by Erin Hynes
Japanese beetles (Popillia japonica Newman)are a destructive garden pest in the eastern half of the US. Adult beetles feed on the leaves and flowers of hundreds of plant species. Larva of Japanese beetle are soil-dwelling grubs that feed on the roots of lawn grasses.
Identification
The adults of Japanese beetles are a little less than 1/2 inch long, and have shiny, metallic green bodies with bronze wings. They are most numerous in late spring and early summer. Females lay eggs a few inches below the soil surface. In mid-summer, the eggs hatch into grayish-white C-shaped grubs with short legs.
Adults eat the tissue between the veins of leaves, leaving just the skeleton of the leaf. Grub damage shows in late summer and fall. The first symptom is thinner, less lush areas of the lawn. The lawn might wilt, even when the soil is moist. If white grub damage is severe, areas of sod have no roots, and you can easily lift sections of sod off the ground.
Crows, grackles and starlings feed on grubs; their frequent presence on the lawn can indicate a grub infestation. Mammal pests such as skunks, raccoons and moles also eat white grubs and can damage the lawn as they dig for their food.
Control
Most lawns have grubs, but only a minority of lawns have enough grubs to warrant controlling them. Before applying a control product, estimate the population density of Japanese beetle grubs in spring (April in the South, May in the North) and again in mid-summer (July in the South, August in the North). To estimate the density, cut out a one-square-foot piece of sod in several areas of the lawn. Count the number of Japanese beetle grubs present — not all grubs are Japanese beetle larvae. Compare what you see to the image at www.ext.colostate.edu/pubs/insect/05601.html. Control is warranted if you find 10 Japanese beetle grubs per sampled area, on average.
Cultural
Although Japanese beetles feed on a wide range of plants, some are more vulnerable than others. Growing resistant plants prevents adult females from visiting your yard and laying eggs in your soil. For a list of vulnerable and resistant plants, see the United State Department of Agriculture’s publication Managing the Japanese Beetle: A Homeowner’s Handbook
Biological
Biological controls are natural organisms that attack Japanese beetle grubs. They‘re environmentally friendly but require more time to take effect. To ensure that these living organisms are viable, apply them exactly as the label directs.
- A specific strain of the bacteria Bacillus thuringiensis (Bt) acts as a stomach poison for grubs. According to the United States Department of Agriculture, it’s as effective as a chemical insecticide.
- Parasitic nematodes in the genera Steinernema and Heterorhabtitis are microscopic roundworms that reproduce inside the grub, eventually killing it.
- Milky spore disease (Paenibacillus papillae) is most effective if applied communitywide. It’s a bacterium that infects and kills young grubs if they ingest it. The population of the bacterium builds up in the soil over several years.
Some communities have successfully introduced beneficial insects that are parasites of Japanese beetles. These parasites aren’t commercially available. Contact your city’s pest-control agency to see if beneficial parasites have been established in your community.
Chemical
Timing of application depends on the white grubs’ growth stage (egg, young, mature); that timing varies by species and region. Your state’s cooperative extension service might have instructions online that specify when to apply a grub-control product. Search online for “grub control extension (your state).”
Always carefully read and follow label directions to prevent damaging nearby plants, contaminating water supplies and exposing yourself to potentially dangerous chemicals. Pesticide labeling and registration can vary by county and state. Consult your state or local cooperative extension service office about regulations governing use in your area.
Curative. To control grubs that are actively feeding, use carbaryl (Sevin) or tichlorfon (Bayer Advanced 24 Hour Grub Killer Plus). Fall application is more effective than spring. Limit treatment to infested areas and water the lawn after applying.
Don’t use a product that contains only cyhalothrin (gamma or lambda), bifenthrin, deltamethrin, cyfluthrin or permethrin. These products kill only insects above the soil.
Preventive. Use preventive products only if your lawn has a history of severe grub infestations. Apply in late spring to mid-summer (exact timing varies regionally) to prevent fall damage. Use a product containing chlorantraniliprole (Scotts GrubEx) or imidacloprid (Merit, Bonide Grub Beater, Bayer Advanced Season Long Grub Control, Bayer Advanced Complete Insect Killer).
Always carefully read and follow label directions to prevent damaging nearby plants, contaminating water supplies and exposing yourself to potentially dangerous chemicals. Pesticide labeling and registration can vary by county and state. Consult your state or local cooperative extension service office about regulations governing use in your area.
Garden writer Erin Hynes has fewer Japanese beetles after squashing every grub she found when she converted half her lawn to garden.
