Japanese beetle (Popilla japonica) is a long, green beetle that is native to Japan. It is considered one of the most devastating pests of turfgrasses, and urban landscape plants in the eastern United States. In the United States, the insect was first discovered infesting a plant nursery near Riverton, New Jersey. It is believed to have been introduced to the United States in a shipment of iris bulbs prior to 1912. Due to the climate in the eastern United States, the lack of natural predators, and the hundreds of plants the adults could feed on, the insect flourished. Throughout the 1950s and 1960s, Japanese beetle populations increased exponentially. The insect has since become widespread throughout the eastern United States, where it devours the leaves, flowers, and fruit of host plants.
Distribution & Habitat
Japanese beetle is indigenous to Japan. In North America, the geographic range of Japanese beetle extends from Ontario to the northeastern United States, south to George and Alabama, and west to Minnesota, Iowa, Missouri, and Arkansas. The insect thrives in moist settings. It often becomes abundant in landscapes with irrigated lawns, and golf courses.
Japanese beetle infests around three hundred species of plants. Trees that are commonly infested by Japanese beetle include American chestnut, American elm, American mountain ash, American linden, black cherry, black walnut, cherry, English elm, flowering crabapple, gray birch, hollyhock, horsechestnut, lombardy poplar, london planetree, peaches, plums, roses, rose of sharon, and sassafras. Japanese beetle also feeds on select weeds and non-cultivated plants, such as bracken, elder, Indian mallow, multiflora rose, poison ivy, smartweed, and wild grape.
The insect’s larvae are small, white grubs. The pupae are yellow. The adults are long, metallic green beetles, with copper-brown wing covers. A row of white tufted hairs projects from under the wing covers on each side of the thorax.
Adults overwinter in the debris that forms beneath host plants. They become active in spring, whereupon they mate. Females vacate host plants in the afternoon. They navigate to moist, grassy areas to lay their eggs. Females lay 40 to 60 eggs. The eggs require adequate soil moisture to prevent them from drying out. In favorable settings, the eggs hatch, revealing masses of white grubs. The grubs develop in the soil for the next ten months, feeding on the roots of turfgrasses, and vegetable seedlings. The grubs grow quickly, reaching nearly an inch in diameter by late August. As the soil cools in fall, the grubs burrow deeper into the earth, where they overwinter. Most grubs overwinter two to six inches below the soil surface. Some may descend eight to ten inches beneath the earth. The grubs become inactive once soil temperatures consistently drop below 50°F. Once temperatures increase above 50°F in spring, the grubs resume activity, and ascend into the root zone, where they feed for four to six weeks. Once mature, the grubs pupate in an earthen cell. By late spring, they emerge as adults. In early summer, the adults navigate to host plants, where they commence feeding. The adults feed on the plant’s foliage from late June to August or September. As the temperatures plummet in fall, the adults drop to the ground, and settle beneath the leaf litter and mulch to overwinter.
Symptoms of Infestation
As the grubs feed within the soil, they consume grass roots, which reduces the ability of the grass to absorb water and nutrients. This causes large dead patches to form in grub infested areas. The adults feed on the leaves and flowers of host plants. Their persistent feeding causes the infested leaves to develop a skeletonized appearance. Dense beetle populations may completely defoliate host plants. The flowers on infested plants are often consumed entirely. In infested areas, the adults can be observed flying around, or hovering near host plants.
- Soil insecticides can be applied to infested turf to control the grubs. Applications should be performed in summer to ensure success.
- The grubs are susceptible to milky spore disease, which is caused by the bacterium Paenibacillus popillae. Milky spore disease is commercially available. It is sold in a powder form, and used for lawn application.
- When planting, select resistant trees and shrubs. American elder, American sweetgum, boxelder, boxwood, butternut, common lilac, common pear, flowering dogwood, mulberry, red maple, scarlet oak, silver maple, white oak, and white poplar are seldom infested by Japanese beetle.
- When beetle populations are low, they can be removed by hand, and doused in soapy water.
- Select plants can be covered with cheesecloth, or other fine netting during the peak of beetle activity. This will prevent the beetles from feeding on the plant’s foliage and flowers. Roses in particular benefit from this method.
- Various insecticides are registered for use on Japanese beetle. Applications should be performed in late spring to deter the adults. Applications should be repeated throughout the growing season to prevent additional infestations from occuring. Susceptible foliage and flowers should be thoroughly drenched.
- Neem oil applications are effective at controlling beetle populations. Neem oil should be administered at the first sign of infestation. When applied to host plants, Neem oil reduces the beetles’ feeding.
- Natural repellents include catnip, chives, garlic, and tansy. These herbs offer limited control, and are not recommended for use on dense populations.
Photo courtesy of Bruce Marlin CC-by-SA 3.0.