Typically, the infestation level of white grubs in turf can be determined the first week of August. Cutting through the turf with a heavy knife and pulling back the sod reveals young grubs. Thresholds vary with turfgrass use, but generally 10 to 12 or more per square foot are enough to cause damage through turf dieback. The numbers are higher in damper areas where irrigation is frequent and near sidewalks and driveways where the soil is warmer from the heat stored in the pavement from the daylight hours.
However, the adult grubs were late in emerging this year. Japanese beetle adults emerged in numbers about a week later than usual in central Illinois, becoming numerous about the first week of July. Even longer delays were observed in northern Illinois. Many areas are seeing fewer beetles than normal, which is probably due to death of overwintering grubs from the deep frost in the soil over winter. However, some areas are reporting very high numbers. With this emergence pattern, damaging numbers are likely to be spotty. Some turf may have few grubs, while other areas may have damaging numbers. Increased scouting is needed to determine and treat areas with high numbers.
Masked chafer adults were even later emerging. Instead of emerging the first week of July and being gone by midmonth, beetles were still being found in central Illinois August 2. In northeastern Illinois, the first were seen July 23, about 2 weeks later than normal. Particularly in northeastern Illinois, the unseasonably cool weather this year has likely contributed greatly to late emergence. Increased rain across most of the state has probably also influenced emergence patterns. The northern and southern masked chafer beetles are the adult stages of annual white grubs, so it will be a while before we know how many grubs will be present. Once laid, eggs hatch in 2 weeks.
Both masked chafer beetles and Japanese beetles tend to lay their eggs in damp soil where there is green grass. In most years, this results in large numbers in irrigated turf, with beetles avoiding dormant, brown, unwatered turf areas. In much of the state, unwatered turf is still green and actively growing. Under these conditions, the beetles lay their eggs over large areas. As a result, most areas will have one to five grubs per square foot, well below the levels required for turf injury. However, raccoons, skunks, and birds damage turf while looking for grubs when numbers are as low as three to five per square foot. In areas where this animal damage commonly occurs, treatment may be needed. Most other areas will probably not need treatment. Scout these areas to be sure. Continue to scout because grubs may not appear until mid-August.
With the late emergence of the beetles, imidacloprid (Merit) and halofenozide (Mach 2) can still be used during the first half of August in central and northern Illinois. These insecticides take about 3 weeks to kill the grubs, but damage should not occur until late August to early September. In southern Illinois, the grubs should have hatched, with damage showing up by mid-August. Treatment with trichlorfon (Dylox) is probably a better option to get quicker control in southern Illinois.