As you work in your garden, you may come across C-shaped, whitish grubs in soil. They have brown to tan heads and six stubby, brown legs. The grubs are the larvae of different kinds of scarab beetles. Most of the grubs feed on plant roots, but some feed on decaying organic matter. Although white grubs may look formidable, an occasional grub usually doesn't cause significant damage to plants. Spring is a good time to assess white grubs' presence and their population level.
Keep Lawns Healthy
The yard area most likely to contain grubs is the lawn. A number of scarab beetles lay their eggs in turfgrass; those beetles include the masked chafer, May and June beetles and the black turfgrass ataenius. A healthy lawn can tolerate some damage from grubs without showing symptoms. So in spring, follow good watering, mowing and fertilizing practices to keep your lawn as healthy as possible. Aerate and dethatch the lawn because aeration destroys a number of grubs.
Monitor for Lawn Grubs
In spring, lawn grubs are large and easily seen. They soon turn into pupae for the resting stage and then hatch as adults in early summer. Although usually the best time for grub control measures is in mid to late summer, spring monitoring will give you an idea of the grub population's number. With a sharp spade or shovel, cut through the grass so you can roll back a flap of the turf and look at the root area. Count the grubs present in 1 square foot of lawn grass roots, using several 1-square-foot samples. If the samples average six or fewer grubs, then the grub population level is low enough for lawn damage from grubs to be unlikely. Home lawns infrequently have severe enough grub infestations to warrant treatment with pesticides. If the level of infestation is near the threshold of six grubs per 1 square foot, monitor the grub population in late summer or fall as well.
Manage Lawn Grubs
If treatment is needed, consider using beneficial nematodes, which are parasites of white grubs and kill them. The microscopic wormlike creatures can kill 50 percent or more of white grubs without killing naturally occurring parasites and predators that also reduce grub levels. Purchase nematodes from a reputable supplier, and keep them alive until you use them. Apply nematodes when the grubs are active and present, which is generally late spring, summer or early fall. Determining which kinds of nematodes are effective against the kinds of grubs in your lawn is necessary. Don't apply the nematodes unless temperatures are above 60 degrees Fahrenheit. During hot weather, wait until cooler evening temperatures before their application. Mix the live nematodes with water, and apply them with a hose-on sprayer, following the label directions for the specific kind of nematode you use. Then irrigate the lawn with about 1/4 inch of water. Additional irrigation sessions may be required to keep the soil moist but not soggy so the nematodes can move through it freely. Another nematode application two weeks after the first one may be needed.
Monitor Non-Lawn Areas
White grubs you find in compost heaps or piles of plant debris are probably larvae of green June beetles, large, metallic-green and brown insects. The adults fly in summer, eating ripe and overripe fruits. The grubs eat decomposing plant material rather than live roots. In spring, remove piles of old plant material, and turn compost piles, removing and destroying the grubs as you find them, to lessen the number of grubs that will become green June beetle adults.
- University of California Integrated Pest Management Online: Lawn Insects
- University of California Integrated Pest Management Online: Masked Chafers (White Grubs) -- Cyclocephala Spp.
- University of California Integrated Pest Management: Turfgrass -- Monitoring and Treating Insects and Mites
- Texas A&M System AgriLife Extension: White Grubs in Texas Turfgrass
- University of California Integrated Pest Management Online: Beneficial Nematodes
- University of California Integrated Pest Management Online: Green Fruit Beetle -- Cotinis Mutabilis
- PlazacCameraman/iStock/Getty Images