But it is NOT the BT in Dipel. It is a different variety that acts on different larvae.
Your absolutely right. BT has many subspecies (kurstaki, etc.) and each is formulated to kill a specific range of insects and/or grubs.
BT works by selecting or working on the insects internal system being triggered by a very specifuc ph level. The stuff has for the most part been proven bee safe by tests.
But many are now questioning if external factors such as chemicals and pesticides, bee treatments, etc., have the ability to change the bees internal ph level making them susceptable to BT.
I had mentioned this in one of the "Thinking outside the box" threads in the natural beekeeping forum on this site.
As with many things.....what was normally found safe for bees, has been time and time again, was found later to be a problem.