A、By flying in circles around a corn field. B、By egging offspring who eats the insects. C、By inspecting individual corn leaves.

Cultural controls are the oldest methods that have been used to manage pest populations. However, with the development of synthetic pesticides these controls were rapidly abandoned or de-emphasized and research on them was largely discontinued.(19)Today, we are going to talk about a natural way some plants respond to being invaded by pests. The plants react by emitting a chemical signal, which acts like a call for help. Let’s take corn plants for example. Sometimes,(20)a kind of insects that cannot fly chew on the corn leaves; when this damage happens, the plant releases a chemical scream that attracts a kind of bees. The bees respond to the signal by flying to the chewed on leave, and laying their eggs on the insects.(21)The insects die in the next few days as the bees’ offspring will eat the insects. Thus the corn plant prevents all its leaves from being eaten by the insects. This chemical scream is specific. It’s only released after the plant has damaged by the insects. A plant damaged in other ways does not give off the same signal, nor does undamaged plants. Cotton and probably many other plants use similar type of defense against pests.(22)By enhancing this natural response in plants, researchers might reduce, some day even eliminate the need for chemical pesticide, which can cause ecological damage. For example, scientists might breed plants for this screaming trait, or they might transplant specific genes to increase the release of the chemical signals.
19. What’s the main purpose of the talk?
20. What causes the corn plant to emit the chemical signal?
21. How do the bees protect the damaged corn plant?
22. What is the meaning of enhancing this natural response in plants?