’t run away or fight back in ways that we can plainly see, plants aren’t helpless victims when animals try to eat them. Many plants can quickly kick their chemical defenses into action in response to insects biting into them or even crawling on them. The caterpillars' poop is part of a strategy for taking down the plants’ defenses: It appears to trick plants into fighting off the wrong attacker, and induces them to mount a defense against fungal pathogens while suppressing their anti-herbivore defenses.
For their new study, Luthe and her team collected armyworm frass, extracted the proteins from it and slathered them on wounded corn leaves. The next day, they looked at what kinds of defense chemicals the corn had produced. They found that plants had reacted as if they were dealing with a fungal infection, and that their pathogen defenses were triggered. That’s great news for the armyworms, since a buildup of the hormone that controls fungal defenses inhibits corn from producing another hormone that controls its herbivore defenses. Caterpillars that ate corn leaves that had been rendered defenseless by a 24-hour frass treatment ate more and grew faster than caterpillars that were fed leaves treated for only eight hours.
The researchers aren’t sure what it is in armyworm’s frass that dupes the plants into fighting the wrong battle. It might be a protein that’s produced by the caterpillars, or by microbes living in their guts. It could also be a protein produced by the corn itself that gets ingested by the caterpillars as they eat and then gets turned against the plant. The researchers want to work on isolating the compound responsible for the caterpillar’s trick in future studies.