Iowa Insect Information Notes

White Grub Treatments in Late Fall

White Grub Treatments in Late Fall

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.

Treatment for white grubs in late summer is problematic.  It is not an automatic decision to choose to use an insecticide for white grubs.  By September white grubs are fully-grown and thus harder to kill.  The best treatment may kill only 60% of the grubs.  Severe damage to turf may have already occurred.  If raccoons have found the grubs they will continue to return and cause additional destruction.  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 with adequate watering.

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, trichlorfon (Dylox or Bayer 24-Hour Grub Control) is the fastest-acting, most effective homeowner insecticide for curative grub control.  By the time damage is apparent it is much too late for preventive white grub products such as Merit and Grub-X.  These must be applied before August 1.  Insecticides must be watered in to be effective.  Use at least ½ inch of irrigation immediately following treatment and continue to water damaged turf to promote recovery.

Updated 10/10/2007 - 7:46am