We’ve all seen the images of receding glaciers and stranded polar bears that accompany talks of climate change. But rising carbon dioxide levels also have subtler and less familiar effects, and may prove to be a boon for many animal groups. Plant-eating insects, for example, have much to gain in a high -CO2 future as rising concentrations of the gas can compromise the defences of the plants they feed on.
Plants and herbivorous insects are engaged in a silent war that we are rarely privy too, where chemicals act as both weapons and messengers. Munching mandibles trigger the production of signalling molecules like jasmonic acid that announce the presence of invaders to other plants of the plant, neighbouring individuals, or even parasitic wasps, which attack the pests and turn them into living larders for wasp eggs.
The battle isn’t over even after parts of the plant are eaten. Beetles, for example, rely on enzymes called cysteine proteinases to digest the plant proteins they swallow, and free up valuable amino acids for the insects’ own growth. But when plants detect jasmonic acid signals, they produce chemicals called “cysteine proteinase inhibitors” (CystPIs) that block the insects’ digestive enzymes and prevent them from fully digesting their meals.
But these defences may buckle as carbon dioxide levels rise. Jorge Zavala and colleagues at the University of Illinois found that increasing levels of CO2 reduce the ability of soybeans to use jasmonate signals. That shrinks their stockpiles of defensive CystPIs and makes them more vulnerable to hungry pests including two voracious species of beetle – the western corn rootworm (Diabrotica virgifera) and the Japanese beetle (Popillia japonica). Given that soybeans are an increasingly important food crop, it’s in our interest to stop insects from eating them so that we can instead.