The Japanese Beetle, Popillia japonica, is a highly destructive insect that has become an invasive pest across much of North America. While the metallic-green and bronze adult beetles are visible, the most significant damage is caused by the larval form, commonly known as a white grub. This larval stage spends the majority of its life cycle underground, feeding voraciously on the roots of turfgrass and other plants. Understanding the appearance of this grub is the first step in managing this pervasive agricultural and lawn pest.
Physical Traits of the Japanese Beetle Grub
The Japanese Beetle grub is a type of white grub, characterized by its soft, creamy-white body and distinctive C-shaped posture when unearthed from the soil. This curled appearance is characteristic of the scarab beetle family. When newly hatched, the larvae are tiny, but they grow quickly, reaching a maximum length of up to one inch (about 30 millimeters) when fully mature in the late fall.
The grub’s head is a distinct reddish-brown or yellowish-brown color, which contrasts sharply with its pale body. It possesses chewing mouthparts and three pairs of segmented legs located on the thoracic segments just behind the head. The posterior end often appears slightly grayish or dark, which is the contents of its digestive tract showing through the translucent skin. This overall morphology is shared by many other species of white grubs, making precise identification challenging without closer inspection.
Where Grubs Live and the Damage They Cause
Japanese Beetle grubs spend approximately ten months of their one-year life cycle buried in the soil, typically within turfgrass or ornamental planting beds. Females prefer to lay eggs in moist, well-maintained lawns, which provide the ideal environment for the hatching larvae. The grubs primarily feed on fine grass roots just beneath the soil surface during their major feeding periods in the late summer and fall, and again briefly in the spring before they pupate.
This root-chewing activity severs the connection between the grass blades and their water source, leading to significant turf damage. The first sign of damage often appears as irregular patches of brown, dying grass that resemble drought stress. In heavy infestation, the grass roots are consumed that the turf can be easily lifted or rolled back like a piece of carpet, revealing the grubs underneath. Secondary damage occurs when mammals like skunks and raccoons tear up the lawn while digging to feed on the grubs.
How to Tell Japanese Beetle Grubs Apart
Since many beetle larvae look nearly identical, the most reliable way to confirm a Japanese Beetle grub is by examining the raster. The raster is the pattern of hairs, or setae, found on the underside of the last abdominal segment. This pattern is unique to each species of white grub and serves as a technical fingerprint for entomologists.
The Japanese Beetle grub is distinguished by a V-shaped or horseshoe-shaped arrangement of hairs on its raster. Observing this detail requires magnification, often a 10x hand lens, to see the specific alignment of the setae. Other common grubs, such as the Masked Chafer or May/June beetles, have different raster patterns, typically with a scattered or parallel arrangement of hairs. This V-shape is the definitive marker that separates the Japanese Beetle grub from its relatives.