Most animals prefer to eat them. They can't hit back or run away. Yet plants dominate vast areas of Earth's continents.

How Can Plants Have So Many Natural Enemies And Yet Be So Successful?

Of course, some plants have physical defenses such as thorns and spines. But ecologists are finding that many more plants use chemical weapons to defend themselves.

Some plants make their own insect repellents and insect poisons a simple but effective strategy.

Insect Hormones

Other plants make chemicals that closely resemble insect hormones. An insect larva feeding on such a plant may have its development disrupted.

For example, an African plant called the bugleweed has been found to make an imitation insect hormone that causes many insect larvae to grow two or three heads. The mouthparts of a multi-headed larva don't work properly, so the larva starves to death.

Aphid Alarm Pheromone

Even more sophisticated is the strategy used by a type of wild potato to defend itself against aphids.

The plant manufactures the active ingredient of the so-called aphid alarm pheromone, a chemical that an aphid under attack releases into the air to warn other aphids of danger. Aphids run away from the wild potato plant's chemical signal as if it were a danger signal from an aphid.

Plants Are Important Defenders

There are a couple of general lessons to be drawn from the ecologists' ever-growing collection of plant chemical-defense stories.

One lesson is the importance of preserving the great natural diversity of plants, because we can learn so much chemistry from them.

Also, the very specific nature of the chemical weapons used by many plants gives us hope that we humans can develop pesticides that will hurt only their intended targets.