Crops, Beetles, and Carbon Dioxide:
As spring warms the heartland, farmers prepare to plant, and soon the nation's flat brown fields will be green with corn, soybeans, and other staple crops. But as the carbon dioxide in the atmosphere skyrockets and the climate changes, will farmers be able to grow enough crops to feed the nine billion people expected to be living on the planet by midcentury?
It may be harder than many had assumed, according to recent studies on experimental farm plots in Illinois. That's because rising carbon dioxide levels could allow insect pests to take a bigger bite out of crops, thereby reducing yields.
After rising carbon dioxide emissions and climate change were recognized as a looming global problem, agronomists began testing how crop plants would respond. They knew that raising levels of carbon dioxide accelerated the photosynthetic machinery in most crop plants, and they hypothesized that crops would grow faster and yield more, says plant biologist Evan DeLucia of the University of Illinois, Urbana-Champaign (UIUC). What's more, early tests of this hypothesis—in greenhouses, growth chambers, and open-top enclosures in the field—seemed to confirm it. Climate change contrarians crowed, claiming that rising carbon dioxide levels would lead to a "greening earth."
But the key experiments still hadn't been done. Crop plants on farms don't grow in enclosures; they're exposed to the elements, to viral and fungal pathogens, and to their greatest nemesis, insects. To make more realistic projections, DeLucia and several UIUC colleagues built a large-scale field apparatus that used a system of gas tanks, pumps, and computerized controls to test how rising levels of heat-trapping carbon dioxide would affect soybeans and corn—two mainstays of Midwestern farmers—grown in the open field..
The first results from the field station, known as the Soybean Free Air Concentration Enrichment—SoyFACE for short—were jarring. Under these realistic field conditions, elevated carbon dioxide boosted yields of soybean and corn only half as much as it did for plants in enclosures. "Higher carbon dioxide does lead in an agricultural context to more production," DeLucia says. "But that increase in production has been greatly overestimated by experiments done in enclosures."
DeLucia and his team soon uncovered a clue about why this was happening. More aphids and Japanese beetles flocked to feed on the high carbon dioxide soybean plots than to the plots grown under today's ambient carbon dioxide levels. What's more, a laboratory study by entomologist May Berenbaum of UIUC, who collaborates with DeLucia to understand how soybean and its pest insects interact, showed that beetles that fed on leaves from the high carbon dioxide plots lived longer and laid more eggs. "That means crop losses may go up in the future," DeLucia says.
Two researchers in DeLucia's lab, Jorge Zavala and Clare Casteel, then launched a series of studies that helped the researchers understand why. They showed that under high carbon dioxide conditions, plants deactivate three genes that help them produce chemical defenses against insects. One of those defenses is a protein that blocks enzymes in the beetles' stomach that digest food, the researchers reported in Proceedings of the National Academy of Sciences in 2008. With the plants' defenses down, beetles can digest more of them, which helps explain why they devoured the soybean leaves and grew and reproduced faster. The results may help explain the lower then expected crop yields under elevated carbon dioxide.
In a separate study presented at the Entomological Society of America conference in December in Indianapolis, Casteel found that high carbon dioxide conditions also deactivate genes that help soybean plants defend themselves indirectly by pumping out chemicals called "green leaf volatiles," which lure natural enemies that kill insect pests. Soybeans grown under elevated carbon dioxide "appear to be helpless against herbivores [plant eaters]," Casteel says.
Other FACE experiments have studied the effects of tomorrow's atmosphere on wheat and rice, and they have also shown lower-than-hoped-for yields, UIUC's Stephen Long and colleagues reported in Science a few years ago. FACE studies like these help scientists understand how crop plants will respond to the warmer climate expected later in the century. They also highlight the need to reduce heat-trapping emissions to ensure that farmers can continue to provide the world with healthy, nutritious food.
Jorge A. Zavala, Clare L. Casteel, Evan H. DeLucia, and May R. Berenbaum. Anthropogenic increase in carbon dioxide compromises plant defense against invasive insects. Proceedings of the National Academy of Sciences, vol.105, no. 13 (2008), pp. 5129–5133.
Clare L. Casteel, Bridget F. O'neill, Jorge A. Zavala, Damla D. Bilgin, May R. Berenbaum, And Evan H. Delucia. Transcriptional profiling reveals elevated CO2 and elevated O3 alter resistance of soybean (Glycine max) to Japanese beetles (Popillia japonica). Plant, Cell and Environment (2008) vol. 31, pp. 419–434.
Clare L. Casteel, May R. Berebaum, and Evan H. Delucia. Does elevated carbon dioxide universally alter phytohormone signaling? Entomological Society of America annual meeting, December 14, 2009, Indianapolis.