A grub is the common term for the soft-bodied, C-shaped larva of various species of scarab beetles, such as the Japanese beetle or the June bug. These subterranean pests spend the majority of their life cycle beneath the soil surface, where they consume plant material to grow. Their primary food source is the delicate root system of turfgrass and other vegetation. This feeding activity leads to widespread damage in lawns and gardens, typically starting in late summer and early fall.
Defining the Grub Diet and Primary Targets
Pest grubs feed on the fine root hairs and softer vascular root structures of many plants, showing a strong preference for turfgrass. Larvae of species like the European chafer and the Japanese beetle target these roots, which anchor the plant and supply nutrients. By chewing through these essential structures, grubs effectively sever the turf from its source of hydration and nutrients.
This root consumption prevents the plant from accessing the water and minerals necessary for survival, mimicking the effects of a severe drought. When populations become dense, the grass roots are pruned so extensively that the turf loses its ability to stay anchored to the soil. Feeding occurs primarily in the top few inches of the soil profile where root density is highest.
Grubs are not solely focused on turfgrass; they may also damage the roots of vegetable crops, ornamental plants, and tree seedlings. However, the majority of noticeable damage occurs in managed lawns due to the concentration of preferred food sources.
Recognizing the Visible Signs of Damage
The initial symptoms of grub activity often appear as irregular, wilted patches of grass that fail to recover even after receiving water. These areas turn yellow, eventually becoming brown and dead, particularly during late summer or early autumn. The appearance of these patches can be confused with drought stress, but the underlying cause is the missing root system.
A more tactile symptom of a severe infestation is turf that feels spongy or unusually soft underfoot. In advanced cases of root destruction, the grass can be lifted or rolled back easily, much like a loose piece of carpet. This occurs because the grubs have consumed nearly all the roots anchoring the sod to the underlying soil.
Secondary signs of a grub problem are often caused by animals foraging for the larvae. Raccoons, skunks, and birds are natural predators that will tear up sections of a lawn to feed on the protein-rich grubs. Seeing small, random holes or flipped-over sections of turf suggests a substantial grub population is present just beneath the surface.
Determining Infestation Levels
Confirming the presence and severity of a grub infestation requires physically sampling the soil in the affected areas. A common diagnostic method involves cutting and peeling back a square section of turf, typically one foot by one foot, lifted to a depth of two to four inches to expose the root zone.
After lifting the sod, the soil and roots should be carefully inspected and the number of C-shaped, white grubs counted. It is advisable to sample several spots, especially along the edge of damaged and healthy areas, to get an accurate average count. The number of grubs found per square foot determines whether intervention is necessary.
A healthy lawn can often tolerate a low population of grubs, typically around five per square foot, without showing significant damage. The accepted damage threshold that warrants treatment is between six and ten grubs per square foot. If the turf is already stressed from drought or poor soil conditions, even a population as low as five grubs may cause visible injury.
Targeting the Grub Life Cycle
Effective management of a grub infestation relies on understanding and targeting the pest’s annual life cycle at its most vulnerable stage. Most common pest species, such as the Japanese beetle, have a one-year cycle progressing through egg, three larval stages (instars), pupa, and adult beetle. Adult beetles emerge in early summer to mate and lay eggs in the soil.
The eggs hatch into the smallest, or first-instar, larvae in late summer, usually around August. These young grubs feed actively near the soil surface and are significantly more susceptible to control measures than their mature counterparts. Targeting this young larval stage in late summer or early fall is the most effective window for intervention.
As temperatures cool in the fall, the grubs grow larger, moving deeper into the soil to overwinter before returning to feed briefly in the spring. Treatments applied in the spring are generally less effective because the grubs are larger, less active, and soon enter the pupal stage. Aligning intervention with the vulnerability of the newly hatched larvae maximizes success.