The second half of August is when white grub damage becomes apparent in Illinois. Heavily attacked turf areas wilt and turn brown due to the grubs eating off the roots. Because the root mass has been greatly reduced, the turf is easily rolled back to reveal the C-shaped, white grubs in the root zone. At this time, the grubs are about 3/4 inch long.
Japanese beetles have become the dominant white grub in most areas of the state. The adults have been very numerous in southern and northern Illinois, where damage will be likely to occur in nontreated, irrigated turf. In central Illinois, the number of adult Japanese beetles has been very low, about one-fourth of what has been normal for several years. As a result, white grub damage should be spotty. In areas where Japanese beetles have not displaced the masked chafers, their white grubs are also present and causing damage at this time.
Scouting for larvae.
Check for white grubs by cutting through the turf with a heavy knife and peeling the turf back to reveal the grubs. Some grubs hang in the sodís roots, but most are easily seen on the surface of the exposed soil. Use the knife to till 2 to 3 inches deep into the exposed soil to dig up any deeper grubs. In dry soil, the grubs may be 4 to 6 inches deep.
Generally, 10 to 12 grubs per foot square (12 by 12 inches) are enough to cause serious turf damage. Lightly used turf may support 18 grubs or more per foot square without showing damage, whereas heavily used turf may show damage with as few as 6 to 8 grubs per foot square. Skunks, raccoons, and armadillos dig up turf when 3 or more grubs per foot square are present.
Trichlorfon, sold as Dylox, is usually the insecticide of choice when white grubs are present. An advantage of Dylox is that it kills up to 95% of the grubs within 3 days. However, Dylox typically lasts only about 5 days. It is recommended that dry turf be irrigated a couple of days before applying Dylox, to bring the grubs up into the root zone. Apply at least 1/2 inch of water after applying Dylox, to move the insecticide down into the root zone where the grubs are located.