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 highly 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.
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.
However, the onset of dry weather can lead to "sudden" appearance of grub damage symptoms. High population of grubs may also go unnoticed until raccoons or skunks discover the grubs during their nightly forages. Raccoons and skunks will turn over large patches of loose turf, eat the grubs and leave behind a torn-up mess.
Treatment for white grubs in late summer is problematic. It is not an automatic decision to choose to use an insecticide for white grubs. If an insecticide is applied late, the grubs will be harder to kill and severe damage to turf may have already occurred. If raccoons have already found the grubs they will continue to return and cause additional damage. Insecticide treatments after early October are not effective and are not recommended. If you do treat it may not be necessary to treat the entire lawn. Treat grub "hot spots" determined by observation or sampling.
Presently, granular Dylox is the fastest- acting, most effective insecticide for curative grub control. Diazinon is also an effective option for homeowners. Products containing chlorpyrifos (e.g., Dursban) are not consistently effective against grubs. Preventive white grub products such as Merit and Grub-X must be applied before August 1 for best results.
Insecticides must be watered in to be effective. Use at least 1/2 to 1 inch of water immediately following treatment. Continue to water damaged turf to promote recovery. In many cases it may be preferable to repair the damage through seeding or sodding without treating. If the old loose sod is still green it may reattach. Evaluate all options carefully before proceeding.
This article originally appeared in the September 17, 1999 issue, p. 122.