Before applying insecticides, their exact amount required for the particular crop is first calculated. This way plants can get maximum benefit and expenditure is also minimum. Many problems are created if insecticides f are used in excess of what is required or if these are unnecessarily used. e.g. insects become resistant to the use of insecticides, these become habitual rather than dying, friendly insects are also killed, the insects which were not harmful earlier may start harming the crop, whole of the environment gets polluted.

Every year the same insect does not attack the crop to the same extent. On one year the attack is intense on the other year it may be moderate or mild. Therefore, one should take proper care while using these drags. Insecticides should be used only when the number of insects reach a particular level for a given crop. This way plant get more benefit and expenditure is also lowered.

Number of insects should not be allowed to increase from the specified value and do not allow these to damage the crop. Farmers should not get monetary loss due to unnecessary use of drugs. Sometime the signs of attack are counted and not the number of insects e.g. Stemborer in rice. Its attacked sites are counted and not actual number of insects.