Weeds compete with desirable plants for light, water, and nutrients. Insects, bacteria, fungi, viruses, and other creatures eat the leaves, stems, seeds, and fruits of those same desirable plants. Weeds and pests have always been among the greatest challenges for gardeners and farmers, and for most of human history, the only way to deal with them was to use simple, nonchemical methods – hoeing or hand pulling of weeds, for example, removing the eggs, larvae or adults of crop-destroying insects, or pulling up plants that were obviously sick with some kind of disease. 

Over time, particularly in the 19th century, a number of compounds were developed or discovered naturally occurring in plants and animals that served to kill insects, fungi, and other living things. Though there was some awareness of the toxicity of these compounds to humans (many of the substances used were based on arsenic and other poisons), they still seemed like a godsend and were widely used in agriculture.  

 

In the 20th century, more sophisticated understanding of chemistry and biology (together with a huge research push, including the effort to make poisonous gases for use in World War I and World War II) led to the development of much more potent chemicals, including the first herbicides, or compounds that kill plants. As with the simpler chemicals that came before them, these new compounds – now known collectively as “pesticides” – were eagerly embraced by farmers and gardeners. In addition to better controlling insects and diseases, they offered the promise of simplifying weed management, which is some of the most difficult work of growing food. Together with the breeding of new crop varieties and the widespread use of so-called “synthetic” fertilizers (which are made through the petroleum-based capture of nitrogen from the atmosphere and the extraction of other nutrients through mining of the earth’s crust), pesticides led to the “Green Revolution” of the latter half of the 20th century, a time of dramatically increased global food production. This huge increase in food production has helped to feed a global population that went from under two billion in the year 1900 to more than six billion in the year 2000.