Adult annual white grubs--the southern and northern masked chafers--have been declining in numbers; their flight should be coming to an end. Japanese beetle adults were numerous in late June into early July, but their numbers have fallen off dramatically. The question that arises is "Now that we know what the adult white grub flight has been and the weather conditions associated with it, should we treat for grubs and, if so, when should it be done?"
Throughout the state, there have been areas with large flights of masked chafer or Japanese beetle adults. This tells us that the beetles have been present to lay the eggs, which could result in large grub populations in late summer and early fall. Both the masked chafers and Japanese beetle prefer damp soil in which to lay their eggs. Japanese beetle is probably even more attracted to it than the masked chafers. However, throughout most of the southern two-thirds of Illinois, there has been enough rainfall to keep turf very green, whether it has been watered or not. In these conditions, eggs are usually laid over large areas, resulting in a small number of grubs--usually one to five--per square foot.
In northern Illinois, particularly in the northeastern section, rains have not been as widespread and frequent. Probably due to infrequent rains and high temperatures over the last couple of weeks, unwatered turf in Chicago and adjoining areas is becoming brownish. This situation causes the adult beetles to concentrate their egg laying in irrigated turf.
In areas where rains have been infrequent and unwatered turf is dry and brownish, preventive treatments for white grubs are justified in irrigated turf, particularly where there is a history of grub problems. Halofenozide (sold as Mach 2) or imidicloprid (sold as Merit) should be applied by the end of July (or, in northern Illinois, before the end of the first week in August). Both of these insecticides are effective but take three weeks to kill the grubs. Damage is unlikely to occur before the middle of August in southern and central Illinois and probably won't occur in northern Illinois until the third or fourth week of August.
In areas where unwatered turf is green, there are still likely to be spots where grubs are numerous enough to cause turf damage in August through the fall, especially in areas where Japanese beetle occurs. Browning of turf from white grub root feeding will be more likely if there is a prolonged dry period this fall. To keep on top of the situation, scout for grubs in early August, when most of the eggs should be hatched. In southern Illinois, scouting can begin in the last week of July.
Scout for white grubs by cutting through the turf with a heavy knife. Pull back the turf and count the white grubs in the root zone. Lightly till the soil with the knife to check for grubs a couple of inches deeper. This is particularly important if the soil is not moist in the root zone because the grubs will move deeper to find moisture. If you find ten to twelve or more grubs per square foot, treatment is justified. Raccoons, skunks, and birds will cause turf damage searching for grubs. If these animals are numerous in the area and you've experienced this type of damage before, realize that these mammals will dig when as few as three to five grubs per square foot are present.
To treat for grubs found in mid-August or later, use trichlorfon (Dylox, Proxol), bendiocarb (Turcam), or another quick-acting insecticide. These insecticides can also be used in late July in southern Illinois and early August in central and northern Illinois.