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United States Department of Agriculture

Agricultural Research Service

When Insects Attack, Some Plants Fight Back

When you see an insect pigging out on a leaf, you probably think that plant's toast! But scientists know not all plants are easily bullied.

Some use the buddy system. Some cotton plants call friendly wasps to protect them from hungry caterpillars like beet armyworms.

When a caterpillar chomps on the plant's leaves, it leaves behind its spit, or saliva (nasty, nasty, na-a-asty). A chemical in the bug spit tips off the plant that it is under attack.

The plant then releases chemicals of its own, called terpenoids, from its wound. These terpenoids float into the air where they become an SOS. Wasps to the rescue! Terpenoids signal a couple of kinds of plant-rescuing wasps, which make a bee-line (har, har, har. Get it? ) straight to the plant.

By their "first names," the wasps are Cotesia and Microplitis. These wasps don't sting people. And unlike yellow jackets, these wasps don't like soda pop. But they do have an awesome sense of smell. That's how they home in on the cotton plants' chemical distress signal.

It sounds gross, but the wasps also follow the scent of a caterpillar's frass, that is, bug poop. The wasp pounces on the caterpillar and lays an egg on or near its body. After a few days, a tiny wasp larva hatches. And is it ever hungry! It eats the caterpillar alive, gobbling up its blood and fat.

A couple of days later, the maggot spins a cocoon so it can pupate and become an adult wasp. This change takes about 2 weeks. Full grown, the wasps can begin patrolling the cotton patch with their noses. That's helpful to farmers, who would might otherwise have to spray chemicals to protect their crop from pesky caterpillars.

-- By Jan Suszkiw, Information Staff, Agricultural Research Service

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Last Modified: 2/14/2011
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