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Pesticides: Natural Isn't Always Best

By Margaret Geick, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver

Insecticides and other pesticides sometimes are needed as a last resort when efforts to grow a healthy plant fail and pests strike.

If you reach for an insecticide, don't buy into a widespread perception that insecticides derived from plants are somehow "softer" in their effects on the environment than synthesized insecticides.

Rotenone, nicotine, pyrethrum and neem are examples of botanical insecticides. Just because the materials are natural, however, doesn't mean they are always less toxic than the synthetics.

Rotenone is produced from the roots of two tropical members of the bean plant family. It has been used as a crop insecticide since the mid-1800's to control leaf eating caterpillars, and it often is recommended for flea beetle control on early season vegetables. It is six times more toxic than carbaryl, (sevin), a synthetic product, also effective for caterpillar and flea beetle control.

Nicotine sulfate has been used since the turn of the century and is the most hazardous botanical insecticide available to home gardeners. The insecticide is extracted from tobacco by steam distillation or solvent extraction. Highly toxic to humans and other warm blooded animals, nicotine sulfate is rapidly absorbed through the skin. It is six times more toxic than diazinon, a widely available synthetic insecticide sold for control of many of the same pests.

Like most organic pesticides, nicotine and rotenone break down rapidly meaning the highest hazard is to the applicator, birds and other wildlife present at the time of application.

Some organic insecticides are very effective for pest control AND have a high degree of associated safety.

Pyrethrum, extracted from the dried flowers of the pyrethrum daisy, has a rapid "knockdown" effect on many insects. It has very low toxicity to mammals and is best used for exposed caterpillars, sawfly larvae, leaf beetles and leafhoppers. Because of its short persistence, its effectiveness is limited but so are its impacts on natural insect enemies.

Neem is an exciting, new insecticide product. Extracted from the seeds of the tropical neem tree, this plant substance has long been used in Africa as a pharmaceutical and toothpaste. Neem has recently been found to interfere with insect feeding and development. Treated insects rarely show immediate symptoms, and death may be delayed a week or longer. Insects are sluggish after spraying, however, and do little feeding.

Labeled uses of neem are still under development. Its very low toxicity and increasing labeling for a wide number of insects make it an attractive, up and coming insecticide. CSU research tests show good activity against tent caterpillars and elm leaf beetles, and fair results with tussock moth.

The container in which an insecticide is sold will have a label that tells the user not only what pests the product can control, but also safety precautions for the applicator and the environment.

The most important thing in choosing an insecticide is not its origin but its label. ALWAYS READ AND FOLLOW THE LABEL CAREFULLY. It's your best guide to using pesticides without harm.

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Date last revised: 01/05/2010