Larvae of masked chafers and Japanese beetles are responsible for most of the root damage to turfgrass in Kentucky. They are flying now and rainfall provides easy burrowing for females entering the soil to lay their eggs. Soil moisture is also an important factor in egg hatch and survival of newly hatched beetle larvae. Japanese beetles fly during the day so it is relatively easy to identify sites where they may be a problem. However, masked chafers fly at night and are strongly attracted to lights. Some nocturnal scouting or watching outdoor lights can give an idea of their activity.
Many of the insecticides used to control white grubs in turfgrass must be applied now as preventive treatments on high-risk sites with a history of grub problems or where heavy beetle activity is noted. These insecticides should be applied between now and mid-July to catch the small grubs, which are most susceptible to the treatments.
Active ingredients in these products include chloranthaniliprole (Acelepryn, Scotts GrubEx1 Season-long Grub Killer), clothianidin (Arena), imidacloprid (Merit, Bayer Complete Insect Killer, and other brand names), and thiomethoxam (Meridian).
For best results with any grub insecticide, mow the lawn and remove dead grass and thatch before treatment. This allows the spray or granules to penetrate better and reduces the amount of insecticide bound up by surface debris. White grubs are especially hard to control in thatch-filled turf because most of the insecticide binds to the organic matter and fails to reach the root zone.
Water the lawn immediately after treatment to move the insecticide into the root zone where the grubs are feeding. In addition, moisture draws grubs closer to the soil surface, which increases their contact with the insecticide. Use a lawn sprinkler to drench the soil with 1/2 to 1 inch of water. Measure this by placing a disposable pan or rain gauge in the treated area to catch the water that is applied. If supplemental irrigation is not available, try to apply the insecticide just before a good rain.
Prompt, post-treatment irrigation is especially important for spray applications; once spray residue dries on foliage it is more difficult to wash it into the root zone by later watering. Granular insecticide formulations may work better when timely irrigation is not possible. The relatively long lasting preventive grub insecticides tend to be more forgiving than the curative products if post-treatment watering is unavoidably delayed for a few days.
See the following UK Extension publications for more information on white grub control and insecticide alternatives:
- Controlling White Grubs in Turfgrass (ENT-10)
- Insecticides for Control of White Grubs in Kentucky Turfgrass (EntFact-441)
By Lee Townsend, Extension Entomologist