Bacillus thuringiensis var. Kurstaki is a species of bacteria. When ingested by a caterpillar, the bacteria proliferate rapidly in the caterpillar's gut to the point wherein the gut is so full of bacteria that there is no room for food. With the gut completely "stuffed", the larva quits eating and literally dies of starvation. Bt is commonly sold in liquid form to be further diluted with water so it can be sprayed on plants.
Good suggestion, FL .... I had forgotten about Bt.