Grubs are about an inch long, whitish in color with a tan-colored head, and can be found curled in a "c-shaped" position on the soil surface, just below the thatch. Grass that is damaged by grubs can be easily lifted since the roots are gone, and irregular brown patches will appear in the lawn. Injury usually appears in late August, September and into October.
Grubs seem to be periodic pests, attacking lawns irregularly from year to year. Rainfall and soil moisture are major factors that influence whether there will be damaging numbers or not. In summers with normal or above normal rainfall grub populations increase. Well-maintained lawns next to ornamental plantings favored by Japanese beetles are also more frequently attacked.
Lawns with heavy thatch may also contribute to large, localized populations of grubs. Japanese beetle grubs do not need to feed on grass roots to survive; they can also feed on decaying organic matter - thatch.
Preventive treatments for grubs are not recommended; populations may be low in a particular season. However, if you have determined you will need to treat for the next generation of grubs, there are two relatively new insecticides on the market that are quite effective.
One product contains the active ingredient imidacloprid or Merit, which can be applied from early June to mid-August for control of Japanese beetle and masked chafer grubs. Merit has to be applied before the new generation of grubs is discovered in August, and is therefore useful in lawns with a history of grub problems. Do not apply Merit earlier than mid-May, or it may not be effective by the time eggs hatch in late July to early August.
The other product is halofenozide or MACH2 (molt-accelerating compound halofenozide). Wait to apply this insecticide until early June. An advantage of this product is that it can be applied after the new generation of grubs is discovered in August, mid to late in the month. This is a more integrated pest management approach; targeting the pest population directly once it is discovered, and determined that control is needed. Several companies manufacture these products under different brand names.
A special thanks to the Department of Horticulture and Crop Science; Ohio State University.