Japanese beetle, Popillia japonica
The adult Japanese beetle has an oval body and is about 15 mm (0.6 inches) in length. It is generally metallic green with coppery brown wing covers, which do not quite cover the tip of the abdomen. Along the sides are five patches of white hair tufts. The antennae are clubbed at the end and may spread to a fan-like form. Japanese beetle larvae are white grubs that feed on the roots of grasses. They have a creamy white body with a dark head and the legs on the thorax are well developed. Normally the body curves into a C-shape. These features are also typical of other white grubs found in association with turfgrass in Colorado, such as masked chafers and May/June beetles which are discussed in factsheet: ‘Billbugs and White Grubs’. Japanese beetle larvae are slightly smaller than these other species when full grown, but they are best distinguished by their iridescent green and copper color and the pattern of hairs on the abdomen (‘rastral pattern’), which forms a distinctive V-shape.
- All beetles belong to the insect order Coleoptera, which is the most diverse insect order with over 350,000 described species. There are 30,000 species of beetle in the United States alone; however, only a small fraction are pests.
- Adults can feed on over 300 plant species. Preferred vegetable hosts include basil, corn, and soybean. The larvae are soil dwelling grubs that feed on the roots of grasses.
- Japanese beetle larvae can be controlled with certain insecticides or by insect parasitic nematodes. Adults are best controlled by handpicking or using certain insecticide sprays.
- Japanese beetle traps can capture many adults but are not effective in reducing damage to nearby plants.
Japanese beetles are an invasive insect pest in the United States and can feed on a wide range of plants. The larvae are soil-dwelling grubs that feed on plant roots, while adults feed on leaves and flowers. In Colorado, adults cause significant defoliation and damage to basil, corn, and soybean. Image credit: Roger Schmidt, University of Wisconsin-Madison, Bugwood.org
White grubs (larvae) of the Japanese beetle. Image credit: David Shetlar, Ohio State University
Soybean leaf injured by feeding of Japanese beetle. Image credit: Roger Schmidt, University of Wisconsin-Madison, Bugwood.org
Japanese beetle has one generation per year. Adults may begin to emerge from the soil in early June and are usually most abundant in early summer – from late June through early August. As adults, Japanese beetles can be found feeding and mating on foliage and flowers of their host plants. Mated females seek areas where soil is suitably moist to lay a small cluster of eggs among plant roots. A total of 40-60 eggs may be laid by each female beetle, during her four-to-eight-week life span. Upon hatching from the eggs, the grubs (larvae) seek out nearby plant roots and feed. During the time Japanese beetles are in the egg and earliest grub stage they are quite sensitive to drying and may die if soils temporarily dry out during this period. Larvae continue to feed until soil temperatures drop to about 60°F (16° C) at which time the larvae move deeper into the soil where they remain through winter. Activity resumes as soils warm in spring and, after a feeding period of about 4-6 weeks, the larvae form an earthen cell and pupate. A few weeks later the pupal stage is completed, and the new adults emerge.
In vegetables, feeding injury by adults is usually the primary concern. Adults feed on leaves, buds, and flowers of certain vegetables. Feeding is usually restricted to the tender tissues between the larger leaf veins, which results in skeletonized leaves. Persistent and severe feeding on young or stressed plants can lead to stunted growth or death. Mature, healthy plants can tolerate some feeding, but severe injury can reduce plant growth and yield.
During the summer, examine susceptible plants for adults weekly, especially if an infestation was present the previous year. To monitor larvae, soil samples can be taken in the early spring. For information on how to monitor Japanese beetle larvae, consult the webpage on white grubs in turfgrass.
Maintain healthy plants with proper irrigation and fertilization. Certain plants such as rose, apple, stonefruits, willow, elm, grape, birch, maples, pin oak, horse chestnut, and sycamore are highly attractive to Japanese beetle, so avoid planting these near vegetable gardens if possible. Gardens that have basil and beans should be monitored closely for the presence of Japanese beetles. During the warmest part of the day when beetles are most active, adults can be handpicked and placed in a jar of soapy water. Also, allowing the top inches of soil in nearby lawns to dry for a short time can help kill eggs and young larvae since they are very susceptible to desiccation.
Many of the chemical controls for Japanese beetle work to reduce the densities of eggs and larvae in turfgrass. While some insecticides can help control adult Japanese beetles, very few of these are approved for organic pest control, and there is a lot of variation among insecticides with respect to the length of time they persist, what plants they can be used on, whether they act systemically, and their toxicity towards beneficial insects, most notably pollinators. Insecticides that are less toxic to pollinators and persist for only a short period of time can be used on flowering plants when applications are made in the early morning or dusk when bees are not actively visiting plants. While some insecticides are less toxic to pollinators, it is important to note that organic insecticides are not necessarily safer for beneficial insects. Generally, insecticides should only be used as a last resort when pest densities reach damaging levels and there are no alternative control options.
Cranshaw, W. 2018. Japanese Beetle. Colorado State University – Extension. Available https://cmg.extension.colostate.edu/wp-content/uploads/sites/6/2021/04/Japanese-Beetle-factsheet.pdf
University of Missouri. 2018. Organic Management Options for the Japanese Beetle at Home Gardens. University of Missouri – Integrated Pest Management. Available https://ipm.missouri.edu/MEG/2018/1/organic_management_japanese_beetle/
University of Minnesota. 2022. Japanese beetles in yards and gardens. University of Minnesota – Extension. Available https://extension.umn.edu/yard-and-garden-insects/japanese-beetles#preventive-insecticides-1592862
Hodgson, E., D. Alston, and C. Stanley. 2010. Japanese Beetle (Papilla japonica). Utah State University – Cooperative Extension. Available https://extension.usu.edu/pests/uppdl/files/factsheet/japanese-beetle-10.pdf