Lawn grubs are the larval stage of various scarab beetles, found just beneath the soil surface. These small, white, C-shaped creatures with brown heads feed voraciously on the roots of grasses, causing significant damage to lawns. The feeding activity of the grub typically alerts homeowners to their presence, as the compromised root system causes the turf to show signs of distress.
The Grub’s Transformation Cycle
The life of a lawn grub follows complete metamorphosis, involving four phases: egg, larva, pupa, and adult beetle. Adult beetles, such as the Japanese beetle or European chafer, lay eggs in the soil during late spring or early summer. The eggs hatch into the larval stage (grubs), which feed on grass roots for several weeks or months before transformation.
As autumn approaches, grubs burrow deeper to survive winter, moving closer to the surface in spring to enter the pupal stage. This pupation phase is a non-feeding period where the grub is encased in a protective shell while it morphs into its final adult form. This transformation usually occurs in late spring or early summer, and the adult beetles then emerge from the soil to complete the cycle. The entire life cycle typically takes a full year, though some species, like the June beetle, can take up to three years.
Identifying the Resulting Adult Insects
The adult insects that emerge from the soil are usually one of three common species: the Japanese beetle, the European chafer, or the masked chafer. The Japanese beetle is the most recognizable, featuring a shiny, metallic green thorax and coppery-brown wing covers, and is often seen feeding in groups on garden plants.
The European chafer is a golden-tan to light-brown adult, while the masked chafer is light brown and distinguished by a dark band, or “mask,” across the front of its head. These beetles are typically about a half-inch long. Unlike the Japanese beetle, the European chafer adult does not typically cause feeding damage to garden plants. The presence of these adult forms in early summer indicates that the egg-laying period is imminent, restarting the damaging grub phase.
How Damage Shifts Between Larval and Adult Stages
The most severe damage to the lawn is caused exclusively by the larval stage, as the grub feeds directly on the grass roots. This root consumption inhibits the turf’s ability to absorb water and nutrients, leading to visible signs of stress, such as irregular brown patches. A heavily damaged lawn will feel spongy underfoot, and the turf can often be easily lifted from the soil because the root system has been severed.
The damage shifts significantly once the grubs transform and emerge as adult beetles. For species like the Japanese beetle, the adult stage introduces a new problem, becoming pests to ornamental plants, roses, trees, and shrubs. These adults feed on foliage, leaving behind a lace-like, skeletonized appearance on the leaves. European chafer and masked chafer adults cause little feeding damage; their primary activity is mating and laying eggs, which renews the threat of root-feeding grubs in the lawn.