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Caterpillars Bypass Corn Plant Defenses to
Grow Bigger and Faster, New Research Shows
Caterpillars trick corn plants into lowering their defenses so that they can grab a larger bite to eat.
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.