Grubs in turf and garden

White grubs are the larval stage of insects known as May or June beetles. While most beetles do not cause significant damage, this is one that is capable of severely damaging turf and some ornamentals.

Once a year, from April through May, adult beetles emerge from the soil to mate. Females return to soil to lay eggs and within about two weeks the eggs hatch into small white grubs that feed on grass roots. Female beetles dig two to five inches to lay eggs and each female can lay up to 30 to 40 eggs. Most of the damage occurs during mid-summer through early fall when the larger, ever-developing grubs are actively feeding.

Turfgrass damaged by white grubs has a reduced root system and is easily pulled from the soil. Severe damage may allow you to roll up the turf, similar to rolling up a carpet. If the damage is not extensive, the turf will recover, developing new roots by early spring.

Additionally, grubs feed on ornamental grasses in a well-amended garden bed, or on nut grass. This is where I ran into trouble, allowing nut grass to take over a raised vegetable bed after the harvest season.

After digging out all of the nut grass, the resulting grubs feeding on the roots are shown in the photo. As you will note, some are over one inch in length, making treatment difficult. Since these grubs are in a confined space, I simply removed them, damaged each and fed them to the birds. In a turfgrass situation, treatment may be necessary.

Before you get out an arsenal of chemicals, first confirm whether you need to treat.

Examine several soil sections with a shovel, digging a wedge into the lawn about four inches in several areas. If you find five white grubs per square foot, treatment is justified and the best time to begin checking the lawn for grubs is in June, not July, as I have done. Remember, you don’t have to kill every grub, just reducing their numbers can allow the grass to recover on its own.

There are a couple of beneficial nematodes that are effective.

Beneficial nematodes in the genera Steinernema and Heterorhabditis are tiny worms that attack white grubs. These microscopic worms are purchased in garden stores, through catalogs, or the Web. (Simply, search “beneficial nematodes” on the Internet.) Nematodes must be supplied with moisture to help them move down into the soil, so provide one fourth inch of water just before applying and one fourth inch of water after nematodes are sprayed on the lawn. These worms pose no threat to humans or plants and are an environmentally sound alternative to pesticides. In colder regions a microbial pesticide, Bacillus popilliae, or milky spore disease, is recommended for white grub control, but researchers at Texas A&M University have not found them effective in Texas.

The insecticide imidacloprid is best for white grub control for small and medium-sized grubs and may kill some larger than one-half inch.

Imidacloprid trade names include Merit, Bayer Advanced Lawn Season Long Grub Control and Scott’s Grub-Ex. For very small grubs, the insect growth regulator, halofenozide, is effective. The trade names for this insecticide are Mach 2, Spectracide Grub Stop and Hi-Yield Kill-a-Grub. Both of these can be applied early and persist in the soil. Although June is the best time to detect grubs, both beneficial nematodes and imidacloprid will reduce grub numbers now, before they can cause severe damage.

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Barbara Storz serves South Texas residents as the area Extension Horticulturist. You can reach her at (956) 383-1026 or by e-mail at b-storz@tamu.edu.