8 Tips For Preventing Japanese Beetles From Overtaking A Garden

Control Options Depend on Landscape, Timing
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How to

Japanese beetles eat a wide variety of landscape plants.

Just when a garden looks good, ravenous Japanese beetles can promptly emerge in the heart of summer to devour the gardener's favorite plants.

Discovered in the U.S in 1916, the Japanese beetle (Popillia japonica) is widespread in many states east of the Mississippi River (except Florida). The insect has been established in Wisconsin since the 1990s.

Both adult and the grub stage of Japanese beetles cause serious damage to many landscape plants (more than 320 species) and turf grasses. The adult beetles skeletonize the leaves by chewing between the veins of the leaf tissues, devour the flowers (roses) and fruits (raspberries), and ultimately weaken the plant. Roses, birch, linden, grapes, raspberries, Norway maples, beans, apples, plum, crabapples, elms, beech, asparagus and rhubarb are some of its favorite delicacies.

Huge swarms of these beetles can emerge in Wisconsin, with peak activity for six to eight weeks. Adult beetles are active during the daytime and can fly an average of 1 to 2 miles. The female beetle lays eggs on the ground, which hatch in about two weeks into white grubs that damages turf grass roots.

An adult Japanese beetle is one-half inch long with a shiny metallic green body, coppery brown wings and a small tuft of white hairs surrounding its sides and posterior.

Management options

Control options for adult Japanese beetles and their grubs vary depending on the landscape plants. No spray treatment is needed for mature trees and shrubs because they have more tolerance to the feeding damage caused by the adults and will leaf out again the next year.

Here are several tips on how to minimize these pests:

  1. Small landscape plants such as roses, vegetable crops, strawberries and raspberries can be protected using floating row cover (white polyester spun bonded fabric) from afternoon until late evening hours. This fabric can be draped over the plant and pinned it to the ground. However, it should not be used on blooming vegetable crops like pumpkins and squash, as they require bees for pollination.

  2. Hand-picking and drowning the beetles in soapy water is an option if their population is low.

  3. For larger populations, standard contact insecticides can be used. Acelepryn (the active ingredient is chlorantraniliprole) is relatively new in the market and is known to provide good control on Japanese beetles as a foliar application; it is less toxic to bees compared to standard insecticides containing active ingredients like carbaryl, imidacloprid, permethrin, bifenthrin or malathion. Because these standard insecticides harm bees, they should be sprayed on small-size woody ornamentals and perennials only after they bloom. The product label includes instructions and a bee toxicity warning.

  4. On fruits and vegetables, standard organic products such as neem oil and spinosad can be sprayed after the blooming period. The product label has instructions and information on the post-harvest interval period and safety. For maximum control, these products should be sprayed during the afternoon when the beetles are in peak activity. The application should be repeated once every five to 10 days until mid-August.

  5. Japanese beetle traps should not be used for control, as they will attract thousands of beetles to a garden and can lead to more damage.

  6. Turf should not be irrigated during the beetle's active season to help prevent the insect from laying eggs there.

  7. A preventive grub insecticide (with an active ingredient containing imidacloprid, halofenzide, clothianidin or thiamethoxam) can be scheduled for application to lawns before the end of July to prevent eggs from hatching. A granular formulation is best. Before application, flowering weeds such as clovers should be mowed in the lawn to prevent bee toxicity. Alternatively, acelepryn insecticide can be used as a preventive application. Immediately after application, the area should be watered with one-eighth inch to leach the product into the thatch layer.

  8. By mid-August, a curative insecticide (carbaryl, clothianidin or trichlorfon) can be used to control young grubs in a lawn. Flowering weeds should be mowed before application and the lawn should be lightly watered afterward.

University of Wisconsin-Extension Horticulture publishes detailed information about gardening and landscape plants, and local Extension offices can provide additional advice.

Vijai Pandian is a horticultural agent and educator for the University of Wisconsin-Extension Brown County. This article is adapted from an item originally published by the Green Bay Press Gazette.

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