We have had scattered reports of turf damage due to white grubs around the state. These typically show up when an area experiences droughty conditions, and the grubs make their presence known with wilted turf. In other cases, wildlife such as skinks, raccoons, armadillos, and birds damage turf to get to the grubs.
Our timely rains throughout most of the summer have helped prevent heavy grub damage. During the first half of July, when the Japanese beetle, northern masked chafer, and southern masked chafer adults were laying eggs, many areas of the state had soft soil and green, actively growing turf due to timely rains. This resulted in egg-laying being more widespread than that which occurs in typical years, in which egg-laying is concentrated in irrigated turf.
Even with these moist soil conditions and actively growing turf during egg-laying, some areas ended up with fairly high grub numbers, typically in the range of 15 to 25 grubs per foot square. Although we use a white grub threshold for treatment at 10 to 12 Japanese beetle or masked chafer grubs per foot square, the effect of grubs on turf is variable. Soil type, degree of use, and other factors apparently enter into the amount of damage that occurs. Heavily used turf may show grub injury with fewer than 10 grubs per foot square. Alternatively, over 30 grubs per foot square have been found in healthy turf under light-use conditions. There appears to be other factors that enter into white grub damage that we do not understand.
Mammals and birds commonly damage turf with as few as three grubs per foot square. While searching for and feeding on grubs, a single skunk in one night can make about 100 holes through the turf that are 2 to 3 inches in diameter. Raccoons peel back the sod in areas that are usually 4 to 8 inches wide to expose the grubs. Armadillos dig holes several inches deep and several inches wide to feed on grubs. Armadillos entered Illinois several years ago and are most common in the southern third of the state. However, several have been found in the rest of the state, including northeastern Illinois. Insectivorous birds, such as starlings, blackbirds, cowbirds, and robins, peck holes through the thatch to feed on grubs. Areas that have been heavily worked by birds look brown from hundreds of tiny divots of thatch having been pulled up. Where the grubs are numerous, robins in particular chicken-scratch, scratching away the turf in patches that are several inches across in searching for grubs.
Insecticidal control of these older, larger grubs is more difficult than earlier in the growing season. Trichlorfon and clothianidin are more effective than several other insecticides labeled for white grub control in turf. Trichlorfon is sold as Dylox. Clothianidin is sold as Arena and is also sold as Aloft in a mixture with bifenthrin. Insecticidal nematodes, particularly Heterorhabditis bacteriophora, Steinernema glaseri, and Steinernema feltiae, are also effective against these larger white grubs. All of these treatments should be watered in thoroughly after application. The nematodes should also be applied to wet turf because they die if they dry on the turf. As trichlorfon lasts only 5 days after application, water dry turf areas a couple of days before application to bring the grubs up into the root zone.