Milky spore, or milky spore disease, is a bacteria applied to lawns as an organic pesticide to control the Japanese beetle grub. The Japanese beetle grub is a large white worm that eats the roots of common lawn grasses and causes extensive damage as the grass dies, leaving large barren patches in the lawn.
Milky spore is a bacteria technically known as Paenibacillus popillae. It is a white powder that is applied to grass. The spore kills the Japanese beetle grub after it is ingested by the beetle during the feeding process. The bacteria was found in New Jersey in 1933 and became a commercial product for Japanese beetle grub control in 1948.
Once milky spore is ingested by the Japanese beetle grub it reproduces and destroys the internal organs of the grub, turning the grub opaque before it dies. Once the grub dies and decays, the spores are released into the surrounding soil where it attaches itself to the grass roots and kills other grubs after they feed. Occasionally, the spores simply pass through the grubs and do not harm them. Milky spore remains inactive in the soil until it is eaten by the grub.
By controlling the Japanese beetle grub, milky spore lowers the population of grubs to the point where the grubs cannot kill large patches of grass by eating the roots. The chances of milky spore eliminating all the grubs in a lawn is small, but once it is in the soil, the population of grubs is kept in check.
Milky spore grows best in soil temperatures between 60 and 70 degrees F, and few places have a constant soil temperature in that range. In cooler climates it may take longer to spread through the soil. Also, the temperature requirements may create a challenge for spore production because grubs are most active in spring and fall when soil is either heating up beyond the ideal range or cooling down.
Milky spore only spreads through the soil when it is consumed by grubs that die and release new spores into the soil. However, it binds to the top layer of soil where the grubs damage the grass and can remain in the soil for more than 10 years. It is more expensive than chemical control methods, and in cooler climates it may not begin to control the Japanese beetle grub for several years after application. However, because it lasts for so long in the soil it does not need to be reapplied every year. Other kinds of grubs, not controlled by milky spore, may attack the lawn even if the milky spore has controlled the Japanese beetle grub.