Japanese beetle and masked chafer adults are attracted to turf with moist soil. The females tunnel into the soil to lay their eggs; moist soil is surely easier to dig through than hard, dry soil. In addition, the eggs dry up and die under very dry soil conditions. The eggs also die when soil temperatures are around 90°F. Higher soil temperatures are typically associated with drier soils, as well as high air temperatures.
After a wet spring with lush grass, much of Illinois has turned typically dry and hot for the white grub egg-laying period. Under dry conditions, the adult beetles seek golf courses, irrigated lawns, and other watered turf to lay their eggs. This results in high white grub populations in these areas in August, causing turf damage in late summer and fall. When there is sufficient rain and moderate temperatures in July, the turf stays green and actively growing in unwatered areas. The beetles lay their eggs over large areas, with the result that few areas have grub numbers high enough to cause damage.
Both the northern and southern masked chafers occur throughout Illinois. These 1/2-inch-long, tan, stocky June beetles emerge and start laying eggs in late June in southern Illinois, in the first week of July in central Illinois, and in the second week of July in northern Illinois. If one goes out about 10:30 p.m. and shines a light just above the turf, these beetles can be seen flying low over the turf. Because the adults do not feed, egg-laying occurs for only about 2 weeks before the adults die. Their larvae are commonly referred to as annual white grubs.
Japanese beetle adults emerge throughout the state slightly earlier, having emerged in southern Illinois in the third week of June, central Illinois in the last week of June, and northern Illinois at the beginning of July. Although the adults feed and are numerous for 6 weeks or so, eggs appear to be laid primarily during the first half of July. This results in Japanese beetle white grubs’ appearing in August at about the same time as annual white grubs.
In areas with hot, dry weather, it is likely that grub numbers will be high this year in irrigated turf. During July, apply imidacloprid (Merit) or halofenozide (Mach 2, Grubex) to prevent later grub problems. It is best to water the insecticide in if rainfall does not wash it into the root zone within a few days. Insecticide sitting in full sunlight is likely to break down under the ultraviolet light. Another reason to water it in with at least 1/2 inch of water is that no grubs are being killed while the insecticide is sitting on the grass blades and thatch. Nonirrigated areas, those receiving infrequent watering, and locations in the state where rainfall has been plentiful may not have enough grubs to cause injury. Scout these areas in early August and treat spots with high grub numbers with trichlorfon (Dylox).