And the answer to my question doesn't appear to be there, nor in the
half-dozen other websites I looked at.
First off, living things die, diminish, bring down the final curtain, or
join the choir invisible, they don't break down.
Bacillus thuringiensis disappears from plant surfaces rather quickly
however they are moderately persistent (whatever that means) in soil
(their natural habitat). They are fried by ultraviolet light. http://cecalaveras.ucdavis.edu/bt.htm
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