A large variety of birds, insects, rodents, worms and microorganisms destroy crops in the field. Any organism which destroys crop plants is called a pest. Insects are the most destroy of all pests. They normally attack leaves and stems. Some, however, many destroy pods, seeds or roots. Fungi and bacteria attack all parts of plant, while viruses affect mostly the leaves. Birds and rodents eat mainly grains.

Diseases caused by bacteria, fungi, nematodes and viruses can be controlled by taking the following steps.

1) selecting disease –resistant varieties

2) Sowing disease –free seeds and treating seeds with antibiotics or fungicides.

3) Destroying weeds that act as alternative hosts of disease –causing microorganisms.

4) Destroying plants which sow symptoms of disease.

5) Spraying fungicides or bactericides over disease crops to prevent the disease from spreading.

The steps taken for controlling insect pests are as follows.

a) selecting resistant varies

b) Practicing mixed cropping and crop rotation.

c) Using pesticides

Birds are usually scared away, while rodents are either trapped of killed with the help of rodenticides.

Weed control

Weeds are unwanted plants which grow along with cultivated plants. Grasses, wood sorrel, pennywort, convolvulus, chenopodium and black nightshade are some common weeds.

Weeds have to be removed from the crop field because they complete with the crop for nutrition, water and space. The process of removing weeds is called weeding. Weeds can be uprooted manually or with the help of a harrow or rake. A harrow is like a big comb with a long handle. It is attached to a plough, and unwanted plants get uprooted as its spikes run between the rows of crops. A rake is similar, but can be drawn by hand.

Chemicals called weedicides or herbicides are also used to get rid of weeds. They kill the weeds, but not the crop plants.

Disadvantages of pesticides

a) Pesticides, weedicides, fungicides and bactericides are poisonous chemicals, so they must e used carefully, only when necessary and in the recommended concentration. Care must be taken to ensure that they do not come into direct contact with the body. They nose, ears, mouth and hands should be covered while spraying these chemicals and they must be stored safely, out of the reach of children.

b) Fruit and vegetables may get contaminated by the pesticides sprayed over them. So they must be washed thoroughly before being eaten.

c) A major problem with pesticides is that they are non biodegradable and persist n the soil and in the water bodies into which they are washed off. They contaminate groundwater, and kill soil organisms and pollinating insects. They also affect host f organisms, including human beings, by entering food chains.

Biological control

The disadvantages of pesticides have led agriculturalists to recommend a natural method of controlling pests. In this method, called biological control, the natural enemies of pests are used to control them. Weeds too can be controlled this way. However, care has to be taken while selecting the natural enemy of the pest to be controlled, so that the organism chosen does not become a pest in turn.