We need a bit more information i..e. are you asking about viability of
the spores or about the effectiveness of the toxins (in the parasporal
body). And are you concerned about residual effectiveness on plant
leaves or in the water (Bti against mosquitoes)? Of course, the
companies that have been producing Bt for the past 40 years or so have
been interested in these questions and have attempted to prolong
effectiveness. There are a number of papers, particularly in the older
literature, concerning things like UV inactivation of toxins but I don't
know of any phys/chem studies of the effects on the proteins
themselves. The answer to your question may depend on the depth to which
you are pursuing the question.
