Course 26.3Lesson 1 of 3
Chemical pesticides harm people and animals and pollute the soil and the water. Biological control uses living things instead: ladybirds and dragonflies, the spores of Bacillus thuringiensis, the fungus Trichoderma and baculoviruses. Each one attacks a narrow range of pests, so it fits into integrated pest management.
Try the code and the simulations beside the lesson.
A farmer who grows cabbage finds holes in the leaves. Caterpillars have been eating them. Insects, fungi and weeds damage crops all over the world.
For many years, farmers have used chemical pesticides and insecticides. They work, but they are toxic. They harm people and animals, and they have polluted the soil and the ground water.
The other way is biological control. It uses living organisms to keep pests and diseases down.
The simplest biocontrol agents are animals that eat the pest.
Bacillus thuringiensis is a bacterium. The short name is Bt. Farmers buy its dried spores in sachets. The spores are mixed with water and sprayed on plants that insects attack, such as cabbages, other brassicas and fruit trees.
It kills the caterpillars of butterflies and moths, and it leaves most other insects unharmed.
The toxin is a protein. It is harmless until it reaches the alkaline gut of a larva. The next chapters of the book tell how the gene for the toxin is put into crop plants to make Bt cotton. Here the whole bacterium is sprayed.
Trichoderma is a genus of free-living fungi. They are common in the soil around roots. They are effective biocontrol agents of several plant pathogens. They grow on the pathogens or stop them from growing.
A note on the name. NCERT also gives Trichoderma polysporum as the source of the drug cyclosporin A. Do not mix the two uses. As a biocontrol agent, Trichoderma protects plants.
Baculoviruses are viruses that attack insects and other arthropods. The majority of those used as biocontrol agents are in the genus Nucleopolyhedrovirus. They are species specific and narrow spectrum. This means that each one attacks a few kinds of insect, and no others. They have no bad effect on plants, mammals, birds or fish, or even on insects that are not the target.
This is a great help when useful insects are being protected, as in an integrated pest management programme, or when a place that is easily damaged is being treated.
Integrated pest management, or IPM, uses several methods together. A farmer watches the pests, uses biocontrol agents first, and uses a chemical only when it is really needed.
More about IPM. This part is not in the NEET syllabus, but A-Level, IB and AP Environmental Science ask it. The farmer counts the pests on a sample of plants. If the average is below a level at which the harm to the crop is worth the cost of action, the farmer waits and watches. If it is above, the farmer acts, and uses an agent that harms as few other organisms as possible.
The graph is computed from a model. Without control the pests grow until food runs out. With an agent released early, they never reach the harmful level.
You should find that a larger share killed each day brings the larvae below the level sooner. A real spray does not kill the same share every day, and new larvae can arrive from other fields.
The program prints the number of larvae on a plant for each day after a spray, and the day on which they fall below the level.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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