The ubiquitous garden grub is a pest no matter what. It chews the roots of lawns and many plants. In its adult stage, this Japanese beetle chews the foliage of trees, shrubs, vegetables and flowers. Because the pesticides used to treat grubs are highly toxic, organic methods are preferable for the vegetable gardener. Methods used to reduce grub problems include handpicking the larval grubs and adult Japanese beetles -- and treating the soil with diatomaceous earth, milky spore and beneficial nematodes, all of which are not harmful to earthworms, bees, humans or pets.
Apply diatomaceous earth. Use a dust mask if applying dry dust. Diatomaceous earth is an abrasive dust found in sedimentary deposits formed from the skeletal remains of algae. It is non-toxic to humans, pets and earthworms, but take precautions to limit eye and mucous-membrane exposure.
Inoculate the soil with milky spore. Milky spore is a bacteria that gets ingested by grubs, but does not affect earthworms. The grubs die and release more milky-spore bacteria into the soil for other grubs to ingest. Because this method takes one to three weeks to affect grubs and depends on the spread of the bacteria when grubs die, this method of organic control is a long-term solution. Optimal results are seen one to three years after initial application.
Introduce beneficial nematodes. As long as high temperatures and drought conditions do not exist, beneficial nematodes kill grubs by infesting and reproducing within the grub. Because nematodes are host specific, they also do not harm worms.
Handpick the adult beetles. When Japanese beetles are present, handpicking them is an effective way to reduce theirs numbers without harming any worms. Simply remove them with your fingers. Alternatively, put the beetles in a container with soapy water or kerosene and a lid to prevent them from escaping.
Practice companion planting. Companion planting uses other plants to benefit each other. Some plants have also shown to deter or kill grubs and Japanese beetles. These include garlic, tansy, rue and geraniums. By establishing these plants near others which attract the Japanese beetle, you reduce the damage they cause while allowing the earthworms to remain undisturbed. Larkspur has been shown to attract and poison Japanese beetles.