PLANT-DERIVED INSECTICIDES



OUTLINE
Reading
CHAPTER 13 IN THE TEXT, and lecture material

Introduction
Many insecticidal compounds are known from plants. Most plants make defensive compounds called allomones. 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 Plant-derived insecticides have largely been replaced by synthetic materials, but there are some advantages to the naturally occurring materials.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.

Some Insecticides
A series of compounds found in members of the genera Derris, Lonchocarpus, Tephrosia(and in 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 ellipticain 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.
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. 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 they enhance the activity of the insecticide) are added.

Pyrethin producing Chrysanthemum species

Other plant derived insecticides
Ryania (Flacourtiaceae) is also used occasionally. Haplophyton cimidium (Apocynaceae) is the cockroach plant of Mexico. Tobacco (which contains nicotine) is another major source of insecticides. Nicotine especially effective against aphids.

Calabar bean





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Fish Poison notes


Revised April 2005

© David S. Seigler, Integrative Biology 363, Plants and Their Uses, Department of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA. 217-333-7577. seigler@life.uiuc.edu.