We have had several reports of scattered grub damage, particularly in northeastern and central Illinois. Some of these reports have been verified as true white grubs. Reports have mentioned counts as high as eight to ten grubs per square foot, enough to cause heavy root pruning and turf browning. In other areas, turf managers are seeing mainly skunk, raccoon, and bird damage. This type of wildlife damage occurs at grub levels of three to five per square foot.
True white grubs belong to the genus Phyllophaga. This genus contains many species that vary in appearance as well as life cycle length. True white grubs are also known as three-year white grubs because several species have a three-year life cycle. However, there are other species of true white grubs with longer and shorter life cycles. The most common species that are important to turf in Illinois have three-year life cycles and emerge in the spring as adults that are about one inch long and dark to reddish brown.
Earlier this spring, we had a large emergence of these beetles. They feed at night on the leaves of oak, crabapple, ash, and other trees. Because they feed as adults, eggs are laid over a period of several weeks. This results in some larvae being much larger than others, a characteristic recently reported by turf managers.
The grubs feed on roots all summer, go deeper in the soil for the winter, and return to the root zone in the spring to feed for another summer. They do this for two cycles, until mid-summer of their third year. They then pupate in the soil, becoming adults in early fall but staying underground throughout the third winter, emerging the following spring. These grubs are responsible for the sighting of large grubs in the soil at various times during the growing season.
Types of grubs can be differentiated by looking at the raster pattern, a series of thickened setae or fine spines on the underside of the last abdominal segment. Annual white grubs (Cyclocephala) have no distinct pattern, Japanese beetle grubs have a V-shaped pattern, and black turfgrass Ataenius have two padlike structures rather than spines. True white grubs have two parallel rows of spines that may be long or short, and stay parallel or fan out posteriorly or anteriorly. Even if the spine rows fan out, they are parallel for much of their length. The patterns of these parallel rows of spines and the number of spines in the rows are used to help identify individual species of true white grub.
If grub numbers are high and enough damage is occurring to warrant control, use a shorter-acting insecticide such as bendiocarb (Turcam, Intercept), trichlorfon (Dylox), or diazinon. Treatment at this time with bendiocarb or diazinon may also control annual white grubs; if that type of grub is numerous and you use trichlorfon, the treatment will need to be repeated. In either case, scout for annual white grubs in early August in areas where you have to treat now for true white grubs. It is still too early to treat with any of these insecticides and be certain of a high level of control.