Agriculture: Pests and pesticides

Michael is in his garden when he notices some of the leaves have tiny holes in them. When he looks closer, he realises a green caterpillar is munching on the leaf. This caterpillar is called a hornworm. Hornworms live and feed on tomato plants and other crops. This damages the crops.

Hornworms are pests. Pests are destructive organisms that attack and damage crops. Pests can be microorganisms, such as fungi or bacteria; small animals, such as insects or snails; bigger animals, such as birds or rats. Even certain plants can be pests! All these can affect crops and decrease the amount of produce harvested — the yield.

So, farmers and gardeners have developed different methods for getting rid of pests. One method is to use substances that are poisonous for specific types of pests. Such substances harm the pests, and often kill them. These are pesticides. Pesticides vary in terms of how they are produced, how long they remain in the environment, and how many different pests they affect.

Some organisms, for instance tobacco, produce substances that kill pests. We can extract these natural substances and use them to protect crops. These are natural pesticides. Many pesticides, however, do not occur naturally. We manufacture them through chemical processes.

These are synthetic pesticides. Both natural and synthetic pesticides contain toxic substances. Some pesticides affect only a few specific pests, such as beetle larvae or fly larvae. These are narrow-spectrum pesticides. Other pesticides are poisonous to a wide range of pest species.

These are broad-spectrum pesticides. Apart from eliminating pests, broad-spectrum pesticides might also unfortunately harm pests’ natural enemies, or insects that pollinate plants. So pesticides kill pests very efficiently, and allow farmers to significantly increase yield. But pesticides have downsides too. Pesticides can be harmful to human health, especially the health of those who handle them.

Some pesticides remain in the environment for a long time, accumulate in other organisms or cause pollution. If the same type of pesticide is used over a long period, pests can become resistant to it, so that pesticide is no longer effective. Some farmers try to control pests without using pesticides. One way they can do this is by attracting pests’ natural enemies. Natural enemies can be birds that feed on caterpillars, or worms that live in soil and eat pest larvae.

They eliminate pests but don’t harm crops. Most pests attack one or a few specific types of plants. If that type of plant is grown in the same field continuously, it can attract huge quantities of pests, which can cause great damage to the crops. So, farmers can divide fields into smaller plots, and rotate crops throughout the year. This confuses pests, and doesn’t let them spread too much over a large area.

Growing a variety of plants in the same area simultaneously has another advantage. Some plants release substances that deter certain pests, or attract their natural enemies. And this protects other plants that grow nearby too! Some pests, like weeds or snails, can also be removed by hand. There are a few things Michael can do to protect his tomatoes from hornworms.

He can spray organic or synthetic pesticides that kill them, or he can pick off the caterpillars by hand. He can also plant dill, mint or thyme next to his tomatoes. When these herbs flower, they attract wasps that kill off hornworms. Not only will Michael get rid of pests, but he will also have fresh herbs to hand!