There may be an invisible foe causing damage to your lawn, and you’ll never suspect until patches of grass die. Or, you may get night time visits from the neighborhood skunks and raccoons, digging up your lawn looking for a tasty treat. If this sounds familiar, you may have a white grub problem.
White grubs are the larvae of several types of beetles, particularly Japanese beetles, June bugs, green scarabs and chafer beetles. The grubs of all of these pests look very similar, varying mostly in size. They feed on the roots of turf grasses, denying the leaves water and nutrients and causing death to the grass. Damaged areas can be lifted up in chunks like sod.
Almost every lawn has some grubs, and at lower levels the damage they cause is barely noticeable. At higher levels, however, they can kill large areas quickly. To see if you have a problem in your lawn, take a spade and cut 1 ft. x 1 ft. section of grass, peeling it back like sod. Dig up the soil about 4 inches deep and sift for grubs. Repeat in a few locations on your lawn. If you find 5 or more grubs per test hole, you should consider applying a pesticide. (Replace the soil, flip the grass back over it and water well when you are done. Toss any grubs you did find onto the lawn, where birds will find them quickly.)
There’s no need to check for grubs if you had grub problems last year and did not treat for them; you’ll likely have even more this year. There’s also no need to check for them if the damage caused is more than you find acceptable, however much that is.
Control methods need to be applied when the grubs are actively feeding, otherwise the application is wasted. Grubs feed in spring (April-early May) before emerging as adults, and again in late summer (August-early October) when the next generation of eggs hatch and feed before retreating deeper into the soil to overwinter. These are the times to apply grub control. Grubs will also retreat and stop feeding during periods of dry weather, so apply pesticides a few days after rain, or irrigate well a few days before application if it's been particularly dry.
The most effective pesticides for grub control are those containing imidacloprid. Be aware that imidacloprid is toxic to bees, as are all chemical grub controls, and must be applied carefully and according to label directions.
If you want to stick to organic grub control you do have a few options. Milky spore works on Japanese beetle grubs only, and will not control other grub species, but is safe for everything including bees. There are also nematodes that kill all types of white grubs and if applied correctly work well to control grubs and other soil-dwelling insect pests, and are harmless to all other organisms. For the nematodes, the “correctly” part can be tricky as they have a narrow range of preferred temperature and are sensitive to UV rays and drying out. If you want to try nematodes, look for Heterorhabditis as they work best on grubs.
Finally, do not use Japanese beetle traps in the hopes of reducing adult Japanese beetle populations. They work well to attract every Japanese beetle in the neighborhood to your yard, where they will happily lay eggs in your lawn and create a grub problem.