Already more than a century ago, the Japanese beetle (Popillia japonica) was introduced to New Jersey and has since then developed into a major plant pest in the US. Only recently, it was introduced to Northern Italy and continues to spread. The beetle is eager to feed on hundreds of ornamental plants, fruit trees and berries, vegetable plants, forest trees, and arable crops. The adult beetles are only active for a couple of weeks in the summer, but their larvae, typical “grubs”, live underground for the rest of the year, which makes Japanese beetles especially difficult to eradicate.
Like typical beetles, Japanese beetles have hardened fore wings, so-called elytra, which are not used for flying but cover the hind part of the insect’s body. These elytra are copper-brown. With their transparent hind wings, they are able to fly but are clumsy flyers and often bonk into objects. Their oval body is about 10 mm long (that’s smaller than a 1-Eurocent-coin, measured without legs!). The head and thorax (middle part) of their body are shimmering in metallic bronze or green, and their wing coverings are copper-brown. Toward their back end, a row of five patches of white, bristle-like hairs covers each side of their body, and there are two white patches on the tip of their back end. The white patches easily distinguish the adult Japanese beetle from other beetles that resemble it. Have a look at our field identification guide to check characters in detail. The grubs (larvae) that develop in the ground cannot be distinguished from similar native grub species without the use of a binocular.
As its name implies, the Japanese beetle originates from northern Japan and the far east of Russia. There, it is a minor pest as it has several natural enemies. Additionally, the local climate and environmental conditions help to prevent massive population growth. In 1916, the Japanese beetle was first discovered in the US, probably introduced with plant material from Japan. After this first discovery outside of its home range, the beetle moved to several other countries, always causing severe damage. Since it was first spotted in northern Italy in 2014, it has rapidly expanded its range, and population levels have exploded. Only four years later, it was discovered for the first time at the southern border of Switzerland in 2017, and continued to spread northwards.
Adult beetles generally feed on leaves, chewing out the tissue between the veins, only leaving the leaf skeleton. They feed on more than 300 plant species, including vegetable crops, flowering plants, trees, and shrubs. They show a preference for leaves of grapes, fruit-bearing trees, and soy. Besides leaves, they also feed on ripe fruit of mean host plants, for example, plums, cherries, raspberries, blueberries, or apples, and they may even feed on flower petals, for example, rose blossoms. Depending on food supply, they can also feed on the leaves of forest trees, including hazel nut, oaks, or linden.
The damage by Japanese beetles is not limited to their adult form. Their larvae feed underground just below the surface, where they consume roots and thereby reduce the vitality and yield of their host plants. These grubs prefer roots of lush pasture and turfgrass but can also feed on roots of maize, beans, tomatoes, strawberries, or nursery seedlings. Predators such as wild boars or crows use the grubs as a food source and cause secondary damage when digging for them. The digging can cause more disruption to the sward than the initial damage by the grubs.