Japanese Beetles EL 37
by G.R. Nielsen, Former Extension Entomologist, Plant and Soil Science
Introduced into New Jersey from Japan prior to 1916, the Japanese beetle* is
now found from southern Maine to the
Carolinas and the Mississippi River. Its density and distribution is strongly affected by soil types, host plants, soil
moisture, and natural enemies.
Description: The adult Japanese beetle is easily recognized by the metallic green body, reddish-bronze wing covers,
and a row of 12 white spots along the sides and rear. It is about 2-inch long. The Japanese beetle spends about 10
months of the year in the ground, in the form of a white grub. This grub is similar to our native white grub, but it is
usually smaller--about an inch long. It lies in the soil in a curled or "C-shaped" position. Several other related scarab
beetles have similar grub larvae. (See EL #1, "White Grubs" and EL #39, "Rose Chafer.") Scarab grubs can often be
identified by characteristic rows of spines (raster) near the tip (underside) of the abdomen. (See EL #200, "Grub
Damage: Grubs feed on the roots and underground stems of plants, particularly grasses. Often this feeding goes
unnoticed until the plants fail to make proper growth, or die. When grubs are numerous, they can cause serious injury
to turf. Adult Japanese beetles will feed on more than 275 different plants. Damage varies from slight to severe. Wild
blackberry, grape, elderberry, smartweed, and roses often attract masses of adults. These plants often abound in or
near sites optimal for oviposition and larval survival.
The beetles often congregate and feed on flowers, foliage, and fruit of plants and trees exposed to bright sunlight.
Beetles feeding on leaves usually chew out the tissue between the veins, leaving a lacey skeleton. A badly attacked
tree or shrub may lose most of its leaves in a short time. The beetles often mass and feed on ripening fruits. They
seldom touch unripened fruit. They seriously injure corn by eating the silk, preventing kernel formation.
Life Cycle: Adult Japanese beetles appear on the foliage and flowers of favored host plants about July 1 in Vermont.
Unlike May and June beetles, they fly only in the daytime. They are very active on warm, sunny days over a 6- to
8-week period. The female beetles lay their eggs in the soil. The larvae or grubs feed extensively on the roots into the
fall. After passing the winter in an earthen cell at depths of 4 to 8 inches below the surface, the grubs resume root
feeding in the spring. Pupation takes place during June in the soil near the surface.
Natural: Extremely dry weather during summer destroys many of the eggs and kills newly hatched grubs. Wet
summers are favorable for their development and are usually followed by seasons of increased numbers of beetles.
Parasitic insects, birds (especially starlings), moles, skunks, and occasionally raccoons also feed on Japanese beetle
grubs. Japanese beetles and grubs are subject to several diseases. The most important is known as milky disease. It
kills grubs after causing their normally clear blood to become milky in appearance. The milky disease spores live in the
soil for long periods, ready to infect and kill successive broods of Japanese beetle grubs as they move about in the soil,
feeding on plant roots. The disease is harmless to human beings, warmblooded animals, other insects, and plants.
Milky spore disease does not work well under conditions of cool, wet, heavy soils (common to much of Vermont).
Milky spore powder is available commercially. Infected grubs are incorporated into dust mixtures that can be spread
over lawns and other infested areas to kill healthy grubs. Treatments are most effective when they are made on a
community-wide basis. Do not use on areas treated with insecticides.
Mechanical: Japanese beetles can be hand picked and destroyed if not too numerous. Attractants and traps are
available for trapping these beetles. These methods may provide satisfactory control in some gardens. Trapping is
more satisfactory for surveying beetle presence than controlling them. Extensive larval and adult damage may occur
within 10 to 20 feet of traps collecting many beetles.
Cultural: Good horticultural practices, including watering and fertilizing, will reduce the damage caused by these
beetles. Plant corn to avoid silking out during period of adult beetle activity. Destroy favored weed hosts.
Chemical: Larvae in turf: You can protect lawns, golf courses, and ungrazed grassy areas from Japanese beetle grub
injury by applying insecticide granules or sprays. For best results, apply treatments in late summer and water grass
thoroughly with 2 inch of water after application. Annual treatment may be necessary. Contact your local lawn and
garden dealer or local Extension office for current chemical recommendations.
Adults on Plants: Most foliage and flowers can be protected by spraying or dusting with insecticides. But insecticides
will not fully protect roses, which unfold too fast and are especially attractive to beetles. When beetles are most
abundant on roses, nip the buds and spray the bushes to protect the leaves. When the beetles become scarce, let the
bushes bloom again. Timeliness and thoroughness of application are very important. Begin treatment as soon as beetles
appear, before damage is done. Use insecticides only on plants for which they are indicated.
Hazard to honeybees: Many dusts or sprays are highly toxic to honeybees. If application of these materials to plants
is necessary during the bloom period, do not apply during hours when bees are visiting the flowers. If larger than yard
and garden plantings are to be treated, you may need to contact nearby beekeepers in advance so that they can
protect their colonies.
*Popillia japonica Newman; Coleoptera, Scarabaeidae
Edited in January 1997, based on material developed in 1983.
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