Grubs in high numbers severely damage lawns. Many lawn grubs are C-shaped, white or greyish; they are the larvae of scarab beetles. Scarab beetles that lay their eggs in turf grass are the May/June beetle, masked chafer, Japanese beetle and the black turfgrass ataenius beetle. Another type of grub that has damaging effects in lawns is the billbug grub.
Scarab Beetle Grub Effects
A heavy infestation of scarab beetle grubs, also called white grubs, causes irregular patches of dead grass in the lawn. The yellow or brown patches resemble drought damage. Walking on a lawn damaged by white grubs, feels spongy. If you tug on the sod, you can roll up the patches of grass like a carpet, because the white grubs have severed the grass roots below. Usually, lawn damage is noticeable in late summer or early fall. Another sign that your lawn is infected with grubs is that birds, moles, skunks or raccoons dig up your lawn in search of grubs, a tasty meal for them.
Billbug Grub Effects
Billbug grubs also kill grass, but the effects differ. Unlike white grubs, billbug grubs devour not just roots, but also the stems and crowns of turfgrass. Root damage also differs; billbug grubs feed on the upper parts of grass roots. A white sawdust-like powder, which is their excrement, appears in damaged areas of the lawn. Generally, lawn patches cannot easily be rolled up, unlike areas where damage is due to white grubs. Walking on billbug grub damaged areas does not feel spongy.
Armyworms and Cutworms
Armyworms and cutworms are not technically grubs, but they are larvae that live in the soil of lawns and cause damage that resembles that of grubs. Different night-flying moths lay eggs in the grass to produce armyworms or cutworms. After hatching, both larvae begin to eat the blades and stems of turfgrass during the day. At night, armyworms and cutworms hide in the soil of the lawn. Armyworms move around in large groups and create large patches of dead lawn, while cutworm damage is confined to smaller spots. Irrigation and fertilization of the lawn are the best ways to minimize damage.
Treating the Lawn
A low number of grubs in a healthy lawn is harmless. Treatment is warranted when there is visible damage and the grub concentration reaches a critical limit. The treatment threshold for masked chafer and Japanese beetle grubs is 8 to 10 grubs per square foot, for May/June beetle grubs 3 to 5 grubs per square foot and for the small ataenius grubs 30 to 50 grubs per square foot. One billbug grub per square foot of lawn is considered sufficient for starting treatment. Fight grub infestations with cultural methods, biological controls or chemical insecticides. Keeping the lawn watered, fertilized and aerated allows the lawn to better withstand a grub infestation. Biological controls include "Milky Spore" powder and nematodes. You can use insecticides on badly infested lawns, but insecticides also kill beneficial insects.
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