Pesticides

From Natural Extracts to Synthetic Pesticides

Before World War II, farmers relied on naturally occurring substances such as nicotine (from tobacco plants) to control pests. During the war, DDT proved highly effective against malaria-carrying mosquitoes and other disease vectors, so after the war it was widely adopted in agriculture to control insects, rodents, weeds and crop diseases. Because of its serious adverse effects, DDT's use has since been banned in India.

Resistance Drives a Chemical Treadmill

Pesticides are synthetic toxic chemicals with real ecological consequences. Repeated use of the same class of pesticide selects for pest populations that are resistant to it, making the chemical progressively less effective. As DDT resistance grew, the industry introduced other persistent organic toxins such as Aldrin and Dieldrin.

• Most of these organic toxins are water-insoluble and non-biodegradable.
• Being highly persistent, they get passed up the food chain from lower to higher trophic levels — a process called biomagnification — so their concentration keeps rising at each step (Fig. 14.3), until top predators (including humans) accumulate levels high enough to cause serious metabolic and physiological disorders.

In response, less-persistent, more biodegradable organophosphates and carbamates were introduced — but these are severe nerve toxins, more acutely harmful to humans, and there are documented pesticide-related deaths among agricultural workers. Insects have since developed resistance to these too, keeping the industry in a continuous search for new insecticide classes.

Herbicides

The pesticide industry has also turned to herbicides such as sodium chlorate (NaClO₃) and sodium arsenite (Na₃AsO₃). The mid-20th-century shift from mechanical to chemical weed control created a large market, but herbicides are not environmentally benign either:

• Most are toxic to mammals, though generally less persistent than organochlorine pesticides, decomposing within a few months. …