Think your garden is a peaceful place?
There's a never-ending war being waged between your plants and the insects that feed on them.
Plants constantly emit chemical signals to attract predators that can kill pests threatening their survival. And an increasing number of researchers say that plants may actually use chemical signals to "talk" to one another, sending out alarms to neighbors when they're under attack.
"There are plants that shout out loudly for help, while others produce whispers, and others still don't produce any sound at all. What you have is different plants speaking different languages," says Marcel Dicke, an entomologist at Wageningen University in the Netherlands.
A half-dozen research groups around the world are now studying this arcane and controversial corner of plant biology -- which first attracted attention two decades ago with tabloid headlines that read: "Trees Talk."
For that, scientists credit Jack Schultz and Ian Baldwin, Dartmouth researchers whose 1983 study in the prestigious journal Science first gave credibility to the notion. They reported that poplar trees sharing an enclosed chamber with leaf-torn poplars showed more signs of stress than poplars in a control chamber with undamaged trees.
They concluded that the damaged poplars sent out signals that warned other trees about their stressful conditions. Once warned, the neighbors increased production of compounds that help combat stress.
"It was a contentious issue at the time, and it did make a lot of enemies. For one thing, in and of itself it sounds pretty crazy," says Schultz, now an entomologist at Penn State University. "A lot of scientists turned their backs on us."
Schultz is still convinced that trees signal other trees when they're infested with insects, exposed to drought or damaged in some way. They likely communicate through root networks but may also send out isoprene and other carbon-based emissions through their leaves when under stress, he said.
"For one thing, trees spew out huge amounts of chemicals in the air. They're in fairly small concentrations, so you can't see them. But they're there," Schultz said.
Scientists have since begun investigating how other plants communicate. Operating like our central nervous systems, a plant's signaling system controls essential functions, such as leaf development, the spread of roots and the production of tissues to carry water.
"The thing about plants is, they're incredibly active," said Jennifer Thaler, a Cornell University entomologist.
Some 500 pests are known to attack plants, and each pest has 80 to 100 natural enemies, according to Dicke. Potatoes attract spiders to kill Colorado potato beetles. Cotton plants send out signals to predatory aphids to combat other aphids. Tomatoes attract parasitic wasps that will lay eggs and produce larvae that feed on caterpillars' internal tissues.
"There's an arms race going on constantly between the plant and the insect," said Ken Korth, a biology professor at the University of Arkansas.
How far plant signals travel is anybody's guess. James Tumlinson, an entomologist at Penn State, estimates that to reach predatory insects, the signals must travel hundreds of yards in some cases.
But the thornier question is whether plants receive and respond to signals from each other. Many researchers say it happens, while others remain skeptical, saying there's too little evidence for it.
"It's an area that's subject to question. When you get into plants receiving signals, there's still more that needs to be shown," said Wallace Joe Lewis, an entomologist with the U.S. Department of Agriculture research lab in Tifton, Ga.
"We have a lot of information about the signaling systems within plants. But we just don't have that much information about these long-distance signals," added Leslie Sieburth, a biology professor who studies plants at the University of Utah.
But Tumlinson and other researchers say that because plants can't move, they've developed an ability to respond to airborne signals through their stems and leaves. There have been some intriguing study results so far.
Researchers showed last year that damaged corn seedlings release volatile compounds as an early-warning system for neighboring corn plants. Thus warned, the neighbors release their own chemicals, making the plants harder for caterpillars to digest. The findings were published in Proceedings of the National Academy of Sciences.
An earlier study found that damaged sagebrush plants grown in the California desert emit volatile compounds that warn nearby tobacco plants of pest infestations. Researchers at the University of California at Davis found that the tobacco plants responded to the warnings by increasing nicotine levels, making the plants more resistant to grasshoppers and other insects.
Understanding a plant's natural defenses should lead to alternatives to pesticides, researchers say.
"If we understand how the signals work, we may be able to find crop breeds that are better equipped to defend themselves naturally," Tumlinson said.
Wasps to the rescue
Tumlinson, who was co-author of the corn-seedling study, first looked into plant communications in the early 1990s, when he tried to determine how a wasp sorts out airborne signals in its search for the caterpillars it needs to survive.
He found that when a caterpillar attacks a corn seedling, the seedling sends out a preliminary signal that's mildly attractive to wasps, followed by a set of stronger signals.
The two-step process allows plants to store up some defensive chemicals for serious attacks.
"If you're a plant, you only have so much energy to put into your defenses," Tumlinson said.
A key to the process is usually chemicals emitted by damaged leaves or even the presence of saliva from an attacking insect. Caterpillar saliva on leaves induces plants to release chemicals that not only alert neighboring plants to the pests, but also make the plant harder to digest.
"Once the caterpillar chews on the plant, it releases these compounds and sets the whole process in motion," Tumlinson said.
The communication signals are only now being deciphered -- in part because the systems are so complicated, experts say.
Plants send out different blends of chemicals, depending on the species of plant, the type of insect and the section of plant anatomy under attack, such as stems or the leaves. Different genetic varieties of the same plant may also send out different signals.
"Every plant has an enemy and a unique response to that enemy," says Thaler.