Nematodes may be purchased in lawn and garden shops or through biological mail-order catalogs. Bt is a naturally occurring soil bacterium typically used as a microbial insecticide. The Bt strain registered for the Japanese beetle is for use on the grub stage only. Bt is a stomach poison and must be ingested to be effective. Apply it to the soil as you would insecticides.
Effectiveness is similar to that of insecticides.
Check with your extension agent regarding the availability of Bt. This bacterium was first registered for use on turf in suppression of the Japanese beetle grub in the United States in 1948. Upon ingestion, these spores germinate in the grub's gut, infect the gut cells, and enter the blood, where they multiply. The buildup of the spores in the blood causes the grub to take on a characteristic milky appearance. Milky spore disease can suppress the development of large beetle populations.