Grub Damage Gone Bad
John Orick, Purdue Master Gardener State Coordinator, Department of Horticulture & Landscape Architecture
Although this “Picture of the Week” was taken in late October, similar damage to turf areas can be seen in mid to late April as white grubs make their way to the surface of the soil after overwintering deep in the soil profile. They return to near the soil surface to feed marginally on turfgrass roots in the spring before pupating. In this photo, the sole culprits are not just white grubs feeding on turfgrass roots, but secondary pests like skunks and raccoons, which forage for grubs found in the soil just underneath the surface. In the spring as grubs make their way closer to the soil surface; these secondary animal pests dig through the turf to feed on the grubs. It may be tempting to apply an insecticide for grub control in order to remedy this problem. But grub control strategies must be properly timed for the best results. Applications of granular insecticides such as trichlorfon (Dylox) more effectively control grubs in turf when applied in late August to early September. Irrigating the turf before and after the insecticide application is recommended to increase the efficacy of the insecticide by drawing the grubs closer to the surface and then washing the insecticide down to the grubs feeding below. Preventative grub controls like imidacloprid (Merit) or halofenozide (Mach II) are best applied mid-July to give grub protection in turf areas throughout the season.
For the situation shown in the photo, it is best to remove the debris, rake the damaged area (or core aerify in several directions) to prepare a seedbed, and reseed. Cool-season turfgrass species can be reseeded in early spring or preferably sometime between August 15 and September 15. Refer to the following Purdue Turfgrass publications for more information about reseeding damaged turf: Establishing Turf Areas from Seed (AY-3)(pdf file); Lawn Improvement Programs (AY-13)(pdf file); and Seeding a Turf Area in the Spring (AY-20)(pdf file).
(References: Japanese Beetles in the Urban Landscape (E-75-W) by Timothy J. Gibb and Clifford S. Sadof; Spring Grub Control Not Your Best Bet by Doug Richmond, Turfgrass Entomologist, Purdue Turf Tips)
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Photo by John Orick (Home lawn located in Anderson, IN)