White grubs appear to be few and scattered in central Illinois and other locations around the state. Both Japanese beetle grubs and masked chafer grubs (annual white grubs) are being found in numbers of three to four per square foot. In the Springfield area, white grubs are being found in patches of four to six per square foot, but these grubs are large--already approaching one inch in length. At least in central and northern Illinois, the adult masked chafers and Japanese beetles were present one to two weeks earlier than normal, which accounts for the larval grubs being a little larger than usual for this time of year. There are undoubtedly spots around the state where white grubs are numerous, but as yet I have not heard of any. No one attending the University of Illinois Turfgrass and Landscape Field Day on August 19 mentioned grub problems or treatable numbers.
Turf areas of northeastern Illinois, particularly on the south side of Chicago, were dry with the nonirrigated grass appearing brownish in the first half of July when Japanese beetles and masked chafers were flying and laying eggs. So be watchful for grubs in that area. North of Chicago in Lake and McHenry counties, grub numbers appear to be sparse. If you're in the northwestern portion of central Illinois near Peoria, Monmouth, and Galesburg, be watchful for spotty grub problems. In previous years with abundant early summer rainfall like we had this year, that portion of the state still tended to have some treatable turf areas.
If you have high numbers of grub-feeding wildlife, such as skunks, raccoons, or insectivorous birds, you may get damage from these animals with grub numbers as low as three to four per square foot. In these situations, treating with an insecticide that will kill the grubs fairly quickly, such as bendiocarb (Turcam) or trichlorfon (Dylox, Proxol), should effectively head off wildlife turf damage.
Insectivorous birds that will feed on white grubs include starlings, robins, cowbirds, red-winged blackbirds, thrashers, and cuckoos. When grubbing, they poke 1/2- to 3/4-inch holes through the turf. Light damage will appear as scattered dark holes across the turf. When present in flocks, the birds tend to uproot grass plants in their search for grubs. From a distance, the turf area will appear brownish and fluffy, similar to the way it would look after a light dethatching operation.
Mammals cause more impressive-looking damage. Skunks tear out three-inch-diameter divots while searching for grubs. A single skunk will commonly make about one hundred holes in a single night. These holes and their respective divots frequently are concentrated in a relatively small area, making it look from a distance as though someone has been tearing up the turf with a rototiller. Raccoons roll back the turf in sections that are a foot or more square. Moles feed primarily on earthworms and are unlikely to be attracted to an area with white grubs, although grubs do make up a small part of their diet. The champion turf destroyer in North America is the armadillo. Those of you planning on retiring to Florida, Texas, or another southern state can look forward to numerous three-inch-deep holes made by this animal. If these animals replaced their divots and watered them in, we could look at them as worthwhile biological control agents against the white grubs.