Weeds are a major economic and environmental problem throughout the world and pesticides are regularly applied to control them. In the USA, it was estimated in 1980 that 500 major species of weedy plants were responsible for yearly losses of $8 billion (Chandler, 1980). Herbicides to control weeds comprise 47% of the worldwide agrochemical sales while insecticides comprise only 29% (Woodburn, 1995). In the developing world, where herbicides are rarely used, removing weeds by hand can account for 60% of the pre-harvest labor input.

Weeds are not only a problem in agriculture. We consider any plant a weed that is undesired and grows in profusion, often outcompeting desired plants. We often think first of weeds important to agriculture or horticulture, although weedy plants also impact natural areas. Non-native plants that are introduced to a new area often have a competitive advantage over natives because herbivory is absent or has a minor impact (the enemy release hypothesis). In such cases, invasive plant species outcompete native plants. Controlling invasive weeds that have spread to cover enormous areas such as pastures or wetlands is not feasible using direct controls such as synthetic chemical pesticides or cultural control. Aquatic weeds are especially difficult to control; herbicides must be added to water where they are quickly diluted and could later potentially be ingested by humans. Cultural control such as weed removal provides only a temporary solution.