Grass Grub Damage

Root Damage

• Grubs live underground where they feed on grass roots. An infested lawn may feel spongy, and the turf layer rolls back as if it were not attached to the soil. In mid-summer, the first-stage grubs feed near the soil surface and cause minimal damage. As the grubs pass through the second and third stage in late summer and fall, they feed more heavily on the roots and cause damage that becomes visible above ground the following spring. Grubs overwinter 4 to 8 inches under the soil surface and emerge one more time for a brief spring feeding.

Leaf Blade Damage

• Irregular, rapidly expanding patches of brown, dead grass appear in a grub-infested lawn during the spring following the summer and fall feeding. The leaf blades lack nutrients because the grubs have eaten the roots, so the grass turns brown and dies. At first the patches are a few inches wide, but they grow and blend together. You may find grubs feeding at the edges of the brown patches, where the brown grass and green grass meet. Ten grubs per square foot of lawn will damage the roots enough to cause brown patches, notes the University of Illinois Extension.

Indirect Damage

• Raccoons, skunks and other animals eat grubs and damage grass as they dig for their prey. Many of these animals are nocturnal, so you will only see the evidence the next day. Skunks leave holes and 3- to 4-inch areas of dirt in a lawn, while raccoons shred a lawn to pieces or roll up new sod. Repellents are not effective for skunk or raccoon control. You can, however, discourage these animals if you control grubs in your lawn.

Non-Chemical Control

• In late summer, beneficial soil nematodes control second- and third-stage grubs that cause the most damage to grass roots. Heterorhabditis bacteriophora (Hb) nematodes enter white grubs and kill them with a toxic bacteria within 24 to 48 hours. You can purchase sponges that contain millions of these nematodes from mail order suppliers and some garden centers. For every 2,000 square feet of lawn, drop a sponge segment that contains 5 million nematodes in a gallon of water, recommends the Gardening Zone. Wring out the sponge and spray the mixture on the lawn in late afternoon or evening. You can rake, reseed and water damaged lawn areas in early fall.

Chemical Control

• Some insecticides kill grubs which have already damaged the lawn, while others prevent grub damage. You can spread 2 pounds of trichlorfon insecticide granules per 1,000 square feet in late summer at the first sign of infestation, or 1.4 to 1.8 pounds of imidacloprid granules per 1,000 square feet in mid-summer to prevent infestation. If it does not rain within 24 hours after you apply these pesticides, water the treated area. Check product labels for use restrictions in your state. Wear long sleeves and long pants when you apply pesticides, and keep children and pets away from the treated grass until the rain or water has dried.