Since before 500 BC, humans have used pesticides to prevent damage to their crops. The first known pesticide was sulfur. By the 15th century, toxic chemicals such as arsenic, mercury and lead were being applied to crops to kill pests. In the 17th century, nicotine sulfate was extracted from tobacco leaves for use as an insecticide. The 19th century saw the introduction of two more natural pesticides, pyrethrum which is derived from crysanthemums, and rotenone which is derived from the roots of tropical legumes.

In 1939, Paul Müller discovered that DDT was a very effective insecticide. It quickly became the most widely-used pesticide in the world. However, in the 1960s, it was discovered that DDT was preventing many fish-eating birds from reproducing which was a huge threat to biodiversity. Rachel Carson wrote the best-selling book "Silent Spring". DDT was also found to cause birth defects in other animals and humans. DDT is now banned in at least 86 countries, but it is still used in some developing nations to prevent malaria and other tropical diseases by killing mosquitos and other disease-carrying insects.

Pesticide use has increased 50-fold since 1950, and 2.5 million tons of industrial pesticides are now used each year. Glyphosate (the active ingredient in Roundup) is currently the world's most used herbicide.

Cite: Wikipedia