Damage from white grubs in lawns can show up anytime after mid-August. However, our experience in the recent past has been that grub damage does not become obvious until September or even into October. Damage from white grubs is usually localized. It is typical to have severe damage in irregular and isolated spots.
White grub damage may first appear as drought stress (gray-green discoloration and wilting in the hot sun). More severe damage causes the turf to die in large irregular patches that can be rolled back like a loose carpet. High populations of grubs may go unnoticed until discovery by raccoons or skunks. Raccoons, skunks and crows will turn over large patches of loose turf, eat the grubs and leave behind a torn-up mess.
Rainfall and soil moisture are critical factors affecting the extent of grub damage. Adequate moisture in mid-summer will favor beetle activity and grub development. If plentiful rainfall or irrigation continues through August and September (when grubs are actively feeding) damage may not be noticeable because the grass continues to grow and masks the root injury symptoms. Healthy turf can sometimes tolerate 20 or more grubs per square foot before showing signs of injury. The onset of dry weather can lead to “sudden” appearance of grub damage symptoms.
We had treated almost 200 acres of bluegrass turf with a insecticide to prevent white grubs. However we are having some areas where it is failing us and the distributor and the manufacturer have both been contacted. All areas that we see damage to have been treated with a contact grub control and watered in to control the annual white grub.