Japanese beetles are, according to the University of Wisconsin Extension, the “single most important turfgrass-infesting pest in the United States.” These destructive insect pests thrive in most states east of the Mississippi and cause damage to over 300 species of plants. Japanese beetle grubs live in and feed on the roots of turfgrass, overwintering in the soil. When infected with a biological control known as Milky Spore, grub populations are suppressed and beetle damage decreases.
Adult Japanese beetles measure about 3/8 inches long and are a dark, shiny green. Wing covers are tan and metallic-looking.
Small, white C-shaped grubs live in the soil and eat grass roots, restricting water uptake in unwatered grass. In areas of high grub activity, dead patches of grass will appear. Because roots are damaged, the turf can be rolled back like carpet. Birds and rodents dig for the grubs and stress the turf further.
Milky Spore disease was discovered in New Jersey in 1933, and in 1948 it became one of the first microbial insecticides registered in the US. As interest in less-toxic pest control methods grew in the 1980s, milky spore was used extensively. Results were not good and it was discovered that the wrong bacteria was being used. Existing inventory of Milky Spore was removed from the market and replaced with a more consistent product.
Milky Spore powder is applied to the turf in early spring. Grubs eat the powder and the bacterial spores begin to reproduce. Internal fluids begin to turn a milky white (hence the name) and the infested grub dies. Spores disperse into the soil and continue to spread throughout the grub population.
Milky Spore works best when soil temperature is between 60 and 70 degrees Fahrenheit. Colder temperatures will inhibit spore development. Grubs will still be infested, but at a slower rate.
Pros and Cons
Milky Spore is safe to use around food crops, people, pets and wildlife. Spores remain bound to the soil for several years, providing long-term Japanese beetle grub population control.
Be aware, however, that Milky Spore is a population suppressor and not a fast-acting beetle killer. In colder regions, results may take at least 3 years for full effect and milky spore powder is more costly than conventional, more toxic insecticides.