Re: natural vs. synthetic biocides

E. Ann Clark, Associate Professor (ACLARK@crop.uoguelph.ca)
Sun, 16 Nov 1997 06:25:35 EDT

Steve and Rich have raised an interesting point for discussion, and
one that I'd like to hear more about. It is quite entirely true that
plants and perhaps other organisms (I am a plant scientist; like
the hammer, everything looks like tacks!) have evolved an elegant
arsenal of methods of avoiding herbivory - getting eaten. Some of
these involve digestion, like tannins and lignins. Others act as
vasoconstrictors - like alkaloids - which not only reduce
palatability but cause a whole range of upsets in stock unfortunate
enough as to be obliged to eat them (e.g. old style reed canarygrass
cultivars and endophyte-infested tall fescue). There are whole
textbooks on the neat evolutionary counterplay between hervivore and
plant in the game of life.

More recently, Theo Colborn in her new book Our Stolon Future talks
about endocrine disrupters (ED)which are chemicals that mimic such
compounds as estrogen or androgen or other key human regulators,
causing all manner of havoc in developing embryos. Much of the book
is about synthetic ED's, like DDT and PCB's.

However, she goes into some depth about naturally occurring plant
estrogens, and discussing a theory that plants may have evolved the
ability to generate estrogen mimics as a vehicle to deter
reproduction in herbivores. The timing of highest estrogen
concentration in plant tissues, and the type of plant that expresses
this trait (the tasty ones, not the ones already unpalatable to
stock) support the notion that this is the intent. So, here is a
case where "natural" estrogens might cause an effect analogous to
"synthetic" estrogens - Steve's point. Examples include red clover
and alfalfa.

But she goes on to note that there are differences in the herbivore in
how it responds to these outside influences. Herbivores (including
humans) have apparently evolved a system of molecules which destroy
the plant-origin estrogen-mimics, and maintain the correct,
exceptionally low concentration of estrogen for fetal development
(not wholly effective, as producers are still cautioned against
grazing some species during flushing/breeding). So, herbivores have
evolved in response to this selection pressure, just as one might
suspect. Such is NOT the case for the synthetic estrogen-mimics,
which are able to perist in excessive concentrations in the blood
stream and do damage. So, while both natural and synthetic estrogen-
mimics are present, herbivores are able to manage one but not the
other - Rich's point.

It is very important to distinguish between natural and synthetic
products, whether estrogen-effectors or biocides. They are both
intended to "kill" but natural origin stuff is part of an elegant
evolutionary dance between equal partners. The synthetic stuff is a
rude interloper against which natural systems have no defence.

I'd be very interested to hear of other work along these lines. Ann
ACLARK@crop.uoguelph.ca
Dr. E. Ann Clark
Associate Professor
Crop Science
University of Guelph
Guelph, ON N1G 2W1
Phone: 519-824-4120 Ext. 2508
FAX: 519 763-8933

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