Japanese Beetle Grubs

If you have an infestation of Japanese beetles you will want to destroy the grubs before they develop into beetles and feed on your plants. In Canada, the grubs are a problem in Ontario and Vancouver.

During the summer, the beetles mate and lay eggs in the soil. Those eggs hatch in late August, and the young, white,C-shaped grubs become active in the soil, usually eating the roots of turf, until they hibernate for the winter.

In the spring lawns start to turn green. As the turf begins to grow, you may find patches in your lawn where the turf is thin. Common causes are drought stress from the previous summer or a lack of fertilizer.However, sometimes patches of thin or dead turf may be due to grubs.One indicator of grubs in your lawn is when a flock of birds, especially starlings, are observed feeding around the dead patches.

It is important to make sure the problem is grubs. The outbreak phase where turf damage is common may last five to 10 years.

If you see a dead patch, use a shovel to dig up a few Frisbee-size samples of turf around the bare spot to a depth of 2 inches and look for 0.75-inch long, C-shaped white grubs. These are most likely the larvae of European chafers if they are found in lawns without an irrigation system. European chafers can devastate a lawn with little warning because the adult beetles fly at dusk when they emerge in June and early July. They can easily be overlooked because their evening flight to mate and lay eggs occurs after sunset.

Japanese beetle grubs also feed on turf roots in home lawns. Japanese beetles and European chafers lay most of their eggs in July, but Japanese beetles continue laying eggs into August. Japanese beetles like to lay their eggs on irrigated turf. They can be abundant in non-irrigated home lawns if we receive frequent rain in July, but are scarce in lawns that are very dry in July and early August.Grub damage may appear in home lawns from mid-September to November or from March to early May.

No Action Needed

Healthy turf with a few grubs may not need an insecticide.It is important to realize that healthy turf, supported by frequent rain orirrigation, can support a grub population of five or more grubs per square footwith no visible turf damage. A lawn should be mowed at 3.5 to 4 inches inheight and properly fertilized for maximum root growth.

Action Needed

Finding one or two grubs does not indicate you need to apply a grub control product. If the grub population is high or if there is a history of damage in your area, you may want to use an insecticide for grub control. If you confirmed grub damage the previous fall or spring, meaning you found lots of grubs, then you may want to use a preventive insecticide for one or two years to build a more dense turf that will be tolerant of grubs.

Talk to Local Experts

Talk to staff in your local garden centre to find suitable preventative insecticides. Alternatively, they may recommend nematodes.

Preventative insecticides

These products are used to prevent future grub problems, not to control the grubs present in the lawn in the spring. They will not work on grubs found in the lawn from the middle of October through the middle of May. Apply products to the soil in June or July. Don’t forget to irrigate. Apply the preventive insecticide BEFORE the new grubs are there They provide excellent protection against the next generation of grubs.

If you have treated for several years and you do not see evidence of grubs in your lawn or in the neighbor’s lawn, it may be time to stop treating.

Note: Do not use products containing only lambda-cyhalothrin, gamma-cyhalothrin, bifenthrin, deltamethrin, cyfluthrin or permethrin for grub control. Products containing only these ingredients will not work for grub control because the active ingredient binds with organic  material and will not move down to where the grubs are feeding.

Using Nematodes

Some people spray the soil in September with beneficial nematodes which are microscopic worms native to Canada. They are 100% organic, fresh and targeted to Canadian pests. The product will have an expiry date so  check that before you buy. It is safe to handle and there is no harm to pets, plants or friendly insects such as ladybugs.

Why Use Nematodes

Nematodes seek out the grubs and release bacteria to initiate an infection. The infection causes the grubs to die, allowing the nematodes to feed and reproduce.

Nematodes can be applied as long as the temperature is above 10°C (50°F). Apply nematodes in the fall when the grubs are small and close to the surface. The soil must be moist to below the level of the grubs. Apply after soaking rains or water deeply if no rain is forecast. Apply in the evening as the sun can reduce the effectiveness of the nematodes. After applying, water thoroughly to help flush the nematodes into the soil. If it’s hot, water for 2 or 3 days afterward.

References and Resources for more information

Canadian Food Inspection Agency fact sheet


OMAFRA fact sheet, Grubs in Lawns http://www.omafra.gov.on.ca/english/crops/facts/08-023w.htm

North Dakota State University Extension https://www.ag.ndsu.edu/yardandgardenreport/documents/japanese-beetle-alert

Submitted by Chris Harwood