Why should preventive measures and biological …

Preventive measures ensure that the crop plants are protected from the harmful biotic factors (such as diseases, weeds and insects) and abiotic factors (such as drought and salinity). Biotic and abiotic factors can cause huge losses of food grains and need proper treatment to control them. So it is better to take preventive measures and avoid loss in crop plant production.
Using chemical control methods is not advisable since their use creates problems. They are poisonous to plant and animal species and cause environmental pollution. They reach human beings through the food chain. On the other hand, biological control methods are safe for plants, animals, humans and the environment in general.
Hence, preventive measures & biological control methods are preferred for protecting crops.