Milky Spore is a bacterium, Bacillus popillae, which is primarily lethal to Japanese beetle grubs. The life cycle of the Japanese beetle includes the immature beetle, called a grub. Once Milky Spore is applied, the grubs ingest it as a normal part of their feeding. The more spores are in the area, the greater the chance of infection. Each infected grub dies within approximately 3 weeks and releases new spores.