How to Win the War on Bugs? Listen to Plants. They’ve Been Fighting Longer Than We Have.

How can the Earth support 9 billion people?
April 18 2014 11:45 AM

Listen to the Plants

They talk to bugs and birds all the time in a language that could be the future of food.

Adult citrus root weevil, Diaprepes.
An adult citrus root weevil.

Courtesy of Keith Weller/USDA

We are losing the war on bugs. Every year, we dump 1 billion tons of insecticides on them; every year, they eat up to a fifth of the crops we grow. It’s a lose-lose scenario. Insecticides are expensive to make and use: Apple trees, for example, must be sprayed 20 times a year. Poisons have side effects: sick farmworkers, nightmarish industrial accidents, even the die-off of butterflies and bees. Eventually, pests evolve and become resistant to the pesticides, and they stop working.

There is another way. It involves listening to plants, which have fought this battle much longer than we have.

When a caterpillar or a beetle starts chewing on a leaf, the plant responds by synthesizing its own defense chemicals in an attempt to drive away the insect. It also releases a chemical plume into the air, a message that can be intercepted by creatures nearby. Other plants respond to these alerts by producing their own chemical weapons, substances that repel leaf-eating insects. Predators that eat plant pests also detect the signals, using them like beacons to locate their prey.

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This chatter is going on all the time, all around us. A moth or wasp or a bird flitting through a field is barraged by constant status updates about which plants and creatures are nearby and what they’re up to. It’s the biggest and most important conversation on Earth. But until recently we had no idea it was happening, since human noses can’t detect most of these chemical communiqués. (The scent of cut grass is one exception. To us, it smells like summer, but to other plants, it’s a warning that something is attacking.)

The alfalfa plant bug, Adelphocoris lineolatus, is a non-native plant pest.
The alfalfa plant bug is a non-native plant pest.

Courtesy of Scott Bauer/USDA

The first evidence that plants might be able to communicate with one another came in 1983, with a study exploring the defense mechanisms of willow trees. When one willow tree’s leaves got chewed up by caterpillars, researchers discovered, not only that tree but also willows nearby began synthesizing compounds to fend off bugs. It was an amazing find: Somehow, trees could share information, without ears, eyes, or brains. And it soon turned out that most, if not all, plants can send and receive these airborne chemical signals.

Cracking the plant code offers a new opportunity for our agricultural future. Researchers who study plant signals now foresee an era in which crops can identify pests, deploy natural bug-deterring compounds, and summon their own bug-eating bodyguards—all without toxic chemicals.

“We can’t feed the world with this technology at the moment,” says chemist John Pickett, one of the pioneers of this work. “But we’re not going to be able to feed the world tomorrow with today’s technology.”

To reach this future, we will have to reconceptualize the war on bugs. Now, we treat it like a conventional war, where the way to win is to make more and deadlier weapons to kill more opponents. We have to start thinking about it like a war of intelligence, won by spying on our enemies and using their own codes to trick and defeat them.

The first step will be mastering the language. Plants can make thousands of volatile compounds, and just one emission may blend more than 200 different chemicals. Sending the wrong message could be a disaster: In one study, corn plants treated with some volatile compounds actually became more attractive to armyworms and cucumber beetles.

Using techniques like gas chromatography to separate out individual compounds, Pickett’s team at Rothamsted Research in the United Kingdom is identifying several volatile chemicals that many plants respond to, such as cis-jasmone. Well-known in the perfume business, cis-jasmone is partly responsible for the intense sweetness of jasmine flower, but it also works as a signal to activate plant defenses. When Pickett sprayed synthetic cis-jasmone on plants ranging from soybeans to cotton, they began producing compounds to repel aphids and other bugs.  

Plants aren’t the only ones listening to these messages. Predatory bugs and caterpillar parasites sniff out the emissions of a plant being eaten, and like Batman responding to a bat signal in the night sky, swoop in for the rescue. Lima bean plants infested with sap-sucking spider mites attract predatory mites this way, chemical ecologist Marcel Dicke of Wageningen University in the Netherlands discovered. He and others are working to identify which insects respond to which chemicals. Their go-to technique involves cutting the antennae off of predatory insects, wafting plant emissions over the disembodied antennae, and using electrodes to monitor the electrical responses of odor-sensing cells. They have compiled a long list of creatures that recognize plant distress calls, including thrips, predatory stink bugs, beetles, ladybirds, tiny wormlike nematodes, parasitoid wasps that lay their eggs on caterpillars, and even birds.

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