A history of chemicals used as pesticides over the years.
In the mid-1800s there were a number of arsenical compounds used to control insect pests. These were classified into food or internal poisons (stomach poisons), contact poisons, and tracheal poisons (chemicals primarily dependent on poisoning insects in vapor form). The information to follow was originally published by William B. Alwood in Virginia Agricultural Experiment Station Bulletin No. 97, Vol. VIII, No. 2. February, 1899 -- "The Nature and Use of Certain Insecticides."
Lead arsenate was a mainstay in chemical pest control from 1892 until its cancellation in 1988. Its use was most popular prior to the marketing of DDT in the 1940s. It was used on most crops, but was most prevalent on fruit crops such as apples and peaches. Today, landowners are worried about soil residues left from years of application of lead arsenate and other arsenicals on old orchard land.
Some of the early fungicides included lime sulfur, Bordeaux mixture, sulfur and copper compounds. Many of these inorganic chemicals are still in use today. However, most fungicides today are organic man-made chemicals.
One of the greatest discoveries of modern times was that of the plant hormone herbicides (2,4-D, 2,4,5-T, and MCPA). Particularly 2,4-D, which has proven to be a reasonably safe chemical that has been used for over 70 years. Here are a few important links to information on the topic.