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. These weekly columns on science appear in the Regina (Saskatchewan) Leader Post and Red Deer (Alberta) Advocate and on CBC Radio One's Afternoon Edition in Saskatchewan.. They are available for one-time publication or regular syndication to any interested newspapers, magazines or on-line publications. E-mail me for details. Posted March 19, 2001
One day maybe with genetic meddling the plants will come up with a way to ward off humans. I'm glad someone has figured out that there is more to lima beans than bad taste. Carol Ja
