Growing Edge reported some time ago that if BT was applied at a level
that allowed some of the population to survive, after a few generations
the average fatal dosage had increased 15:1.
The lesson is clear that to avoid losing the product's benefits, one must
use it only after identifying the pest as susceptible and then spraying
heavily with fresh solution. BT loses potency in storage, more so after
dilution.
Bob Cruder - bcruder@miaco.com