White grubs continue to be a problem in northeastern Illinois, particularly in southern Cook County and in Will County. Scattered infestations have arisen throughout the rest of the state, most notably in the Bloomington and Peoria areas as well as southwestern Illinois. Soil temperatures are hovering near thresholds at which grubs move deeper into the soil and out of the turf root zone. Soil temperatures in Champaign on October 22 were 54 and 57 degrees F at the three- and six-inch levels, respectively.
Annual white grubs migrate deeper into the soil when the temperature drops below 50 degrees F. Japanese beetle grubs migrate deeper when the temperature drops below 59 degrees F. If temperatures stay warm, grub damage is likely to continue to occur. However, continued air temperatures like we have had for the last couple of weeks should cool the soil enough for migration downward by the grubs.
To determine whether insecticide treatment for white grubs will be effective, cut through the sod and pull it back. If grubs are present in the root zone, then treatment should be effective. If grubs are not present, they have migrated deeper and insecticides will not reach them. If soils warm up during an unseasonably warm autumn or if there is increased soil moisture from rainfall or irrigation, the grubs may migrate back up into the root zone and resume feeding.
To treat for white grubs at this time of year, use a quick-acting insecticide. Trichlorfon (Dylox, Proxol) or bendiocarb (Turcam, Intercept) is likely to be the most effective. Halofenozine (Mach 2) or diazinon may also be effective. Also, there is the possibility that turf may be able to grow roots fast enough to stay ahead of grub feeding if it has sufficient water during cool conditions.
Providing irrigation until the soils cool enough for the grubs to migrate deeper is an option for those who wish to avoid an insecticide treatment. As a result of natural winter mortality, fewer grubs will migrate upwards to feed on this same turf in the spring. Be watchful, however, for a spring drought, which could slow down turf growth enough to allow even a few grubs to cause damage next spring.