Plant-derived Insecticides
David S. Seigler Department of Plant Biology University of Illinois Urbana, Illinois USA
Outline: Plant-Derived Insecticides Important insecticides from plants o rotenoids - New World and Asia o pyrethrins - Near Eastern center o tobacco - New World Ryania speciosa, Flacourtiaceae Antifeedants o neem, Azadirichta indica, Meliaceae
Reading CHAPTER 13 IN THE TEXT, and lecture material
Introduction Many insecticidal compounds are known from plants. Most plants make defensive compounds called allomones. Only a few are important commercially. Plant-derived insecticides have largely been replaced by synthetic materials, but there are some advantages to the naturally occurring materials. For example, these substances are biodegradable. Selectivity is needed. Compounds that are toxic to insects, but not toxic to mammals, are preferable, of course.
Rotenoids A series of compounds found in members of the genera Derris, Lonchocarpus, Tephrosia (and in an Illinois plant, Amorpha -- although that's never been exploited) are known as rotenones. We discussed them previously, as they have been used in "primitive" cultures as fish poisons.
Commercially, rotenoids are isolated mostly from the roots of Derris elliptica in Indonesia and from Lonchocarpus in South and Central America. These compounds are isolated by grinding the plant and extracting with solvents such as hexane or petroleum ether or chloroform. The compounds are oil soluble or lipids. They make up 1-20% of the dry weight of the roots.
Derris elliptica, Fabaceae Pacific Island Ecosystems at Risk (PIER) Photo by Agnes Rinehart
Rabo molle, Lonchocarpus muehlenbergianus, Fabaceae Libro del Arbol, Celulosa Argentina, Vol. 2, 1975
False indigo bush, Amorpha fruticosa, Fabaceae
Pyrethrins Another major series of compounds, the pyrethrins, come from species of the genus Chrysanthemum (some people put these species in Pyrethrum) (Asteraceae or Compositae). These were used as far back as the 1st century B.C. by the Chinese. Insecticidal plants mostly are grown in countries with inexpensive labor and high elevations such as Kenya and New Guinea.
Pyrethrum, Chrysanthemum cinerariifolium, Asteraceae Courtesy Dr. Saifu Dossaji
Harvesting pyrethrum flowers in Kenya Courtesy Dr. Saifu Dossaji
The flower heads contain about 3% crude pyrethrins. The compounds are extracted with lipid-like solvents just as rotenoids above. These plant-derived compounds are the major ingredients of many household insecticides. Often synergists (compounds not active themselves, but that enhance the activity of the insecticide) are added.
Ryania speciosa (Flacourtiaceae) is also used occasionally and an insecticide. A mixture of diterpene alkaloids is isolated and used for specialty insecticide uses. Because the extract is expensive, it is not commonly used. Other plant-derived insecticides
Ryania speciosa, flower and fruit, Flacourtiaceae
Tobacco, Nicotiana tabacum, Solanaceae Tobacco (which contains nicotine) is another major source of insecticides. Tobacco wastes are often extracted and used as a source of nicotine. Nicotine especially effective against aphids.
Tobacco, Nicotiana tabacum, Solanaceae, in Ohio
Calabar bean Calabar bean (Physostigma venenosa, Fabaceae) is (or has been) a trial-by-ordeal drug in the Calabar coast of Nigeria. The active component is physostigmine, an acetyl choline esterase inhibitor. The structure of several commercial carbamate insecticides is patterned after the structure of the plant alkaloid.
Calabar bean, Physostigma venenosum, Fabaceae R. Bentley and H. Trimen, Medicinal Plants, London, Churchill, 1880
Antifeedants Antifeedants are compounds that prevent insect feeding. Although many are toxic, the insects usually don’t consume enough to be poisoned. Only one of these, neem, Azadirachta indica, Meliaceae, is commercially available. The active compound, azadirachtin, is a structurally modified triterpene.
Neem, Azadirachta indica, Meliaceae Courtesy Dr. Ramesh Pandey William M. Ciesla, Forest Health Management International, United States