In much of Illinois, timely rains during June and July reduced the likelihood of white grub injury in turf. When rainfall continues through July, with even unwatered turf being green, adults of both the masked chafers (annual white grubs) and Japanese beetles lay their eggs over large areas. This results in few places where grubs are numerous enough to cause serious injury by feeding on turfgrass roots.
Although grub numbers per square foot appear to be low this year, there will be “hot spots” where grub numbers are high and damage is very likely. Block after block of lawns with low numbers will commonly be interrupted with an occasional lawn or portion of lawn with very high numbers of white grubs. Frequently, these lawns have heavy thatch or have been fertilized with manure or compost. The beetles that produce white grubs in turf are members of the Scarabaeidae, a family primarily known for dung beetles and species that feed in rotting wood. It is thought that lawn-damaging scarabs are attracted to high-organic soils because they still have an interest in decaying materials. Many of these species’ larvae feed and can subsist on the dead organic matter in the soil, transferring over to feed on roots as the opportunity occurs.
To find these “hot spots,” scouting is critical. Using a sharp, heavy knife, cut through the thatch and pull up a square foot of turf. Count the number of grubs in the root zone. If there are fewer than 10 per square foot, treatment should not be necessary. However, raccoons, skunks, and birds will tear up the turf, seeking the grubs as food. Grub numbers as low as three per square foot have been known to attract them. Areas with high numbers of these animals may require treatment to avoid damage.
Raccoon damage appears during the night as strips of sod (6 inches to 1 foot wide) pulled back to expose the grubs. Their damage is very obvious and impressive. Skunks also go after grubs at night. They open circular holes about 3 inches in diameter through the thatch. A single skunk makes about 100 of these holes per night. Insectivorous birds such as starlings and robins scratch open the soil to expose the grubs, causing brown, rough areas.
The treatment of choice for grubs at this time of year is trichlorfon, sold as Dylox. It kills the grubs in 3 days and should be gone in 5 days. Irrigation a day or two before insecticide application brings the grubs closer to the surface, resulting in better control. Insecticidal nematodes, particularly Heterorhabditis bacteriophora (Hb nematodes), are also effective at this time of year. Irrigation before application in the late afternoon or evening with follow-up irrigation after nematode application should provide about 60% control.