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

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.

Leaves grown under high CO2 lose a vital defense pathway, said U. of I. plant biology professor and department head Evan DeLucia, an author on the new study. Click here for more information.

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.”