The poisoning isn't from the spores per se. Bacterial spores, in this case BT, is what's in the stuff you buy and treat with. After application, those spores release (usually by consumption but there may be varieties that work in other ways?) an endotoxin that kills a specific pest, be it a mosquito, cabbage looper or a wax worm--depends on the racial variety of BT. So, it is a pesticide but one that targets specific insects. The folks that make it refer to it as a biological insecticide and that seems accurate--it just works a little differently than others. That said, the poison works just like in most other bacteria that cause death or make you sick. Botulism is a good example. Spores of botulism are on everything we touch, and in many cases consume, but you only get sick (or die) under circumstances that let those spores release the toxin. We pressure can meats and low acid foods to kill the spores because the medium (stuff you're canning) provides the proper environment for the spores to release their toxin. Botulism is one of those toxins that is extremely strong--like a teaspoon in water could wipe out an entire town kind of bad! It's so toxic that determining the cause of death is difficult (for example, crib death--it wasn't that long ago that practitioners had no idea why kids would die but now we know not to give infants honey because botulism spores are everywhere and a kids gut can have the right medium for spores to release toxin) and it has delayed the diagnosis of many deaths over the years. It's a chemical, just not a synthetic one. All that said, I don't think BT is bad; it's certainly gained a rightful place in the control of many insects because we can target which ones it will kill and which ones it won't.
