Both white grubs and parasitic, microscopic nematodes create dead or thin areas of the lawn. Not all struggling lawns are the result of grubs and nematodes, but healthy lawns bounce back from a wide range of problems, including subterranean pests. These pests are nearly impossible to eradicate completely, but with patience and legwork, you can protect your lawn.
Take multiple samples of soil, 3 inches under the soil surface, from various areas of the lawn. Mix the samples and place roughly 2 cups of the soil in a waterproof container, such as a resealable plastic storage bag, and take it to your local county extension office. Ask specifically for a nematode test. Do not allow the samples to dry out or expose them to sunlight, cold or heat; this kills the nematodes.
Look for white, C-shaped grubs just under the turf and within the root zone when digging. Count the number of grubs; more than five grubs per square foot may overwhelm the lawn.
Prevent stress on the lawn. Keep lawns 2 to 3 inches high when mowing. Aerate compacted turf and top-dress the lawn lightly with compost or an appropriate fertilizer in late spring. Parasitic nematodes do not respond to pesticides. However, healthy lawns cope with average nematode infestations or white grub damage.
Water the lawn deeply, allowing the lawn to dry between watering. This encourages root growth deep into the soil where moisture is more abundant and lessens the lawn's stress during drought or hot weather. A well-formed root system also rebounds from nematode and grub damage.
Treat serious white grub infestations with commercially-produced nematodes that prey on the grubs or, in severe cases, pesticides. The nematodes are not toxic to earthworms, people or pets and may overwinter in the lawn, providing protection through the next spring. If using mild pesticides, such as imidacloprid, treat the lawn in June or July, before the eggs hatch. Stronger pesticides, containing trichlorfon or carbaryl, kill young grubs in spring or fall.