Adult annual white grubs (masked chafers) emerged in central Illinois during the last week of June and were reported in northern Illinois a few days earlier. Checking beetle numbers around lights at about 10:30 p.m. will provide insight on both the northern and southern masked chafer emergence in your area. The number of adult Japanese beetles present during the day on landscape plants will give you an idea of their numbers.
The relative number of these adults plus the soil moisture will provide you with the information you need for preventative white grub treatments in turf. All three species feed in the larval stage as white grubs on the roots of turfgrass. The adults are attracted to damp soil, which they tunnel into to lay their eggs.
If your area has received enough rainfall so that nonirrigated grass is green with moist soil and the beetle flight is not unusually heavy, then it is likely that grubs will be present in damaging numbers only in small areas, which are conducive to spot treatments with quick-acting insecticides in August. If only irrigated turf is very green with moist soil and the adult flight is normal to heavy, then those irrigated turf areas will likely have heavy grub numbers. These are the turf areas where imidicloprid (Merit) or halo-fenozide (Mach 2, Grubex) should be applied by the end of this month to prevent grub damage in the second half of August. Many areas of the state have enough soil moisture that white grubs will not be a problem this year. However, that can reverse itself if the rains stop and temperatures are high for the next couple of weeks, and unwatered turf goes dormant.