“I talk to the trees, but they don’t listen to me…” sings one of the characters in the musical Paint Your Wagon. Maybe he’s just not speaking the right language.
Far from being inert lumps, plants can and do communicate–both with other plants, and, interestingly, with insects.
Of course, we’re not talking Shakespearean sonnets or even political speeches here. Plant communication is all based on chemicals.
The idea that plants communicate chemically with one another has been around for a couple of decades, but it’s only been in the last few years that solid scientific evidence has been accumulated to support the notion.
Over three seasons spanning 1996 through 1998, researchers from the University of California in Davis monitored wild tobacco plants growing near sagebrush. They clipped the leaves of some of the sagebrush plants to mimic the damage caused by insects. The sagebrush plants responded with a puff of a chemical called methyl jasmonate. In response, tobacco plants downwind immediately begin boosting the level of an enzyme called PPO that makes their leaves less tasty to plant-eating insects. Within minutes of the clipping of the sagebrush, the plants’ PPO levels quadrupled.
It worked, too. Tobacco plants next to the clipped sagebrush suffered sixty percent less damage from grasshoppers and caterpillars than tobacco plants next to unclipped sagebrush.
Then, last fall, scientists at Kyoto University in Japan let spider mites loose on lima-bean plants and tracked the plants’ responses. They found five different defense mechanisms. First, each injured plant released a chemical that changed its flavor, making it less attractive to the mites (although I personally nd it hard to imagine anything less attractive than the taste of a lima bean to begin with).
Then the plants released other chemicals that drifted away. Other lima bean plants received the chemical and immediately begin giving off the same chemicals, making themselves less tasty and warning still more lima bean plants, before the mites even reached them.
Most amazingly, some of the released chemicals had the effect of summoning a whole new batch of mites–mites that, rather than eating lima bean plants, preferred to eat the spider mites attacking the lima bean plants.
The Japanese researchers even found that the plants could distinguish between insect damage and crushing damage. They crushed some leaves and stems and found that although the injured plants released chemicals, the surrounding plants ignored them, somehow recognizing no real danger existed. (It appears that substances in the attacking insects’ saliva are required to trigger the anti-insect chemical response in the plant.)
Other examples from agriculture are also known. Corn under attack from armyworms, for instance, puts out a chemical signal that attracts a predatory wasp. The wasp lays its eggs inside the armyworm; when they hatch, the wasp larva eat the armyworm.
And a study released last week shows that this kind of signaling exists not only in agricultural situations and in labs, but in the wild–which means it is likely widespread throughout the plant kingdom.
Researchers from the Max Planck Institute for Chemical Ecology in Jena, Germany, discovered that when a species of wild tobacco plant that grows in the southwestern United States is damaged by hornworms (the larva of the hawkmoth) it releases chemicals that attract predatory insects that kill the larva.
Such chemical calls for helps benefit both the plant being attacked and the predators, who would otherwise find it very difficult to find the larva, which are camouflaged and also tend to feed on the underside of leaves, where they can’t be seen. Most likely the plant developed a chemical defense that predators evolved to take advantage of, but the result is the same as if the plant consciously sent out a cry for help.
Agricultural scientists are already looking for ways to use this new understanding of plant communication to improve the way farmers fight pests. For example, these kind of indirect defenses could be genetically engineered into crops so that they could more effectively summon natural predators. That could reduce the need for chemical pesticides–in fact, you’d want to avoid chemicals, because they would kill the predators you need to kill the pests. And whereas pests can develop a resistance to pesticides, they can’t develop resistance to predators.
The guy from Paint Your Wagon had it all wrong. Instead of talking to the trees, maybe he should have been listening to the lima beans.