To defeat the corn plant’s natural defenses against insects, caterpillars produce potent poop that deceives the plants and leaves the caterpillars free to gorge.
The idea of defecating on your own food sounds repulsive to (most) people, but for fall armyworm caterpillars, a little sprinkling of feces adds just the right zest to their diet of corn plants. Researchers at Pennsylvania State University have figured out why these caterpillars like to season the leaves they’re about to eat with their own poop, and the discovery could help with developing pesticides.
It all comes down to the evolutionary arms race between predator and prey. Plants have to contend with a variety of attackers, from large animals like us to the tiniest pathogenic fungi. Although they seem passive, plants have a lot of tricks up their sleeves to deal with these predators, like bitter-tasting or even toxic secretions as well as painful thorns and spines. Corn plants have specialized lines of defense against ravenous insects and fungal infection, but they can only employ one technique at a time. Normally, they can perceive the accumulation of frass (insect feces) and use it as a signal to ramp up their defenses against insects.
Taking advantage of this switch, the fall armyworm caterpillar evolved a way to deceive the plants into thinking they’re being attacked by fungi, even while the caterpillars munch away at their leaves. They imbue their frass with a mixture of chemicals derived from the corn plants, their own genes, as well as other microbes. This potent concoction of chemical “elicitors” creates a decoy that distracts the corn plants from the real menace, tricking them into chasing down a phony fungal threat.
The researchers tested the unique effects of caterpillar frass by preparing extracts to apply to the plant leaves. They compared the growth of caterpillars who ate frass-treated leaves with those eating regular leaves, and found that the caterpillars who ate corn leaves with a side of frass grew much faster than the second group. Next, they exposed the corn plants to fungal spores that cause blight disease. At first, the frass-treated plants responded to the frass proteins by mounting defense strategies against insect-caused wounds. But as the exposure continued, the frass-treated plants began to perceive those proteins as fungal pathogens, and they were able to fight off the infection even better than the untreated plants could.
These results show that the caterpillars apply their frass like a digestive aperitif that prevents the plants from releasing any distasteful compounds that would spoil their meals. Although the caterpillars eat their fill of the plants, the frass also benefits the plants by enhancing their resistance to fungal infections. This could explain why the corn plants have not evolved a counter-defense against the caterpillars’ ploy.
Ultimately, plant scientists may be able to use these caterpillars to design organic, sustainable fungicides without the harmful environmental side-effects of synthetic treatments. The study serves as a reminder that we need to consider all the components of a crop’s ecological position, or risk missing out on useful, natural solutions to our problems.