November 2005


A nation's farmland, its "bread basket," is one of its most important natural resources. Today's farmers grow the foods that feed whole cities of people hundreds or thousands of miles away from barns, silos and patchwork fields of crops. Thanks to advances in agricultural technology, farmers today can harvest more crops per acre of farmland than ever before.

One technological breakthrough responsible for abundant harvests is the development and use of crop protection chemicals. Crop protection chemicals help farmers manage crop "pests," including weeds, insects and fungi (pronounced "fun - guy," fungi is the plural of "fungus") that threaten crops. Used properly, crop protection chemicals help growers provide a safe, nutritious bounty of affordable foods.

Pyrethrins are natural insecticides found in plant oils in the seeds of a particular variety of chrysanthemum flowers that resemble daisies. Pyrethroids are an important group of manufactured crop protection chemicals that are modeled on pyrethrins. Several of the pyrethroids are manufactured to include chlorine in their chemical structures.

Plants' Natural Defenses

Pyrethrum Daisies, are a variety of
chrysanthemum that makes pyrethrin.
(Photo courtesy of Tasmania Dept. of
Primary Industries, Water & Environment).

Over millions of years, most plants and animals have developed various chemical defenses that help them survive in the competitive environment of the natural world. Like many other plants, chrysanthemums, or "mums," as they are popularly known, manufacture their own insecticides to survive insect attacks.

According to Professor Bruce Ames, a well-known University of California biochemist, of all the pesticides found in the foods we eat, 99.9 percent of pesticide residues found in these foods are naturally produced by those plants themselves.i

Pyrethrins: Early Pesticides

Known even to the ancient world to be effective against pests, pyrethrins have been extracted from a variety of chrysanthemum known as pyrethrum daisies for centuries. Pyrethrins have been used against all kinds of insect pests. The Chinese crushed and powdered these plants for use as insecticides as early as 1000 BC. It is thought that the plant was transported to Europe along silk trading roads. And by the 19th Century, ground pyrethrum daisies were found in most European pharmacies.ii

Today, the high-tech agricultural industry grows pyrethrum daisies as crops to harvest the pyrethrin oils they contain. The flowers are cut and dried and then soaked in an organic solvent to extract the valuable oils.
The Australian island state of Tasmania is a major producer of pyrethrum today.

Learning from the Mums of the World

To help farmers provide their crops with added protection from pests, agricultural chemists have developed synthetic crop protection chemicals. Pyrethroids are manufactured chemicals that are very similar in chemical structure to the natural pyrethrins of chrysanthemum flowers. But pyrethroids are chemically designed to be even more effective. For example, while pyrethrins are broken down rapidly by sunlight or other compounds in the atmosphere, lasting only one to two days, pyrethroids can last for a few months before they are degraded. Several of these highly effective pyrethroids contain chlorine. The chemical structure of one of these, esfenvalerate, is shown below. Note the symbol "Cl" for a chlorine atom in this structure.

The Chemical Structure of Esfenvalerate

Esfenvalerate is one of several chlorinated pyrethroids; the hexagons are ring-like structures made up carbon and hydrogen atoms. The chemical elements in esfenvalerate are C, H, O, N and Cl.

Follow-up Activity: Evaluating Risk

There are many ways to consider both the benefits and the risks when we think about farming and food production.

(a) Describe the effect on world food production if synthetic pesticides, such as pyrethroids, were eliminated. Would there be enough food for everyone?

(b) According to health experts, eating more fruits and vegetables, which contain important nutrients, are thought to be some of the best ways to lower the risk of developing cancer and heart disease (other than giving up smoking - Don't Even Start That!). What effect would higher food prices have on the health and nutrition of consumers?

(c) Discuss the benefits of modern science, including the use of crop protection technology, in helping to feed a hungry world.

End Notes

iAmes, B. (1993). Science and the Environment: Facts v. Phantoms, © National Wilderness Institute. 

iiBotanical Resources: Australia, Pyrethrum: The Natural Insecticide, PTY LTD, Tasmania, Australia.

For a list of previous "Chlorine Compound of the Month" features, click here.


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