B.T. is an insecticide that was discovered in 1911. Unlike typical nerve poison insecticides, BT act by producing proteins that react with the cells of of the gut lining of the insects. BT paralyzes the digestive system and makes the insect stop feeding on the plants within hours. So in turn the insect die of starvations. It is not harmful to beneficial insects.
This insecticide is not harmful to humans. The downside to this product is that it can be weakened by the sun as well as it wears off in less than a weeks time.
Please read the link below it is full of great things about BT, just found this as I was composing this article. Hope this helps save someones garden.