Botanically-derived insecticides have gained favor in recent years, due in part to the perception that, because they originate from plant material, they are more safe or "natural". These pesticides are often used for growing crops organically, according to guidelines set forth by certification programs. However, it is important to be aware that they are pesticides, and that they fall under the same state and federal regulations as synthetic pesticides. All pesticides must be labelled for the specific pest(s) on the particular crop(s) for their use to be legal. If the use is not stated on the label, then the pesticide is not legal to employ.
It is important to realize that produce grown using botanical insecticides is not "pesticide free." Botanical pesticides also leave residues, can be disruptive to natural enemies, and may be toxic to humans. Therefore the same precautions must be taken in the use of these materials as with synthetic pesticides. Relying heavily on the use of any pesticide is a rash, unecological practice. Conversely, using botanical insecticides prudently when needed can aid the gardener.
Fortunately for the home gardener, traditional outlets such as garden centers and nurseries are increasingly stocking botanical insecticides; they are more readily available in small containers than in commercial quantities. Pyrethrum based insecticides are widely available but the supply may be limited. Check with your local supply source to see which they carry and what they can special order.
The following are some commercially available botanicals:
Sabadilla: This compound was first used in the sixteenth century, and grew in popularity in this country during the second World War when other botanicals such as pyrethrum and rotenone were in short supply. The insecticidal dust is made from the seeds of a small perennial bulb in the Lily family. The toxic components are lacking in other plant parts (roots, bulbs, stems and leaves). Commercial supplies come from South and Central America. It is interesting that the toxic constituents actually become more powerful after storage; fresh sabadilla extracts have not proven to be a strong insecticide. Sabadilla is a broad spectrum contact poison, and may have some action as a stomach poison also. Sabadilla is toxic to honeybees. It is most effective against leafhoppers and true bugs. It degrades rapidly on exposure to air and sunlight, leaving very little residual toxicity.
Ryania: The insecticidal properties are contained in the stems and roots of Ryania speciosa, a South American shrub. Although it acts as a stomach poison, ryania often depresses the insects feeding initially, so that it undergoes a long period of inactivity before death. Its residual period is longer than the other botanicals. Relative to rotenone, ryania is moderate in acute or chronic oral toxicity in mammals; this is partly why much attention has been given to this insecticide in recent years.
Rotenone: This important, widely used insecticide is derived from several tropical leguminous plants, including derris, cube and timbo. These plants range from the Far East to the Amazon Valley of South America. Many plant species have been shown to contain rotenone. In general, the rotenone principles are contained in the roots, which are dried and powdered to be used as a dust. Liquid rotenone is available commercially.
Rotenone was originally employed by Native American and other indigenous cultures to poison fish. It should by no means be considered "non-poisonous" to animals; in fact, it has been shown to be fatal to mammals if inhaled over extended periods, and may cause numbness of the lips and tongue if exposure during application is constant. Dermatitis may result if rotenone comes in contact with the skin, but it does not appear to be absorbed through the skin.
Rotenone is effective against a wide range of insects and has a short residual life. It acts as a stomach poison and as a contact insecticide. It is not toxic to honeybees, but will kill some beneficial insects. It is registered for use against a number of chewing insects on many vegetables and some fruits. Different brands and formulations of rotenone are labelled for various pests; always check the label before purchase to ensure that the product is legal for the intended use.
Pyrethrum and Pyrethrins: Pyrethrum is the natural product that comes from the ground up dried flower head of the African chrysanthemum, Chrysanthemum cinerariaefolium. Pyrethrins refer to the insecticidal compounds that occur in pyrethrum. Pyrethrins affect the insect on contact, creating disturbances in the nervous system which eventually result in convulsions and death. Low doses, however, often cause temporary paralysis from which the insect may recover. Pyrethrum has almost no residual activity, breaking down rapidly on exposure to sunlight, air or moisture. One study in 1937 demonstrated that finer powders are more rapid in action on insects, but deteriorate more quickly upon exposure to light. Activators (like PBO) or other "synergists" are occasionally added to pyrethrins to extend their use, but many of these are not approved by organic certification programs. The insecticidal activity (if any) of these extenders varies.
Pyrethrum and pyrethrins are widely available, but supplies may be limited. They are effective as broad spectrum insecticides and will control pests such as aphids, whiteflies, stinkbugs, and mites. Be sure the brand that you purchase is labelled for use on the crop and pest in question. Like rotenone, pyrethrum and pyrethrins should be used with caution. Pyrethrum's hazard to humans lies mainly in the form of allergic reactions and dermatitis.
Carolyn Klass, Senior Extension Associate, Department of Entomology, Cornell University, Ithaca, NY 14853
Marcia Eames-Sheavly, Extension Support Specialist, Department of Fruit and Vegetable Science Cornell University, Ithaca, NY 14853