Doyle et al. 1995. Adv. Appl Micro. 40:
Tapp and Stotzky. 1995. Appl and Environ. Microbiology 61:602-609.
Crecchio and Stotzky. 1998. Soil Biol and Biochem. 30:463-470.
These papers show, among other things, that active Bt toxins can
persist for extended periods of time in the soil, bound to clay
particles, where they resist breakdown but retain activity. They
also discuss the potential for less selectivity in Bt action,
specifically because what is put into crop plants is active Bt, not
the prototoxin that is sprayed on in foliar formulations.
Recall that part of the elegant selectivity that allows particular Bt
endotoxins (I think there are about 100 of these) to affect
particular species (e.g. lepidopteran but not coleopteran or vice
versa) involves the processing of the prototoxin in the insect
stomach. Requires particular pH, particularly enzyme, and even
particular receptors in the stomach of the insect in order to be
"activated" and toxic to that particular organism.
All of this is lost when active Bt is dumped all over and into the
ground in decomposing Bt crop residues. So, the concern is a much
wider range of organisms - including soil decomposer organisms - may
be affected - selectivity will be much reduced. And then, of course,
there is the dose issue, which is what your reference alluded to.
Dr. E. Ann Clark
University of Guelph
Guelph, ON N1G 2W1
Phone: 519-824-4120 Ext. 2508
FAX: 519 763-8933
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