Study: Global warming might help insects in war on plants
When University of Illinois researchers found that soybean plants heavily exposed to carbon dioxide at an open-air lab here suffered more insect damage – and that the insects damaging them lived longer – they wondered what was behind the effects.
The result: a UI study indicating that higher carbon dioxide levels associated with man-made emissions, generally thought to play into climate change and global warming, appear to make plants, soybeans anyway, more susceptible to some insect damage by impairing their chemical defensive systems.
Plants exposed to high carbon dioxide levels in the study lost their ability to produce jasmonic acid, the hormone which, when insects attack, starts a chain of chemical reactions in plant leaves leading to the production of a compound called a protease inhibitor, say UI professors May Berenbaum and Evan DeLucia.
Plants use protease inhibitor to protect themselves against pests – Japanese and Western corn root worm beetles in the UI study, published online this week by the Proceedings of the National Academy of Sciences. Insects that ingest the compound, a type of protein called an enzyme, are unable to digest the leaves properly and die sooner.
DeLucia said the enzyme leaves the insects feeling like they've consumed the mother of all Thanksgiving dinners despite having fed minimally, so they stop eating.
Absent the compound, adult insects feast, live longer and produce more offspring – and do more damage. Berenbaum, who heads the UI Entomology Department, DeLucia, head of the Plant Biology Department, and colleagues want to study whether the effect occurs in other plants, whose defense mechanisms may vary.
Still, Berenbaum said the results raise a new concern in the complex web of impacts from climate change. One school of thought has plants benefitting from more carbon dioxide by accelerating photosynthesis and boosting plant growth.
"That's the multimillion dollar question," Berenbaum said when asked if the enhanced photosynthesis might offset damage like that identified in the UI study, as well as increased damage from caterpillars and other insect larvae, aphids and other pests likely to thrive with warming.
"I think (the study) undermines the assumption that everything is going to be rosy," she said.
All the more so if climate change eases the way for invasive species like the Japanese beetle, a relatively recent arrival to Illinois soybean fields, which Berenbaum said only promises to do more damage in the future.
The researchers did their study at the UI's Soybean Free Air Concentration Enrichment facility, or SoyFACE, which can expose plants to varied carbon dioxide and ozone levels without removing them from typical conditions involving rainfall, sunlight and pests. In effect, the facility simulates a field growing amid what is expected to be the atmosphere of the future, 2030 to 2050 at this point.
"The plants are not enclosed," DeLucia said. "There's no wall around them. There's no cage over them. Everything else is normal and open. It's really the reason we could make this discovery."
They figured higher carbon dioxide levels would have a couple of effects by enhancing photosynthesis and altering the nitrogen-to-carbohydrate ratio in the plants.
Caterpillars and other larvae need nitrogen and would likely have to eat more to get it. Meanwhile, the sugary carbohydrate boost promised to be attractive to adult insects such as the Japanese beetle, which Berenbaum called a sugar freak.
They did indeed see more beetles, and more Asian soybean aphids during an outbreak of those pests, along with more plant damage.
But the UI researchers found that plants exposed to high carbon dioxide levels at SoyFACE suffered inexplicably severe damage, beyond even plants in a test plot outside the facility that had their sugar content artificially boosted. Moreover, the pests in the plot bombarded with carbon dioxide lived longer and reproduced more.
That set the researchers, both affiliated with the UI's Institute for Genomic Biology, to looking for answers at a molecular level.