The two nematodes that are most effective against Japanese beetle grubs are Steinernema glaseri and Heterorhabditis bacteriophora. The latter is commercially available.
A University of Minnesota bulletin has this to say about using nematodes to control Japanese beetles:
"A beneficial nematode, Heterorhabditis bacteriophora, attacks JB grubs. Nematodes are microscopic parasitic roundworms that transport and feed on bacteria. When they find a grub, the nematodes penetrate the larva and inoculate it with bacteria, which quickly multiply within the grub's body. The nematode then feeds on the bacteria. Nematodes need to be applied to soil at night and the soil must be irrigated daily to keep it moist so the nematodes stay alive."
Go here for a thorough discussion of Insect-Parasitic Nematodes for the Management of Soil-Dwelling Insects
"Nematodes are shipped in the infectious juvenile stage of their life cycle and can be stored in the refrigerator for up to 2 - 3 weeks. They may come on a sponge, in gels, granules or liquids.
The best time to apply nematodes depends on the weather and grub species. Most summers mid to late August is about right. The grubs should be actively feeding and in their second or third instar when they have larger spiracle openings. The nematodes enter the grubs through the spiracles, mouth and anus and release a fungus which then consumes the grubs.
The nematodes are neutralized by exposure to strong sunlight, so they must be applied in heavily overcast or rainy conditions; at dusk or during rainfall is an ideal time. Nematodes prefer a moist environment. If the ground is very dry, it helps to pre-irrigate the area with at least 0.5 inches of water.
Follow the label and mixing instructions carefully. To treat a 2000 sq. ft. lawn area, mix 1 million nematodes in at least 4 gallons of water that is under 86o F. Carefully pour the nematodes into a hand sprayer, hose-end sprayer or pressurized backpack sprayer and use the spray solution immediately. The sprayer nozzle opening should be at least 0.02 inches and any screens should be removed. Evenly spread the solution over the area to be treated. Continuous mixing should take place to prevent the nematodes from sinking to the bottom. After application apply 0.25 inches of water to wash the nematodes off the grass blades, and then keep the soil moist for at least two weeks to help them get established."