Crop-Damaging Insects Proliferate in Higher-CO2 World

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“The Japanese beetle, as the name suggests, is a relatively recent
arrival in Illinois soybean fields. It is causing considerable damage
now but this study suggests that its ability to inflict damage will
only increase over time.”
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Public release date: 24-Mar-2008
University of Illinois at Urbana-Champaign

Contact: Diana Yates
diya@uiuc.edu
217-333-5802

Insects take a bigger bite out of plants in a higher CO2 world

Atmospheric carbon dioxide levels are rising at an alarming rate, and
new research indicates that soybean plant defenses go down as CO2
goes up. Elevated CO2 impairs a key component of the plant’s defenses
against leaf-eating insects, according to the report.

The University of Illinois study appears this week online in the
Proceedings of the National Academy of Sciences.

Deforestation and the burning of fossil fuels have significantly
increased carbon dioxide levels since the late 18th century, said
plant biology professor and department head Evan DeLucia, an author
of the study.

“Currently, CO2 in the atmosphere is about 380 parts per million,”
DeLucia said. “At the beginning of the Industrial Revolution it was
280 parts per million, and it had been there for at least 600,000
years – probably several million years before that.”

Current predictions are that atmospheric carbon dioxide will reach
550 parts per million by the year 2050, DeLucia said, and the rapid
industrialization of India and China may even accelerate that
timetable.

The new study, led by entomology professor and department head May
Berenbaum, used the Soybean Free Air Concentration Enrichment (Soy
FACE) facility at Illinois. This open-air research lab can expose the
plants in a soybean field to a variety of atmospheric CO2 and ozone
levels – without isolating the plants from other environmental
influences, such as rainfall, sunlight and insects.

High atmospheric carbon dioxide is known to accelerate the rate of
photosynthesis. It also increases the proportion of carbohydrates
relative to nitrogen in plant leaves.

The researchers wanted to know how this altered carbon-to-nitrogen
ratio affected the insects that fed on the plants. They predicted the
insects would eat more leaves to meet their nitrogen needs.

When they exposed the soybean field to elevated carbon dioxide
levels, the researchers saw the expected effect: Soybeans in the test
plot exhibited more signs of insect damage than those in nearby
plots. A closer inspection showed that soybeans grown at elevated CO2
levels attracted many more adult Japanese beetles, Western corn
rootworms and, during outbreaks of Asian soybean aphids, more of
these than soybeans in other plots.

Caterpillars and other insect larvae need nitrogen to grow and build
new tissues, but adult insects can survive and reproduce on a high
carbohydrate diet. So it made sense that more adults would migrate to
the high CO2 plants, DeLucia said.

But did the higher sugar levels in the leaves explain the whole
effect” To find the answer, the team allowed beetles to live out
their lives in one of three conditions: on a high CO2 plant, on a low
CO2 plant outside the Soy FACE plot, or on a low CO2 plant grown
outside the test plot but which had its sugar content artificially
boosted.

“What we discovered was startling,” DeLucia said.

The beetles on the high CO2 soybean plants lived longer, and as a
result produced more offspring, than those living outside the Soy
FACE plot. Even those fed a supplemental diet of sugars did not see
their life span extended.

“So here we were thinking that sugars were the main thing causing the
beetles to feed more on these high CO2 leaves,” DeLucia said. “And
that still may be true, but sugars aren’t what’s causing them to live
longer and have more breeding events and offspring.”

The team turned its attention to the hormonal signaling pathways of
the plants, focusing on a key defensive chemical the plants produced
to ward off an insect attack. When insects eat their leaves, soybeans
and other plants produce a hormone, jasmonic acid, that starts a
chain of chemical reactions in the leaves that boost their defenses.
Normally this cascade leads to the production of high levels of a
compound called a protease inhibitor. When the insects ingest this
enzyme, it inhibits their ability to digest the leaves.

“What we discovered is that leaves grown under high CO2 lose their
ability to produce jasmonic acid, and that whole defense pathway is
shut down,” Delucia said. “The leaves are no longer adequately
defended.”

The higher carbohydrate content of the leaves and the lack of
chemical defenses allowed the adult insects to feast and live longer
and produce more offspring.

“This study demonstrates that global environmental change is
multifaceted,” Berenbaum said. “The impact of elevated carbon dioxide
on crippling the capacity of the plant to respond to insect damage is
exacerbated by the presence of invasive insect pests in soybean
fields. The Japanese beetle, as the name suggests, is a relatively
recent arrival in Illinois soybean fields. It is causing considerable
damage now but this study suggests that its ability to inflict damage
will only increase over time.”

The researchers, both of whom also are affiliated with the
university’s Institute for Genomic Biology, will now seek to
determine whether the same process occurs in other plants.

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EMBARGO FOR RELEASE UNTIL 4 P.M. CDT MARCH 24 (MONDAY)

Editor’s note: To reach Evan DeLucia, call 217-333-6177; e-mail:
delucia@uiuc.edu.

To reach May Berenbaum, call 217-333-7784; e-mail: maybe@uiuc.edu.

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