Using its poo, a caterpillar that feeds on young corn leaves tricks the plant into thinking that a fungus is attacking it. That way, the plant mounts a defense against the (possibly non-existent) pathogen, while suppressing its defenses against the actual plant-eater. The findings were published in the Journal of Chemical Ecology last month.
The larvae of fall armyworms (Spodoptera frugiperda) are voracious eaters, and they prefer munching on leaves confined in the whorls of maize (Zea mays). They typically defecate nearby in the crevasses where the leaves meet the stalks (pictured above). And over time, copious amounts of caterpillar poo, called frass, accumulate in the enclosed whorl tissue. But why shit where you eat?
When herbivores feed on plants, not only do they cause damage, they also deposit chemical substances that might manipulate the plant’s response to being eaten. So, Penn State’s Dawn Luthe and colleagues wanted to see what chemical compounds in the frass were signaling to the plant. The frass is made of molecules derived from the plant, the caterpillar itself, and its associated microbes. The team found that proteins from caterpillar frass initially trigger wound-response defense genes in corn, but then shortly after, genes that defend against pathogens were induced.
“Plants cannot defend against both pathogens and insect attackers simultaneously,” Luthe explained in a statement. “They must switch on either their pathway to defend against herbivores or their pathway to defend against pathogens.” These pathogens are typically fungi or bacteria.
Then, the researchers applied frass extract to corn leaves and compared the growth of caterpillars that ate those leaves to the growth of caterpillars eating untreated leaves. They also measured the activity of a fungal pathogen in response to this frass treatment by inoculating leaves with spores of Cochliobolus heterostrophus, which causes leaf blight. The elicitation of pathogen defenses by frass proteins, they found, was correlated with increased herbivore growth and reduced fungal pathogen prevalence. More frass means bigger caterpillars and fewer fungi.
"The plant perceives that it is being attacked by a pathogen and not an insect, so it turns on its defenses against pathogens, leaving the caterpillar free to continue feeding on the plant,” said lead author Swayamjit Ray of Penn State. “It is an ecological strategy that has been perfected over thousands of years of evolution.”
Image in text: A caterpillar eating a corn leaf. Penn State