This is the time of year when we start making predictions on the numbers of white grubs and their damage that is likely later this summer. At this time, it is still a little early to tell. White grubs in Illinois turf are primarily of two types, the Japanese beetle and the masked chafers. There are southern and northern masked chafers, with both being common throughout Illinois. The masked chafers are commonly called annual white grubs, but the Japanese beetle also has a one-year life cycle.
Japanese beetle adults emerged in central Illinois about 3 days later than usual in late June. They emerged at about the same time in northern Illinois, making their emergence closer to normal in that part of the state. The first reported masked chafer was found in central Illinois on July 5. We have no reports from other parts of the state.
Although we had unseasonably warm weather in April in Illinois that accelerated spring insect emergence, we also had 2 to 3 weeks of unseasonably cool weather in late May. This cool weather tended to balance the warm weather for summer-emerging adult insects such as white grubs. It appears that we had more late cool weather than early warm weather this year.
Rainfall is another major factor in white grub numbers. Until the first week of July, we had warm, dry conditions, and the unwatered turf in much of the state was dry and dormant. However, in early July, much of the state experienced 1-1/2- to 2-inch rainfalls that served to green up the cool-season turfgrasses just in time for white grub egg-laying.
In summary, we have a large Japanese beetle adult emergence in the state, but it is too early to tell about the masked chafers because of their late emergence. If we have continued rainfalls through the first half of July, grub eggs will be scattered over all turf, with grub damage likely only in isolated spots. However, dry weather over the next 10 days is likely to cause grub eggs to be laid primarily in irrigated turf, resulting in heavy grub damage there in late August through September.