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White Grubs in Lawns

Concerns are often raised about white grubs in lawns and ways to manage them. Late summer into early fall is the time white grub problems are most likely to show in lawns here in northern Illinois. While a relatively small percentage of lawns will actually get white grubs in a given year, this insect can cause serious damage when a significant population occurs on a lawn or other turf area.

Since white grubs are often asked about, this month’s Lawn FAQ features white grubs in lawns and summarizes the most common questions regarding this pest of home lawns.

• How do I know if I have grubs in my lawn?
• Why does my lawn have grubs but not my neighbor?
• How can I predict if my lawn will have grub damage this year?
• Can I prevent grub damage?
• What about using organic controls for grub control?
• What should be done after grub damage has occurred?

How do I know if I have grubs in my lawn?

White grubs feed on the roots of grasses, so lawns will show wilting and browning of irregular shaped areas. Certainly there could be many reasons for lawns browning, especially in late summer when most grub damage occurs. Always check the root zone of affected areas for the white, c-shaped grubs. Carefully pull back the sod in suspect areas, in particular the marginal areas where brown grass meets green grass, and look for the grubs. Usually a population of about 10 or more grubs per square foot will lead to browning of the lawn.

Peeling back a damaged lawn area has revealed these white grubs in the soil as the cause.

Keep in mind other factors that can lead to poor rooting and are mistaken for grubs. For example, lawns in shade areas often have weak roots and are pulled-up easily. Grubs do not typically appear in shade lawns. Also, many lawns were easily pulled up this spring and grubs were blamed. Once grass dies, regardless of the cause, roots will rot away and the grass is very easy to tear out. So trying to diagnose grub damage from the previous season as the cause of a dead lawn area in spring is very difficult to do, even if limited roots are found in an area of dead grass.

Raccoons and skunks have damaged this lawn area looking for grubs.

Another sign of grubs is damage from skunks and raccoons digging up lawns in search of grubs to eat. This usually happens at night. Moles may or may not be feeding on grubs so are not a reliable indicator of grub problems.

Why does my lawn have grubs but not my heighbor?

Keep in mind the adult stage of the grub life cycle is a beetle, which can fly. Random chance is part of the answer. But adult beetles usually lay eggs in full-sun lawn areas with adequate soil moisture. The masked chafer (annual white grub) and Japanese beetle lay eggs in July. So if the weather has been dry but your lawn is watered and surrounded by dry lawns, it is a prime target for egg laying.

How can I predict if my lawn will have grub damage this year?

It is difficult, as insects can go in cycles and many factors influence the chances of grubs appearing in your lawn. Lots of adult beetles on the lawn in July is one indication. Masked chafers, the adult of the annual white grub, are tan beetles active shortly after sundown. Japanese beetles fly during the day and feed heavily on many ornamentals. Noting these adults and then having irrigated lawns surrounded by drier turf increases the chances of grub damage to your lawn. Watch lawns closely starting about mid-August and continuing into September for wilting and browning areas, and then check the root zone for grubs.

The masked chafer is the adult beetle which lays eggs becoming the annual white grub in lawns.

Japanese beetles, which have been increasing in Illinois, feed on many plants as adults and lay eggs in lawn areas which may become a grub problem later in the season.

Can I prevent grub damage?

There are some options to consider. One option is allowing the lawn to go into dormancy if conditions dry due to lack of rainfall in July, reducing the odds of grub damage since the adult beetles look for green lawns with good soil moisture for egg laying. The downside is the lawn will be brown and dormant. Also, this may not be reliable if rainfall keeps grasses green throughout July, although if all lawns are green, the chances of significant grub damage on any one lawn are low since the adult beetles tend to disperse and lay eggs over a much broader area under this scenario.

Another option is to closely monitor the lawn as we advance into late summer and be ready to act if grubs start to appear. Watch for grass areas going off-color and just starting to brown, in particular those areas that have been irrigated. Check the root zone for small white grubs. Insecticides such as diazinon or trichlorfon (Dylox) can be applied when grubs are first noticed to prevent large-scale damage.

Other insecticides such as imidacloprid (Merit) or halofenozide (GrubEx) can be applied prior to noting damage, such as in late July to lawns likely to show damage (adult beetles present, irrigating lawn). All of these insecticides should be watered into the soil for best results. Lawns should also be watered prior to application.

With all insecticides, read and follow label directions. To confirm what a product contains as active ingredient, check the active ingredient section on the front of the label.

What about using organic controls for grub control?

Heterorhabditis bacteriophora nematodes have shown good results for white grub control. Nematodes are very small unsegmented worms. This particular species will search out white grubs and after entering the grub, release bacteria that kills the grub. This product is available in mail order catalogs, often sold as Hb nematodes. This product should be applied late in the day to lawns with adequate soil moisture and then watered in immediately.

What should be done after grub damage has occurred?

If significant grub damage has occurred, the lawn will need some renovation work in early fall. Rake away dead debris. Water the lawn area, as some of the grass with damaged roots may recover, especially if the weather cools down. Bare or thin areas may need some reseeding. Labor Day is a good target date for lawn renovation work in northern Illinois.