Milky spore is a natural bacterium that can be an effective control of Japanese beetle grubs, but it usually takes several years for it to become established in the soil. And it requires some grubs in the soil to live on, so don't use it in combination with chemical grub controls. Milky spore is ineffective on other types of grubs.
Beneficial nematodes are microscopic worms that live in the soil. They seek out grubs, and after entering their bodies, release bacteria that kill the grubs. The two major types of insect parasitic nematodes are Steinernema spp. and Heterorhabditis spp. The nematodes are applied as a live product, so make certain if you use them that they have been handled carefully to keep them alive, and that their shelf life has not expired. These nematodes are effective on all types of grubs in the soil.
Weather and irrigation practices can work in your favor to control grubs, too. Grub populations are usually highest when soil conditions are consistently moist. In dry years or if you withhold irrigation water during hot summer months, many grubs will not survive. This strategy may work well for bluegrass lawns that recover well from summer dormancy.
Apply curative chemical grub controls at the correct time, when grubs are small and actively feeding near the soil surface. This is usually from early July until mid-August. Chemicals labeled for curative control are carbaryl, halofenozide, and imidacloprid.
Because grub populations vary so much from year to year, preventative chemical controls are seldom justified. But if your lawn has been attacked by grubs consistently, a preventative insecticide application in late May or June may be warranted. imidacloprid and halofenozide provide extended, preventative grub control.
Water in chemical controls to move the chemical into the soil where it can act on the grubs. Watering the lawn after chemical application not only moves the product down to the thatch layer, it also stimulates the grubs to move upward in the soil, closer to the chemical.