The larvae of Japanese beetles and the masked chafers, annual white grubs, hatched in central Illinois the first week of August. By the second week of August, these insects should be hatched throughout the state. It is too early to tell how high the numbers will be. Grubs collected on August 5 were still first and second instar larvae, about 1/4 inch long. If you have not applied imidicloprid (Merit) or halofenozide (Mach 2), you can probably still do it if you apply it immediately. Damage is unlikely to show up until about the third or fourth week of August, and these two chemicals take about three weeks to start killing grubs.
It is also the correct time to apply the shorter-acting insecticides for grub control in irrigated turf. Bendiocarb (Turcam), diazinon, or trichlorfon (Dylox, Proxol) can be applied at this time to kill the hatched grubs. Remember that diazinon is not labeled for application to golf courses and sod farms. Water in any insecticide with at least 1/2 inch of water.
Turf areas that are not irrigated or have not been irrigated regularly should be scouted for grub numbers. Cutting through the sod with a heavy knife and pulling back the sod reveals the white grubs in the root zone. If the soil is dry, the grubs may be a couple of inches deep into the underlying soil.
Ten to twelve or more white grubs per square foot are enough to cause turf injury, and treatment is warranted. Grubs are not only more likely in watered areas, but also in open areas away from trees and along sidewalks, driveways, streets, and other paved areas.