Annual white grub adults were first reported in central Illinois on June 30. This is a couple of days earlier than their usual appearance on July 2, but generally they are right on time. These tan, 1/2-inch-long June beetles are present for about two weeks. During this time, they lay eggs that produce white grubs that feed on turf roots in late summer and fall. Japanese beetles also apparently lay most of their eggs during this same time period.
Observe the heaviness of flight of both of these beetles and also the appearance of unwatered turf during this time. Flight of Japanese beetles can be assessed by how many are feeding on the leaves of plants during the day. Scout for annual white grub adults after dark, particularly between 10:30 and 11 p.m. At this time, both the northern and southern masked chafer species of annual white grub are likely to be active. Shining car headlights or a flashlight across the surface of the turf will reveal the beetles flying low over the turf. Using these methods, you can get an idea of the number of beetles present.
If unwatered turf is dormant and somewhat brownish, egg laying will be concentrated in irrigated turf. If the unwatered turf has received enough rainfall so that it is still green and lush, eggs will be laid in both irrigated and unirrigated turf. In this case, the eggs will be so spread out that damage is unlikely unless there is a heavy flight of beetles.
Dormant unwatered turf and/or a heavy beetle flight tells you that it is proper to apply imidicloprid (Merit, Grubex) or halofenozide (Mach 2) to irrigated turf before the last part of July. If unwatered turf is green and lush, as it appears at this time in east-central Illinois, it is usually wiser to wait until early August when the grub eggs have hatched. Then, scout and spot-treat infested areas with a shorter-lasting insecticide.