The creamy-white, C-shaped creature known as a white grub is the larval stage of various beetles belonging to the family Scarabaeidae, often called scarab beetles. While underground, these grubs cause significant damage to turf and plant roots. Once they complete metamorphosis, they emerge from the soil as adult flying insects. The specific adult form depends on the grub species, but all adults mate and lay the eggs for the next generation of grubs.
Identifying the Specific Adult Beetles
The generic term “white grub” refers to the larvae of several different beetle species. The three most common and damaging adult forms are the Japanese beetle, the June bug, and the European chafer. Correctly identifying which beetle is present is the first step toward effective management.
Japanese Beetle
The Japanese beetle (Popillia japonica) is readily identifiable by its metallic green head and thorax, contrasted with copper-colored wing covers. This relatively small beetle measures about half an inch in length and features five distinct tufts of white hair along the sides of its abdomen. Japanese beetles are active during the day, feeding heavily on over 300 types of ornamental plants, trees, and shrubs from late June through August.
June Bug
June bugs, also known as May beetles (Phyllophaga genus), are larger than the Japanese beetle, often growing up to an inch long. These adult beetles are typically reddish-brown to near-black, possessing a stockier, heavy-bodied appearance. They are nocturnal fliers, often drawn to lights at night, making their feeding activity harder to observe.
European Chafer
The European chafer (Rhizotrogus majalis) is similar in size to the Japanese beetle but is a dull cinnamon or tan-brown color. This coloring helps distinguish it from its more colorful counterparts. European chafers are nocturnal, emerging just after sunset to swarm and mate in trees, with their primary adult activity concentrated in a short three-week span in late June.
Understanding the Metamorphosis and Timing
The transformation from white grub to adult beetle is a process of complete metamorphosis involving four stages: egg, larva (grub), pupa, and adult. The timing is species-dependent, but most damaging grubs, like the Japanese beetle and European chafer, complete their life cycle in one year. This annual cycle ensures new adults emerge every summer to continue the infestation.
Adult beetles lay eggs in the soil between June and August, which hatch into tiny first-stage grubs. These grubs feed on grass roots through the summer and fall, growing through three larval stages (instars). When temperatures drop, the grubs burrow below the freeze line to overwinter, remaining dormant until spring. In spring, the mature grubs feed briefly before entering the pupal stage, a non-feeding resting period lasting several weeks. The new adult emerges from the soil in late spring or early summer, though June bug grubs are an exception, spending two to three years underground.
How the Adult Beetles Cause Damage
The damage caused by adult beetles differs distinctly from the root-feeding that occurs during the grub stage. Adults are primarily foliage feeders, targeting the leaves, flowers, and fruits of plants above ground. This feeding activity is a significant source of plant stress and aesthetic damage in landscapes and gardens.
Japanese beetles are notorious for skeletonizing leaves by chewing the soft tissue between the veins, leaving behind a lacy network that quickly turns brown. They feed intensely during the day, often aggregating in large groups on plants like roses, grapes, and linden trees. Other adults, such as the June bug, cause less noticeable damage by feeding mainly at night, resulting in large, irregular holes in leaves and blossoms. European chafer adults cause no significant feeding damage; their primary impact is completing the life cycle by mating and laying eggs.
Preventing Adult Beetle Infestations
Managing the adult beetle population is a direct way to break the life cycle and prevent the next wave of root-damaging grubs. Control strategies should begin when the first adults emerge, usually in late June or early July, before the peak egg-laying period begins.
Hand-picking is an effective non-chemical method for managing Japanese beetles on smaller plants. This involves shaking beetles off plants into a container of soapy water, preferably in the morning when they are less active. For widespread infestations, targeted insecticides can be applied to susceptible plants to kill feeding adults on contact. Application timing is important, focusing on the weeks when adults are most active and prior to major egg-laying.
Pheromone traps are available to capture adult beetles, but they should be used with caution as they can attract more beetles into the area than they catch. A better non-chemical approach is using fine-mesh row covers or netting over susceptible plants during the adult flight season. This prevents female beetles from burrowing into the soil and depositing eggs.