University of Illinois Extension

 


Bruce Spangenberg,
Extension Educator, Horticulture
Rockford Extension Center

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FAQ's on White Grubs in Lawns

As we advance into summer, concerns are often raised about white grubs in lawns and ways to manage them. Here is a summary of frequently asked questions on white grubs in lawns.

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.

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, brown areas in 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.

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 (refer to the next article on dealing with moles).

Why Does My Lawn Have Grubs But Not My Neighbor?

Remember 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.

Can I Prevent Grub Damage?

There are some options to consider. One option is allowing the lawn to go into dormancy as conditions dry in July (assuming there is little rainfall), reducing the odds of grub damage. Another is to closely monitor the lawn as we advance into late summer and be ready to act if grubs start to appear. Insecticides such as diazinon (still available in 2001) or trichlorfon (Dylox) can be applied when grubs are first noticed to prevent large-scale damage. Insecticides that take about 3 weeks to kill grubs, such as imidacloprid (Merit) or halofenozide (GrubEx, GrubBGon) can be applied prior to noting damage. Apply these in late July to lawns likely to show damage (adults present, irrigating lawn). All of these insecticides should be watered into the soil for best results. Lawns should also be watered prior to application.

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.

 

June - July 2001: Summer Mole Problems | Summer Lawn Care Tips | Immigrant Plants | FAQ's On White Grubs in Lawns

 

Past Issues

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