There have been several reports of large white grubs in turf areas, particularly in northern Illinois. The larvae being reported are over 1 inch long. These are probably the larvae of May beetles, also known as true white grubs or 3-year white grubs. These larger grubs are probably in the third year of their life cycle and will pupate later this fall. They could also be in the second year of their life cycle and will be back in these locations next year unless controlled.
True white grub larva.
Unlike other white grub species in turf, May beetle grubs tend to feed more on dead organic matter. They are commonly found in flowerbeds or other heavily mulched areas, causing no damage to the roots of plants in that bed. As far as root-feeding is concerned, they feed on other roots besides those of turf. For instance, they commonly damage potatoes and carrots.
True white grub raster.
Like other white grubs, the larvae of May beetles are white and C-shaped. Mature larvae may approach 1-1/2 inches in length. The raster pattern on the underside of the last abdominal segment appears as a zipper, being two parallel rows of thickened setae, or thin spines.
These large grubs are difficult to control. Perhaps the most effective control is insecticidal nematodes. Heterorhabditis bacteriophora is one nematode species that is effective and widely available. It should provide about 60% control if applied late in the day to wet turf that is immediately irrigated. Insecticides are not going to be very effective against these large white grubs. Trichlorfon (Dylox) is probably the most effective but considerably less effective than the insecticidal nematodes. Imidacloprid (Merit) and halofenozide (Mach 2) are likely to be even less effective than trichlorfon.