It is recommended that preventive treatments for white grubs in turfgrass be applied during July. The delayed emergence of Japanese beetles in most of Illinois (except for the northern part) has delayed the ability to predict white grub infestations this year.
Japanese beetle emergence appears to be heavy through most of the state. The only exception appears to be east-central Illinois from Iroquois County south to Effingham County and from the eastern edge of Illinois to central Champaign County. In that area, emergence appears to be greater than last year but still about one-third of normal.
In predicting white grub injury, rainfall pattern is probably more important than the number of Japanese beetle adults. Throughout most of Illinois, we had unseasonably high rainfall amounts through the spring into the first half of July. Female Japanese beetles tunnel into the soil of turf areas to lay their eggs. They are attracted to moist soil that is easier to dig into. In addition, Japanese beetle eggs shrivel and die in dry soils, and any grubs that do hatch have a high mortality rate in dry soils. At the other end of the moisture spectrum, the eggs and larvae have a high survival rate in soils almost to water saturation. In addition, there is also some indication that female Japanese beetles are attracted to green or brown, dormant turf.
Due to this spring's higher than average rainfall that has continued into July, even nonirrigated turf is green and the soil moist. Typically under those conditions, Japanese beetle (as well as masked chafer) females lay their egg in all grassy areas, including roadsides, parks, field waterways, and nonirrigated lawns as well as irrigated turf. As a result, white grub numbers are usually moderate in turf and too low to cause obvious damage.
As a result, preventive insecticide treatments such as imidacloprid (Merit) and halofenozide (Mach 2) are usually not needed in years with the rainfall we’ve had this year. One should still scout for white grubs in August to determine whether spot treatments or remedial treatment will be needed. Generally, the eggs have hatched by middle to end of the first week of August, but damage does not occur until near the end of August into September.
During the second and third weeks of August, cut through the turf with a heavy knife and pry it up to look for white grubs in and just below the root zone. Most grubs will be just under the turf and exposed when it is pulled back. A few will be hanging in the turf roots, and some will be 2 to 3 inches deeper into the soil. If the soil is dry, they may be 4 to 5 inches deep.
Ten to 12 (or more) Japanese beetle, masked chafer, and other large white grub species per square foot are enough to cause turf damage in most situations. If the turf is heavily used, the number may be 8 to 10 grubs per square foot. Lightly used turf can host several more grubs per square foot without showing damage.
Areas with damaging grub numbers should be treated with trichlorfon (Dylox) to provide quick control. Trichlorfon kills the grubs in 3 days, whereas imidacloprid and halofenozide typically take about 3 weeks to kill them. Quick control is particularly important in situations in which the client may pull up the turf to see whether you have controlled the grubs.
Areas of northwestern Illinois seem to experience white grub injury regardless of the weather. The area from Peoria through Monmouth and Galesburg tends to have grub damage annually. In that area of the state, preventive treatment with imidacloprid or halofenozide is recommended regardless of the rainfall pattern.