Piles of dung attract flies.

The new study. . . used . . . an open-air research lab that 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.

This is silly. Sweeter leaves high in energy attract more insects when placed in the middle of lesser plants. In reality, all of the plants would be growing in CO2 rich environments, and so get their fair share of abuse.

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.

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.”

No. What they discovered was that low nitrogen plants are less able to defend against insect predation. This has long been known.

The issue is nutrient balance. CO2 is a nutrient just like water and the other 16 primary and secondary nutrients. The task for growers is to have them all available in the proper ratios for plant health and productivity. If one nutrient, CO2 in this case, is increased then others may also need to be increased to maintain balance.

Another consideration is adaptation. A realistic test would slowly increase levels over time since it is known that everything from soil microorganisms to the plants themselves adapt. There are even epigenetic effects that change seed, and so affect later generations. This matters greatly for annual plants such as soya. It is one of the known issues for growers that are remote from seed suppliers, especially when the climate zones vary. Locally adapted seeds do better.

I want to believe that the researchers are not as ignorant as the news account makes them sound.