Researchers from Penn State College of Agricultural Sciences have been studying the dozens of chemical signals that pass between insects and plants. In doing so they have stumbled across a unique signal that caterpillars can produce in order to allow them to munch away at the leaves of the corn plant. Interestingly, the chemical is extruded from the caterpillar via its feces.
The caterpillars in question are fall armyworm larvae and the deposit they leave is called ‘frass.’ Frass accumulates near the bottom of the plant, where crevices in the leaves meet the stalks.
The findings that caterpillar poop has a modifying effect on corn plants has answered a riddle in biology: why do plants react against some insects and not others, especially in the case of aggressive and damaging attacks from caterpillars?
The answer is that the caterpillar does not switch off all defences. Instead, it gets the plant to orientate its defences against a different threat. Corn plants can produce chemical defences against insects or fungi but not both. Caterpillar frass triggers the plant into sensing that it is under threat from fungal pathogens. The plant alters its chemical pathways to fungi, leaving the caterpillar free to roam.
In some areas where fungal threats are greater, the researchers are considering whether the active substance from the frass can be used as a novel anti-fungicide to protect crops from molds. In early studies, the active substance proved effective against the fungus that causes leaf blight in corn (Cochliobolus heterostrophus). This was based on a small field study.
The findings have been published in the Journal of Chemical Ecology. The research paper is “Maize Plants Recognize Herbivore-Associated Cues from Caterpillar Frass.”