Japanese Beetles

By R. Chris Williamson 
(608) 262-4608

The Japanese beetle was accidentally introduced into the U.S. in 1916 and has become one of the most destructive pests of turfgrass and woody-ornamental plants in the eastern United States. Millions of dollars are spent each year to control the beetle adults and larvae, or grubs, and to replace and renovate damaged turf and plants. The adult beetles attack over 300 known species of ornamental plants. To make matters worse, beetle grubs are also destructive. They feed on the roots of all cool-season turfgrasses and ornamental plant roots. Their feeding can cause severe damage or kill the plants.

Homeowners, gardeners and turfgrass managers will notice adult Japanese beetles emerging in early July. The feeding damage they cause will be obvious soon thereafter and grub damage will start to show up in mid to early August, according to University of Wisconsin-Madison/Extension turfgrass and ornamental entomologist Chris Williamson.

Japanese beetle adults are shiny, metallic green, oval, and approximately a quarter inch long. The adults usually feed from the top of the leaf, leaving only a lace-like skeleton of veins. They prefer certain plants, such as lindens, grapes, Norway maples, purple-leaf plums and roses, and the adults are particularly attracted to flowers and fruits.

The beetle grubs feed below ground on the roots and rhizomes of common turfgrass species and cultivars and can eliminate a plant's entire root system. The first evidence of injury by grubs is a patch of pale, discolored and dying grass that resembles drought stress.

These small damaged areas grow larger as the grubs expand their feeding range. The turf will feel "spongy" under foot and can be easily lifted off. Foraging by raccoons, moles and skunks or gathering flocks of birds, especially starlings, are strong indicators of a grub infestation.

The Japanese beetle has a one-year life cycle. Adults emerge from the soil beginning in mid to late-June, and peak adult activity occurs in mid-July in Wisconsin. Mating and egg-laying begin soon after emergence.

Through plant selection and cultural, biological and chemical controls, turfgrass managers and homeowners can reduce the impact of Japanese beetles on turf and ornamental plants.

According to Williamson, a member of the UW-Extension Urban Agriculture and Horticulture Team, "The use of resistant plant species when planning a landscape or replacing plant materials is essential to managing Japanese beetle adults. They are attracted to certain plants and these often sustain heavy feeding damage."

Plants such as grapes, multiflora rose, sassafras, smartweed, and Virginia creeper may attract the adult beetles, which then lay more eggs in the turf around these plants.

Commercial traps are available; however, research has shown that they do not protect a landscape from damage. In fact, Williamson said, these traps attract more beetles than they catch, so plants near the traps are likely to sustain greater damage than if no traps were used.

A number of insecticides are labeled for control of Japanese beetle adults. Homeowners have a limited number and selection of products compared to what is available to commercial or licensed applicators. A homeowner can select from carbaryl (Sevin), acepahte (Orthene), cyfluthrin (Tempo), and permethrin. These are foliage sprays only and may require weekly applications. Licensed applicators have more products available.

Williams recommends withholding water to help reduce grub populations, as eggs and young grubs cannot survive in relatively dry soils. However, providing adequate moisture in late August and September can also help the turf tolerate and even recover from grub damage.

Biological products exist for control of Japanese beetle grubs; however many have shown inconsistent performance. Such products include Milky disease spore, insect-infecting nematodes, and fungal pathogens such as Beauveria bassiana and Metarrhizium.

Most soil insecticides provide adequate control of Japanese beetle grubs, as well as other white grub species, according to Williamson. However, these often require specific factors, such as accurate timing, watering the treatment into the turf, and the presence of minimal thatch. Until now, the most common approach to grub control was to apply short-residual insecticides after the eggs had hatched and before grubs had caused visible damage.

This approach is termed "curative" control, and the ideal treatment time is early to mid August, Williamson said. Such curative treatments can be applied later even after the damage appears, but larger grubs are more difficult to control. Homeowners only have a few options for effective grub control, including diazinon and carbaryl (Sevin). Because spring grubs are distributed throughout the soil, curative spring grub control applications are not recommended.

Due to the development of improved grub control products, preventative treatments of long-residual insecticides are now available. Many turfgrass managers and homeowners prefer them as a control or management strategy. Williamson said this approach seems to provide added value from the standpoint of an "insurance policy" against potential grub damage.

For More Information contact UW Extension

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