The larvae of the Japanese Beetle are a type of white grub. A grub is the common name for the larval stage of a beetle, and many species of scarab beetles, including the Japanese Beetle (Popillia japonica), spend a significant portion of their lives as these subterranean larvae. These creamy-white, C-shaped creatures are among the most destructive turfgrass pests across North America, causing substantial damage before they transform into the brightly colored adult beetles. Understanding this life stage is the first step in managing this pervasive insect, which was first accidentally introduced to the United States in 1916.
The Life Cycle of the Japanese Beetle
The Japanese Beetle undergoes a complete metamorphosis, transitioning through four distinct stages: egg, larva (grub), pupa, and adult, usually completing this cycle within a single year. The process begins in mid-summer, typically from late June through August, when adult female beetles lay their eggs in the soil, preferring well-irrigated, grassy areas. A single female can deposit between 40 and 60 eggs during her short egg-laying period.
Within about two weeks, the eggs hatch into small, first-stage larvae that immediately begin feeding on the fine roots of grasses just below the soil surface. As temperatures cool in late fall, the nearly full-sized grubs burrow four to eight inches deep to overwinter in a non-feeding state. In the spring, usually around May, the grubs migrate back toward the surface to resume feeding before entering the pupal stage. The transformation to the adult metallic green and bronze beetle occurs in the pupal case, and the adults emerge from the soil in early summer.
Key Identification Markers of the Grubs
While most white grubs look superficially similar—creamy white bodies, a brown head capsule, and a characteristic C-shape posture—the Japanese Beetle grub possesses a specific feature for identification. The most reliable marker is the raster pattern, the unique arrangement of hairs and spines found on the underside of the last abdominal segment. Examining this area, often requiring a 10x hand lens, reveals a distinctive V-shape or triangulated pattern of bristles.
The mature larva can grow up to one inch in length. The hind end of the grub often appears slightly darker than the rest of the body due to the contents of its digestive tract visible through the translucent skin. This combination of the C-shaped body and the V-shaped raster distinguishes the Japanese Beetle grub from other common lawn pests.
Distinguishing Japanese Beetle Grubs from Other Lawn Pests
Accurate identification of the grub species is important because different species have varying vulnerabilities and life cycles, affecting the timing of control measures. The Japanese Beetle grub is often confused with the larvae of the Masked Chafer and the May/June Beetle, as all three are known as “white grubs” and share the same general color and C-shape. The primary difference lies in the raster pattern, the arrangement of hairs on the grub’s posterior end.
The Japanese Beetle grub’s V-shaped raster is unique among common turf pests. The Masked Chafer grub, which also has a one-year life cycle, is identified by a pattern consisting of two parallel rows of short, stout spines among scattered hairs. May/June Beetle grubs, which can have a life cycle lasting up to three years, exhibit a “zippered” or bifurcated anal slit with two parallel rows of hairs.
Seasonal Activity and Damage Caused by Grubs
Damage caused by Japanese Beetle grubs is concentrated during two periods of the year. The most significant turf damage occurs in late summer and early fall (August through October) when newly hatched larvae are actively feeding and growing rapidly before overwintering. They feed by chewing off the fine roots of the grass, impairing the plant’s ability to absorb water and nutrients.
This root pruning causes the turf to show symptoms of drought stress, resulting in irregular brown, dying patches of grass. In severely infested areas, the lack of roots can cause the turf to feel spongy underfoot and be rolled back like a loose carpet, exposing the grubs beneath. Secondary damage is caused by predators like raccoons, skunks, and birds, which rip up the weakened turf searching for the grubs. Feeding resumes in the spring, typically from late March through May, but this period is shorter and causes less noticeable damage before the grubs pupate.