Japanese Beetle Reduction Methods

Japanese beetles can be a scourge of the garden and landscape, but what can you do to keep these pests at bay?

About Japanese Beetles

Japanese beetles (Popillia japonica) spend their early lives as underground grubs eating turf grass roots. They prefer well-watered, healthy perennial ryegrass and hard fescues in full sun. Emerging as adult beetles in mid-June through July, they begin feeding on over 200 varieties of plants, including shade and fruit trees, shrubs and ornamentals. They also mate and the females lay 50-70 eggs in the soil.

The eggs hatch in the fall and white C-shaped grubs begin eating roots. Autumn is the best time to check your lawn to see if the grubs are present. Dig several one-foot squares 6″ deep in your lawn, turning over the turf and looking for these distinctive grubs. If you find them, taking action immediately can help control the infestation.

Reducing Japanese Beetle Populations

Non-chemical preventative treatments include spraying beneficial nematodes such as Heterorhabditis or Steinernema onto moist lawns and soil in September. Nematodes, naturally occurring soil organisms, are parasitic to soil grubs and many insect larvae, including Japanese beetles. Spray in the evening and ensure the soil is moist to at least 6″ deep. One product, Lawn Guardian, contains two types of nematodes; one lives deeper in the ground to give a “double whammy” to the feeding grubs.

Natural predators include ground beetles, ants and Tiphia, a parasitoid. Applying Bacillus popilliae Dutky to the soil causes “Milky Spore Disease” to the grub. Chemicals to control the grubs include trichlorfon, imidacloprid, halofenozide or thiamethoxam, so look for pesticides that include these compounds to help eliminate Japanese beetles. Neem oil can also be helpful to control these pests. As always, read and follow the directions carefully when using any type of chemical pesticide.

In the garden, row covers can help minimize Japanese beetle populations during the growing season, but this can also reduce crop productivity as fewer flowers are pollinated. Still, if an area is heavily infested with Japanese beetles, a smaller crop may be a better alternative than accidentally nurturing these pests. If only a few beetles are present, hand-picking them off plants and killing and disposing of the insects – toss them in a bucket of soapy water – can keep the populations manageable.

For the latest information and updates on Japanese beetles, as well as more control tips, contact your favorite garden center or County Agent.

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