Melinda's How-To: Managing Japanese Beetles - Pasquesi Home and Gardens
Melinda's Gardening How-To: Managing Japanese Beetles

Melinda’s How-To: Managing Japanese Beetles

…while munching away on more than 350 varieties of trees, shrubs, flowers and edible plants.  The culprit, Japanese beetles. The adult beetles eat holes in leaves and flower petals.  The immature stage of a Japanese beetle is a grub. They feed mostly on the roots of lawn grass. This reduces the grass’s ability to absorb moisture causing brown patches in the lawn. They also attract raccoons and skunks that dig up the lawn looking for a meal of these tasty insects. So what’s a gardener to do?  Here are a few things that can help.


Start by Identifying the Suspects

  • Adult Japanese beetles appear in summer. They are about ½ inch long, coppery-green with tufts of white hair along their back end.
  • The immature stage, known as grubs are C-shaped, worm-like insects about 1-inch long. They are creamy white with a grayish cast on their back end. The larvae of other beetles look very similar. A row of V-shaped spines on the underside of their hind end can help distinguish them from other grubs.

Know Your Enemy – Understand Their Life Cycle

  • Beetles begin feeding in the garden around late June and are most numerous in July and August with a few lingering into September.
  • The beetles feed and mate throughout this time. The females lay their eggs in grassy areas.
  • The eggs hatch in late summer and the young grubs feed on the roots of the nearby plants.
  • As temperatures cool – the grubs move down deeper into the soil for the winter.
  • As soil warms in spring – the grubs move back up nearer the soil surface and start feeding mainly on grass roots.
  • The grubs pupate, transform, into beetles in late spring and the cycle starts again.
  • Keep in mind the adults can fly several miles. So new populations can move into your landscape every year.

Controlling the Beetles – A Few Options to Consider 

  • Provide plants with proper care, tolerate the damage and wait for natural predators like turkeys, sparrows and finches to eat a few of these pests. A healthy perennial plant, tree or shrub can usually tolerate the damage.
  • Knock the beetles into a can of soapy water. You’ll capture the most with the least amount of effort in the morning or evenings when they tend to congregate and move slower.
  • Start applying Neem, an organic insecticide, at least 20 days before you expect damage. Repeat as directed on the label.
  • Insecticide that are applied to the leaves and labeled for Japanese beetle control on the affected plant are used once the beetles are present and feeding. Be sure to read and follow all label directions. Never spray when bees are out foraging as insecticides harm them and other beneficial insects.
  • A soil applied systemic insecticide containing Imidacloprid is applied in advance and protects the leaves but not the flowers from the beetles. Follow label directions for timing of application for the specific plants you are treating.

Controlling the Grubs – A Few Options to Consider 

  • Make sure Japanese beetle grubs are the cause of your lawn damage. Remove the damaged sod and look for the grubs. Check several locations. If you find three to four Japanese beetle larvae per square foot of lawn in these areas, you may choose to treat.
  • Proper timing is the key to successful treatment. Mid July through the end of September when the grubs are small and near the surface is the best time to treat the lawn.
  • Milky spore disease is an organic option that may provide some relief. This disease, Bacillus popillae, takes several years to increase in numbers and infiltrate the soil. Do not apply other pesticides to the lawn that may harm the Milky Spore. And be prepared to tolerate some damage since the milky spore needs a food source, the grubs, to survive and multiply.
  • Soil applied grub controls are available. Apply at the proper time and at the proper rate for best results. These products also kill beneficial insects in the soil so be sure to read and follow label directions carefully.


Written by gardening expert, Melinda Myers. Melinda Myers is a nationally recognized gardening expert with more than 30 years of horticulture experience. She is a wealth of knowledge and we are pleased to share Melinda’s Gardening How-To with you!

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