Caterpillars Bypass Corn Plant Defenses to Grow Bigger and Faster, New Research Shows

Caterpillars have a stealth way of deceiving corn plants and getting them to lower their defenses just long enough to enjoy a good bite. According to researchers from Penn State's College of Agricultural Sciences, fall armyworm caterpillars' deceive corn plants by depositing their feces -- or "frass" -- onto the leaves of corn plants; the frass contain proteins that trigger wound-response defense and a pathogen-defense genes in plants instead of insect repelling VOCs.

To put it simply, when caterpillars release their frass, they buy themselves more time to snack.

"It would be disadvantageous for the insect to deposit cues that could enhance plant defenses against it, so we investigated what chemical compounds in the frass were signaling the plant," Dawn Luthe, professor of plant stress biology at Penn State, said in a news release. "It turns out that the caterpillar frass tricks the plant into sensing that it is being attacked by fungal pathogens and mounting a defense against them, thereby suppressing the plant's defenses against herbivores. Plants cannot defend against both pathogens and insect attackers simultaneously -- they must switch on either their pathway to defend against herbivores or their pathway to defend against pathogens."

Generally speaking, chemical signals pass between plants and the insects attacking them. When plants sense the saliva from chewing insects, they release volatile organic compounds (VOCs) to repel the insects and prevent them from causing too much damage.

When defending against pathogens, a plant may use a series of artillery. One of their defense mechanisms is called basal resistance. This is when plant cells strengthen to prevent any impending disease the pathogen may contaminate them with. Plants can also prevent infection through a process known as RNA silencing, where they digest genetic strands of a virus as it attacks. When the plants are confused by the chemicals in the caterpillar's frass, they can't respond quickly enough to defend against the munching insects.

For their study, the researchers applied frass extracts to corn plant leaves and compared the growth of caterpillars that ate these leaves to those that ate untreated leaves. From this they concluded that when the caterpillars weaken the plants' defenses, they are able to eat more and grow larger and faster.

"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. It is an ecological strategy that has been perfected over thousands of years of evolution," Swayamjit Ray, lead researcher and a doctoral student at Penn State, said in a statement.

The study's results may lead to safer pesticides that help plants boost their resistance to tricky pests.

The findings were recently published in the Journal of Chemical Ecology.

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