White grubs, the larval form of scarab beetles, munch your lawn's tender roots throughout the spring. The C-shaped grubs pupate and hatch in early summer, becoming busy June bugs and chafer beetles laying more eggs, despite their deceptively lazy droning. Eradicating these grubs is not possible, any more than killing every pesky fly in your yard. However, a healthy lawn outlasts the larvae, repairing the damage as quickly as it occurs.
Ensuring lawn health is the first step to grub control. Lift a section of turf using a shovel, concentrating on sections bordering dry, yellowing or sparse areas of lawn, and count the number of grubs. A healthy lawn supports up to ten grubs per square foot, but most lawns stabilize around five grubs per square foot. Aerate lawns and top-dress lightly with compost. Water lawns deeply and less often to encourage root growth, and mow them 2 to 3 inches high. The University of Illinois warns that a green, often-watered lawn is a magnet for egg-laying beetles, especially during dry summers.
Parasitic nematodes are a secondary protection against grubs in healthy lawns or an effective primary treatment for infested lawns. They are cousins to beneficial earthworms, are harmless to people, pets and wildlife, but begin killing white grubs within hours of their application. Nurseries and big-box stores often sell nematodes as "Red Ladies," a commercial name for live nematodes, or as granulated dormant nematodes sold off-the-shelf and designed for use with a hose-end sprayer. A treatment early in the spring introduces these helpful, microscopic predators into the soil, where they may even overwinter and control grubs the following season. Follow the application directions carefully; sunlight and temperature extremes kill the nematodes quickly. Never apply nematodes on a sunny day.
If improving lawn health and applying nematodes are ineffective, use an egg-killing pesticide. According to the University of Connecticut Cooperative Extension, clothianidin, halofenozide and imidacloprid are effective preventative pesticides sold under many brand names. Apply pesticides before the eggs hatch for the best results. Eggs hatch at different times in different climates: for example, in the northeastern United States, apply pesticides in June through mid-July. Check with your local extension office for the best dates in your zone, and don't procrastinate. Irrigate with ½ inch of water afterward to push the chemicals into the soil.
Pesticides for grubs are much stronger than the egg-killing pesticides. Many, such as the commercial brands Dursban and Diazinon, are no longer available for home use. If using a larval pesticide, water the lawn well to bring the grubs to the surface before treatment. Always follow the directions carefully.