This is a naturally occurring material based on the bacteria Bacillus thuringiensis subspecies kurstaki. These naturally occurring products were discovered in the 1960’s and have been used ever since for worm and caterpillar control. When the insects ingest the product, it causes a disease in their gut, causing them to stop feeding and eventually die. The bacteria in Monterey B.t. is 98.35% active (at least 6 million viable spores per mg.)