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Master Gardeners of Goodhue County

Master Gardeners of Goodhue County

Managing Japanese Beetles

Terry L. Yockey, Goodhue County Master Gardener

Last season Japanese Beetles invaded Minnesota in a big way and unfortunately now that they have arrived, they are here to stay. Though they are fairly new to Minnesota, the first transplanted Japanese beetle was found in a New Jersey nursery in 1916. As the name implies they are native to Japan where they have several natural enemies that keep their population in check. Here in the U.S., however, Japanese beetles continue to spread to new areas every year and have become a highly destructive plant pest.

Life Cycle

In mid to late-June female beetles emerge and begin feeding, mating and burrowing into the ground where they lay their eggs. The eggs hatch into grubs in about two weeks. The grubs grow quickly feeding on the roots of turf grass and are almost full-grown in August. As fall approaches and temperatures cool below 50⁰F the grubs dig deeper into the soil to overwinter. When spring arrives and the soil warms again they begin to move upward and resume eating. In mid-June the grubs pupate and the adult Japanese beetles emerge one to three weeks later.

Biological and Non-Toxic Controls

Adult Japanese beetles feed on over 300 different plants including trees, shrubs, annuals and perennials. Once Japanese beetles become established they are very difficult to eradicate so vigilance and using a variety of different control methods is crucial to their management.

Here are six non-chemical control methods:

  1. Hand-picking: When the females arrive in June it is imperative to begin immediately hand-picking when they first start eating your plants. These early beetles are scouts so it is important to destroy them before they draw other adult beetles to your plants. Adult Japanese beetles can fly as far as a mile and a half to a new host and are attracted by leaves and flowers that have already been eaten on by other beetles so it is also advisable to remove any damaged foliage. Hand-pick the beetles in the early morning or late evening when they are sluggish and will easily drop off shaken foliage into a peanut butter jar or coffee can filled with soapy water.
  2. Tolerate some tunnels: Moles may dig up your lawn and garden, but on the positive side they also eat Japanese beetle grubs.
  3. Nematodes: Using parasitic nematodes, especially the strain Heterorhabditis bacteriophora, has been shown to have some effect controlling the grubs.
  4. Milky Spore Disease: MSD is a bacterial infection fatal to Japanese beetle grubs; however the jury is still out as to its efficacy.
  5. Protective covering: Use Reemay or other spun-bonded material to protect plants that are especially susceptible to beetle damage.
  6. Plant non-attractive plants: Japanese beetles favor roses, hibiscus, grapes, hollyhocks and other mallows so choose plants they do not like such as poppies, hosta and coreopsis. There is a long list of their likes and dislikes in the USDA booklet "Managing the Japanese Beetle: A Homeowner's Handbook" that can be found online at www.aphis.usda.gov/lpa/pubs/pub_phjbeetle04.pdf.
  7. Attract beneficial insects: Some tiny parasitic wasps (not the large wasps that sting!) lay their eggs in white grubs and when they hatch their young devour the grubs from the inside out. To attract these beneficial wasps plant nectar-rich herbs and other flowers that have small shallow-faced flowers and of course, do not use insecticides in your yard and gardens.
  8. Pheromone traps: Many people swear by the traps and they do attract hundreds if not thousands of beetles, which may be the problem. Just like having the only "bug zapper" in the neighborhood and consequently all the mosquitoes, you will attract not only most of the Japanese beetles in your neighborhood, but pretty much all of them for miles around. If you do decide to hang a trap make sure it is at least fifty feet away from your garden or other desirable plants and only put it out for a few days every couple weeks.

There are several chemical insecticide sprays on the market that are effective against Japanese beetles, but most are toxic and harmful to birds, butterflies and other backyard wildlife. For more information on using chemical controls for Japanese beetle adults and grubs, please see the UMN Extension fact sheet at www.extension.umn.edu/distribution/horticulture/dg7664.html.