Bug Bodyguards

by Romilla Karnick – Part-time science educator
11/22/02

Wild tobacco plants in the Great Basin Desert, Utah, may seem silent, but when attacked, these plants defend themselves by calling for help.

According to a study by researchers Ian Baldwin and Andre Kessler of the Max Planck Institute for Chemical Ecology in Germany, when tobacco plants are invaded by hungry herbivores, they release a chemical SOS that invites their insect allies to attack those herbivores.

A caterpillar makes a quick snack of a tobacco leaf Photo courtesy of: Charles Hedgcock RBP, ARL Division of Neurobiology, University of Arizona, Tucson

Tobacco plants often fall victim to herbivores such as caterpillars, leaf bugs and flea beetles. But when the plants are attacked by one of these enemies, they release a chemical signal in the form of a volatile organic compound (VOC). Humans can't smell the signal, but bugs can.

So, what is a volatile organic compound? Well, let's break it down to figure it out. An organic compound is a molecule with a main structure made up of carbon atoms. And, a volatile solution is one that can easily be turned into an aerosol – a substance that evaporates easily.

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Once this chemical is released into the air, the wind can carry it for hundreds of kilometres. When plant ‘bodyguards' such as parasitic wasps and flies sense the signal, they head for the tobacco plants in anticipation of a meal.

While the plants are waiting for their bodyguards to arrive, the VOCs continue working by discouraging the herbivore from laying eggs. Once the insect allies arrive, they descend on the existing eggs and larvae. This lowers the herbivore's rank in the food chain and protects the plant.

Although researchers have known for a while that plants are not powerless, this study is the first to confirm that plants in the wild use chemical signals to protect themselves.

Before Baldwin and Kessler's study, scientists were aware that if some plants were attacked, they released poisons that warned their predators to back off. However, these direct mechanisms were not always foolproof. Many plant-attacking insects and mammals adapted to the poisons by developing a more tolerant digestive system that allowed them to continue eating the plants.

A recent study by entomologists May Berenbaum and Xianchun Li at the University of Illinois has shown that some insects produce detoxifying enzymes. When the insect's body detects a poison, it switches on a special gene that releases a neutralizing protein. Of course, this newly discovered gene doesn't protect the insect from its predators.

Releasing chemical signals instead of poisons allows plants to defend themselves indirectly – letting other organisms take care of the attacking herbivore. So, next time you catch a caterpillar lunching on a leaf, you might soon spot the plant's bug bodyguard rushing to the rescue!