You can treat infestations of white grubs (larvae of June beetles, European chafers, Japanese beetles, rose chafers, etc.) in the lawn with nematodes, small parasitic worms that infect and kill young larvae. There are various species of nematodes that will do the job, mostly in the genera Steinernema and Heterorhabditis. They work by penetrating their host and injecting bacteria that kill them.
Note that beneficial nematodes are harmless to beneficial insects, such as bees and ladybugs, as well as earthworms, and also to birds, reptiles, amphibians, mammals, etc., although they also help control other harmful soil insects such as weevils, fleas, borers, wireworms, onion maggots and fungus gnats.
In the Northern Hemisphere, nematodes are best applied at sunset or on a cloudy day between mid-August and early October in cooler climates, mid-July through August in mild ones, as that is when the grubs are freshly hatched and most vulnerable to infestation. The soil should be warm (70 to 86˚F/21 to 30˚C) and moist (apply after rain if possible, otherwise water deeply the day before).
You should be able to find beneficial nematodes in your local garden center. If not there are many mail order sources on the Internet. In years past, you had to ask for them, as they were stored out of sight in a refrigerator (likewise, they have to be stored in a refrigerator at home until you’re ready to use them). However, recent technological developments in storage methods have led to some brands that don’t require refrigeration, although they must not be stored in sunlight.
To apply, mix the product in water and spray on the lawn, following the instructions on the product purchased (they’ll vary a bit depending on the species of nematode and the way the supplier has packaged them). Water the lawn after the application and as needed over the next 14 days so the soil remains moist, especially in heavy or clay soils, as this ensures that the nematodes will descend deep enough into the soil to reach their prey.
Combined with other control methods (such as turf enriched with compost, mowed high, and mixed with other plants such as clover), you’ll find nematodes quite effective… in the medium and long term. But not necessarily the following year! That’s because nematodes are especially efficient in killing freshly hatched white grubs in the first year of their cycle, the ones that hatch in July (mild climates) or August (cool climates). They’re not so good at controlling 2nd-year grubs (those that live two years underground, as is the case with June beetles), the ones that will cause the most damage the following year. That’s why it is often in the third summer you’ll see the full results of your treatment, because that year’s generation will have been largely eliminated. Once introduced, beneficial nematodes can remain in the soil for many years, providing long-term control.
Note that it is not necessary to eliminate all the white grubs from a lawn to get excellent results. A healthy lawn can tolerate a modest population of white grubs without showing the typical symptoms (brown patches in the lawn). That’s what you should strive for: a natural balance in your lawn’s environment, one that will give you a healthy lawn without disrupting Mother Nature’s cycle.