Script 36.6


Most insects, plants, and animals are friends of farmers. For instance, of 100 insects that might be found in a farmer’s field, only one kind of insect will be a pest and the other 99 are friendly insects.

You can reduce the number of pests by putting these friends to work for you. The technique, called ‘biological pest control’ does not kill all the pests so some crop damage may still occur. However, the yield may not be affected very much. For example, insects such as leaf caterpillars chew on the leaves of corn plants. This makes the plant look sick but it may not be seriously injured and continues to grow.

Biological pest control works better if no chemical pesticides are used. Biological pest control does not pollute your field or garden or your water. It is safe for people, plants, and animals. Chemical pesticides, on the other hand, can remain in soil and water and also in the air you breathe, on the food you eat, and in mother’s milk. Biological pest control usually kills only bad insects. Other pesticides kill both good and bad insects.

You use biological pest control much less frequently than chemical pesticides which must be applied regularly. And, if you apply just one chemical on your pests, sooner or later it stops killing them. This means that you must find new pesticides all the time. With biological pest control, pests are much less likely to become resistant to their natural enemies. Over a long period, biological pest control is not as costly as spraying chemicals. Farmers do not have to buy expensive chemicals or the equipment.

Biological pest control is sustainable because it uses what happens in nature. Once you establish the system, it works by itself. You can save your energy. However, you will need to be patient to see the results of this method. It may take some time to achieve a natural balance between pests and their natural enemies.

If the natural enemy of a pest lives nearby, then biological pest control will be simple. If the natural enemy is not native to your region, it may be available commercially or from your extension worker. For example, a spray called ‘BT’ contains a bacteria, Bacillus thuringiensis, which infects the digestive tract of certain insect larvae. There may be a small cost at the beginning of your control program. As more people use biological pest control, more natural enemies of pests will be living in nearby fields.

What are natural enemies?
Pests have three types of natural enemies predators of insects, parasites, and disease causing organisms such as viruses and fungi.

Predators eat insects. Predators include spiders, ladybirds, wasps, praying mantis and dragonflies. Each predator can eat hundreds, even thousands, of insects in its lifetime. These predators do not damage crops.

Most parasites of insects are other insects that lay their eggs inside the eggs or the body of a living insect or animal called a host. Examples are parasitic wasps such as Trichogramma spp. The parasite feeds on its host during its growth until it finally kills it. Then the adult parasite emerges from the dead host and searches for insects to lay eggs in. Using parasites is a slower way to kill pests but it is effective.

Some viruses and fungi carry diseases which infect insects and kill them. Because the disease is carried among the insect pests, whole populations can be wiped out quickly.

You can attract these natural enemies to your garden or field in several ways.

Mixed crops
You can start by growing a variety of crops and plants together. Mixed cropping imitates nature and does not favour one pest species over another. If you have only a few crops they will attract fewer insects. These insects can become pests if their population gets too high.

Intercropping is one form of mixed cropping. For example, plant corn in one row and beans in the next. This reduces the chances of one pest taking over your field and causing a lot of damage. In China, farmers intercrop sorghum between cotton plants because sorghum attracts the natural enemies of cotton. In the Philippines, farmers control corn stalk borer(Ostrinia furnacalis)by mixing corn crops with groundnut crops. The groundnut crops provide hiding places for predator spiders(Lycosa spp.)that attack stalk borers. If you plant strong smelling herbs among crops and around crops, you will find that pests are less likely to eat in those areas. If you grow coriander(Coriandrum sativum)among crops you will attract the parasites of pea aphids. Also, the flowers of many herbs attract good insects like wasps and honey bees. Besides attracting natural enemies, intercropping controls weeds because they cannot find room to grow and spread. It keeps the soil healthy too.

Companion planting
Some plants help each other when grown together. This is called companion planting. Some plants such as tomato or tobacco produce chemicals that repel insects so these plants help other crops to grow better. Speak to farmers or extension workers to find out which plants help each other. You may also want to try different combinations of plants yourself. It is also important to use plants that have been growing in your area for many generations because they are more likely to fight off local diseases and pests.

Crop rotation
Crop rotation also makes life difficult for pests. By growing different crops each season you discourage pests from invading your field every year because you disturb their feeding habits. If you change the time you sow or harvest, you can also escape the peak period of some pests. This means that you need to know something about how a pest develops so that you can predict the best time to plant to avoid or control the pest. As they grow, insects change their appearance. Some insects go through three or four stages before reaching their adult shape. What you may think are different insects may actually be the same insects at different stages of growth. In India, farmers plant their pea crops later in the season because the colder temperatures discourage growth of the fungus that causes root rot(Fusarium solani).

Other helpful hints
Unmanaged forests are a natural example of biological pest control at work. You should try to imitate a natural forest. In such a forest, trees and bushes protect your crops from wind that may carry pests. Birds nesting in bushes and trees around fields eat large quantities of insects and worms. Learn from what you see happening in nature.

You might also consider keeping ducks or hens. They like to eat ticks, slugs, worms, and caterpillars they find under fruit trees and among crops. It is also important to provide refuges such as rocks, logs, and wooden boards for frogs, toads, lizards, beetles and other creatures. They will help control the insects in your field.

Because you are no longer spraying pesticides, remember to cut and destroy the infected or damaged parts of plants or fruit trees. This reduces the spread of pests from one plant to another and across a field.

By carefully managing and observing crops and pests, we can reduce the number of pests on the farm and in the garden.


This script was researched by Elisabeth Abergel, Plant Geneticist, Department of Environmental Studies, York University, Toronto, Canada. It was reviewed by Stuart Hill, Entomology Department, Macdonald College, Montreal, Canada.

Information Sources

Managing pests and pesticides in small scale agriculture. (1989, 204 pages). Centre for Development Work, The Netherlands (CON). P.O. Box 211, 6700 AE Wageningen, NL.

Manejo integrado de plagas insectiles, (1992, 95 pages). ALTERTEC, Apartado Postal 2, Momostenango, Totonicapán, Guatemala, C.A..

Pests, predators and pesticides: some alternatives to synthetic pesticides, (1991, 105 pages) by J. Conacher. Organic Growers Association of Western Australia. P.O. Box 213, Wembley, W.A., 6014, Australia.

“Natural enemies of insect pests” in Regenerative Agricultural Techniques: Trainer’s Kit. International Institute of Rural Reconstruction, Silang, Cavite, Philippines.