White grubs typically infest lawns any time after mid-August, but damage to an established lawn may not be visible until September or October. Heavy infestations can kill localized areas of your lawn, so it can be alarming to see an infestation of grubs in your sod. However, their presence doesn’t always mean that the sod must be replaced -- or even that your lawn problem is caused by the grubs.
The Problem With Grubs
Grubs cause symptoms similar to drought stress. As the grubs feed on your sod’s roots, the grass begins to wilt and turn brown. If grub numbers are high enough, you may see entire patches of dead sod, which typically lifts around the edges because the roots have been eaten by the grubs, so it has nothing to anchor it to the soil. Sometimes small mammals like skunks will pull back the sod to eat the grubs, and this damage is often more extensive than damage caused by grubs alone. If the roots are still present, though, these areas can be repaired by tamping the sod back down and watering the area. Soil moisture is a key factor in the extent of grub damage. Lawns that receive adequate moisture may tolerate infestations without problems, because the grass is healthy and continues to grow. However, if drought conditions occur, the damage symptoms may appear rapidly.
White grubs are the larval stage of beetles in the Scarabaeidae family, which includes the Japanese beetle, European chafer, and June beetle. These grubs feed on grass roots, as well as on the roots of trees and shrubs in the fall and spring. No matter what species you’re dealing with, white grubs are milky white in color, with C-shaped bodies, brownish heads and six spiny legs. They’re small, initially, and depending on species, may range between 1 and 2 inches in length at maturity.
Deciding to Re-Sod
It is unlikely grubs found in new sod will cause problems. Lawns that receive daily irrigation can handle infestations of 30 grubs per square foot without showing signs of stress. Lawns that are drought stressed, though, may not survive more than 10 grubs per square foot. To make sure you aren’t dealing with a heavy infestation, take some samples by digging a 6-inch square piece of sod from at least four different areas of you lawn. Count the number of grubs you find in the soil and roots on the underside of each square and add these totals together. This will give you an average number of grubs per square foot of lawn.
Installing New Sod
If your sod is severely damaged, and much of the root system has been destroyed by grubs, it should be replaced. Sod can be replaced any time of the year as long as the soil remains consistently moist to ensure root establishment. Cut out the dead sod and replace it with a new piece. Tamp the new sod down firmly, or go over it with a roller so that all parts of the underside of the sod contact the soil, and then water the area thoroughly. Another option is to remove the dead sod and plant lawn seed. Scatter the seed evenly over the soil and rake it in to ensure the seed is in contact with the soil. Cover with a layer of topsoil or compost and roll the area lightly. Keep the seedbed moist until the seeds germinate. The best time to replace sod with grass seed is in the spring, because the majority of grub damage will occur at this time.
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