To really sympathize with rice—and to understand why it's developed tricks for bossing insects around—I need you to imagine you're a plant throwing a party.
Have you got it? Let's say it's a sushi-making party, since, you know, you're a rice plant and you already have one ingredient.
Oryza sativa and you're growing in a field somewhere in Asia, and you're enjoying your party with the other rice plants. But then a notorious moocher shows up: the brown planthopper, Nilaparavata lugens. And it brings a whole crowd of its friends.
Next thing you know, the moochers are stealing all your sushi and eating it. Which is to say, they're causing severe crop damage. Brown planthoppers are one of the world's biggest rice pests. They feed on sap from the stems, carry viruses that infect rice plants, and lay new generations of eggs on the leaves.
Since that kind of behavior can ruin a party, you (the rice plant) want to drive the freeloading insects out. You can't physically remove them, so instead you change the tone of your party to something that's not at all their taste. Let's say you switch off the classical music and crank up some heavy metal.
Really, the rice plant emits a chemical called S-linalool into the air when it senses the familiar chewing. Brown planthoppers don't care for the molecule. But if any of them stick around, they'll be sorry. That's because the loud music simultaneously attracts the rice plant's real friends, who love both heavy metal and laying their eggs inside the eggs of brown planthoppers.
These metal fans are the parasitic wasps Anagrus nilaparvatae. Their larvae grow inside the planthopper eggs where they're laid, consuming the unhatched planthoppers from the inside out. And the same chemical signal that hustles the mooching planthoppers out of the party summons the wasps to punish any that stay behind.
Yonggen Lou at the Zhejiang University in China, along with other researchers, found out what the rice plants were up to by spying on them both in the lab and in the field. They knew already that rice plants emit S-linalool when they're chewed on by planthoppers. In fact, all kinds of plants are known to release certain compounds in apparent self-defense. But to understand the crashed house party one step at a time, the scientists created genetically altered rice plants that couldn't make S-linalool at all.
Starting with regular, non-altered rice plants, the researchers showed that S-linalool (the heavy metal music) was turned on whenever brown planthoppers fed on the plants—or when humans repeatedly stabbed the rice stems with tiny needles, imitating planthoppers in search of sap.
Next the researchers released groups of brown planthoppers into cages holding the rice plants. They saw that females preferred to hang out and lay their eggs on the genetically altered plants that couldn't make S-linalool. This means regular, unaltered plants, which could crank up the music when they wanted to, ended up with fewer pest eggs.
To explore the tastes of the parasitic wasps, researchers didn't let them see the plants or the planthoppers at all. Instead, they put the wasps in tubes and gave them whiffs of chemicals previously released by rice plants. The wasps followed the smell of regular plants that had been chewed by planthoppers—and released S-linalool—but weren't interested in the smell of genetically altered, non-heavy-metal-playing plants.
When the same genetically altered plants were growing in a field, scientists found more than twice as many female planthoppers on them (along with their eggs) than on the regular, S-linalool-producing plants. And the insects on the altered plants had significantly fewer parasitic wasps attacking them. There were also fewer predatory spiders on those plants. The rice plant, despite being stuck in one place and seeming pretty passive, is dictating in detail who's invited to its party.
Not all compounds emitted by plants are for deterring pests, though. The researchers also studied a second chemical that comes from the rice plant called (E)-?-caryophyllene. Presumably it's helpful to the plants, because they make it all the time. But brown planthoppers are attracted to it—as are their parasitic wasps. At the sushi party, let's call it the beer.
Yonggen Lou thinks farmers might be able to take advantage of the compounds rice plants naturally emit. For example, they could grow rice around the outside of a field that's genetically engineered to produce only (E)-?-caryophyllene (the beer) but not S-linalool (the loud music). These plants would attract both brown planthoppers and their pests. The rest of the rice plants in the field would be engineered in the opposite way, cranking up the heavy metal without providing any beer. Since pests would gather at the edges of the field, where the more attractive molecules were in the air, farmers could reduce their pesticide use and protect their crop. They're the ones, after all, who would really like to be in charge of the guest list.
Xiao Y, Wang Q, Erb M, Turlings TC, Ge L, Hu L, Li J, Han X, Zhang T, Lu J, Zhang G, Lou Y, & Penuelas J (2012). Specific herbivore-induced volatiles defend plants and determine insect community composition in the field. Ecology letters, 15 (10), 1130-9 PMID: 22804824
Image: Brown planthoppers by IRRI Images (Flickr)