MAY
2003

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

05-08-03

Plants Battle Insects in Chemical Warfare

The eternal cycle of life on Earth begins with plants. Yes, plants. These green, leafy things capture the energy in sunlight through the magic of chlorophyll and use it to build plant tissue from water and minerals. That means plants can survive without other living things.

Animals, the other major group of living things, can't produce tissue from minerals and water. So animals need help from other living things to survive. That help is in the food they eat. Animals get the energy for life from their food. Some animals feed on dead stuff, either plant or animal in origin. Other animals feed on living things. We call such creatures carnivores or predators. Still other animals are plant feeders, which are known as herbivores.

It is an "eat-or-be-eaten" world out there in nature. As a general rule, living things try not to become meals for animals. The benefits for such a relationship are decidedly one-sided. Predation is of benefit to the eater but a real bummer for the animal that becomes a meal. So much so that potential prey do what they can to avoid providing food for another animal.

Avoiding predation is no simple matter. Some animals try to escape by running or flying. Hiding to avoid detection is also a useful tool. So is having a shell or a very thick skin. Still other animals pretend they are fearsome and to be avoided. A few prey animals are themselves dangerous.

Plants, on the other hand, can't run or hide when a vegetarian meal-seeker shows up. To borrow a line from that famous Muppet frog Kermit, "It isn't easy being green!" Especially if you are a plant with all of that energy just waiting to be transferred to some animal.

Plants are by no means helpless in the matter of defense against herbivores. In fact, those green energy producers are down right ingenious when it comes to avoiding meal seekers.

Some plants have spines and thorns. These projections are useful to ward off big plant eaters, including mammals like cows and deer. Some plants grow so tall that it is hard for land-dwelling leaf eaters to reach a meal. We call these plants trees.

Insects destroy more plant tissue than any other group of animals. In fact, about half of all described insect species feed on plants. This includes insects that are leaf feeders, stem borers or root feeders. Insects that suck sap from plants are also herbivores. Even those that cause galls on plants are technically plant feeders.

The major approach that plants use to avoid plant-feeding insects is, in general terms, known as chemical warfare. Plants produce what have come to be called noxious phytochemicals. These bad plant chemicals are harmful to animals that feed on the tissue.

Noxious phytochemicals are in many varieties of plants. One such chemical is called cucurbitacin. Cucurbitacin is found in plants like cucumber, squash, watermelon and gourds. These plants belong to the Cucurbitaceae family. Cucurbitacin is one of the bitterest substances known to humans. These plants have used the chemical to keep many animals, including lots of insects, from feeding.

Humans have developed plants with low levels of curcurbitacin for our food. The slightly bitter taste you get from the skin of a cucumber or the rind of a watermelon reminds us that a little of the bitter substance remains. The same is true of the rind of lemons or limes. The bitter chemical is limonene.  

Many of these chemicals have found other uses among humans. For instance, caffeine in coffee or tea or nicotine in tobacco has come to be used as a stimulant. Some, including nicotine and pyrethrin from the chrysanthemum plant, have found use as insecticides. Others are dangerous poisons to any animal, such as cyanide and strychnine.

This chemical approach to warding off plant feeders generally has worked for plants. But as is always the case in nature, some plant feeders have managed to overcome the tactic. A few insects have adapted to such chemicals and even thrive on them.

One example is the cucumber beetle, which loves to eat plants with cucurbitacin in them. But that is no surprise to gardeners who engage in the battle of the beetles feeding on cucumbers every year! Oh well, gardeners and Mother Nature can't win them all when it comes to insects feeding on plants.

 

Writer: Tom Turpin
Editor: Olivia Maddox