Crops are vulnerable to extensive damage from numerous sources: disease; pests; adverse environmental conditions; unfavourable weather; weeds. Lack of crop protection, therefore, seriously reduces potential yields.
The problems of crop protection have changed dramatically since 1945. There is now a whole arsenal of chemicals with which to combat agricultural pests and diseases, but this development has itself many drawbacks. Such sophisticated techniques are available only to a minority of farmers; in most parts of the world the standard of crop protection remains abysmally low. In addition, modern crop protection methods have been criticized for relying too heavily on chemical control. Biological controls, both natural and contrived, have been neglected. In some cases involving misuse of agricultural chemicals, crops must be protected from the very measures intended for their protection. Meanwhile previously localized pests and diseases continue to spread worldwide.
Recognition of the importance of insect pests dates back about 100 years, although from earliest times farmers must have noticed the damaging effects of pests, diseases and the environment. Early writers could do little more than describe the life cycles of insects as there were few, if any, means of dealing with them. Early twentieth century attempts to take direct action against pests were limited, in the USA to controlling scale insects on citrus crops with oil sprays, and in Europe to moth control with lead arsenate. Few other chemicals were available and the means of applying them in sprays to trees was, by modern standards, quite primitive. Fumigation of citrus trees was also practised in the USA and in the Mediterranean region by tenting trees and releasing hydrogen cyanide gas into the tents.
As knowledge about pests and diseases gradually accumulated, so did an awareness that this knowledge should be made available to the farming community. In the USA in 1887, the Hatch Act provided for Agricultural Experiment Stations in each state and for the information to be passed on to farmers and agricultural students.
In 1970 insects, diseases and weeds caused pre-harvest losses worldwide of 34% ($ 70 million) of the total potential value (estimated at $ 208,000 million). This loss was borne largely by developing countries in Africa, Asia and South America. To these figures must be added the postharvest losses of about 20%, due mainly to insects and rodents.
The Food and Agriculture Organization of the United Nations has published world figures which indicate that 35% of the wheat which could theoretically be harvested is lost as a result of pests and diseases. The corresponding figure for potatoes is 40%, for sugarbeet 24%, for apples 30%, for cotton 60% and for tobacco 62%. Even where modern methods of crop protection are employed, losses are still very heavy.
It was reported in 1992 that there is a link between stunted plant growth and higher ultra-violet radiation caused by the depletion of the Earth's protective ozone layer. Test plants were irradiated for six to eight weeks with 5 to 10 per cent more ultra-violet-B light, which is normally screened off by the ozone layer. Some of the crops declines were as much as 70 percent. The damage occured at molecular level, affecting the productive apparatus of the plants, because of an energy overload turning into heat which destroys proteins important for plant production. It was estimated that in reality some 20 percent more UV-B gets through during peak times.