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Re: weed chemicals

From: Bill Shear <BILLS@hsc.edu>

>I had to throw in my two cents worth, probably shouldn't.  Aren't most
>pesticides just a higher concentration of what is already in plants?  I
>believe most plants already have a defense against insects and fungus, and
>out here anyway, the pesticide used is a stronger concentration of naturally
>occurring toxins.

Mike, this is a complex question.  An insecticide like pyrethrin is indeed
derived from plants--chrysanthemums and relatives.  But most insecticides
and nearly all herbicides are novel compounds, or artificially produced
analogs of natural substances.  Several popular herbicides, for example,
mimic plant growth hormones and cause plants to "grow themselves to death."

Plant defenses against insects are many.  My favorite example comes from
balsam trees and some other conifers, which produce an analog of the insect
"juvenile hormone" so that insects that feed on them never mature to
reproduce.  Other plants have immune-like responses against fungi.  Still
others attacked by insects not only produce their own defenses, but also
give off chemicals that alert others of their species in the immediate
area, so that they also begin to make defensive chemicals.

There is a constant evolutionary arms race going on between plants and
their enemies.

It's a fascinating world!

Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
FAX (804)223-6374

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