The white grub is the larval stage of various scarab beetles, including common lawn pests like the Japanese beetle, masked chafer, and June beetle. These creamy-white, soft-bodied insects are found just beneath the soil surface, where they target the root systems of turfgrass and other garden plants. Grubs are a widespread pest, and their presence can lead to significant damage as they consume the plant material anchoring the turf. However, not all grubs are destructive, and their impact depends heavily on their number and species.
Understanding the Grub Life Cycle
The typical grub life cycle follows four stages: egg, larva (grub), pupa, and adult beetle. Adult beetles emerge in the late spring or early summer, mate, and lay their eggs in the soil, often favoring moist, healthy turf. The eggs hatch into larvae in mid-to-late summer, and this grub stage is when the damage begins.
The larval stage is characterized by a stout, C-shaped body with a brown head capsule and three pairs of legs near the head. As the grubs feed voraciously on roots and organic matter, they grow through developmental phases called instars. By late fall, as soil temperatures drop, the grubs burrow deep beneath the frost line to overwinter, returning to the root zone for a brief feeding period in the spring before transforming into pupae and emerging as adult beetles.
How Grubs Damage Lawns and Gardens
Grubs cause primary damage by chewing off the grass roots just below the soil surface. This root consumption prevents the turf from taking up necessary water and nutrients, which first leads to gradual thinning, yellowing, and wilting, often mistaken for drought stress. As the infestation progresses, the grass dies in irregular patches that turn brown.
A distinctive sign of severe grub damage is the “spongy” feel of the turf when walked upon, followed by the ability to pull the dead grass back easily, like a loose carpet, because the roots have been completely severed. Furthermore, grubs become a high-protein food source for predatory animals such as skunks, raccoons, and birds. These animals actively dig and tear up the lawn in search of the larvae, causing secondary damage that can often be more visible and destructive than the grubs’ feeding alone.
Assessing the Need for Action
Not every grub requires treatment, as a healthy lawn can tolerate a small population without visible injury. The recommended way to confirm an infestation and determine the severity is to perform a small-scale inspection. This involves cutting and peeling back a one-square-foot section of turf in a damaged or suspected area, 1 to 2 inches deep, to expose the soil and roots.
The threshold concept helps determine if treatment is warranted, as applying control measures unnecessarily can be costly and ineffective. If you count fewer than five grubs per square foot, no action is needed. However, finding a density of six to ten or more grubs per square foot usually indicates a significant infestation that will cause noticeable damage and warrants a response.
Effective Control Methods
Controlling a grub infestation requires a strategic approach, with the timing of application being the most important factor. The two main categories of control are biological and chemical, each with its own advantages and timing requirements.
Biological/Natural Control
Beneficial nematodes, specifically species like Heterorhabditis bacteriophora, are microscopic roundworms that naturally seek out and infect grubs. These organisms release a bacterium that kills the grub within 48 hours, providing a fast-acting solution. Nematodes are most effective when applied in late summer or early fall when the grubs are small, actively feeding near the surface, and the soil is moist and between 50 to 70°F.
Another biological option is Milky Spore, a naturally occurring bacterium (Paenibacillus popilliae) that is highly specific to the Japanese beetle grub. While it is a long-term control method that can take years to establish a sufficient presence in the soil, it provides sustained protection once effective. Optimal application of Milky Spore is during the summer months when the grubs are present and feeding.
Chemical Control
Chemical insecticides are divided into two categories based on their timing and action. Preventative products, which often contain active ingredients like imidacloprid or chlorantraniliprole, are systemic and work best when applied in early summer (June to mid-July). This timing ensures the chemical is present in the soil when the beetle eggs hatch, killing the small grubs before they cause significant root damage.
Curative products, which typically use fast-acting ingredients such as carbaryl or trichlorfon, are necessary when a high grub population is already causing visible damage. These are contact insecticides applied in late summer to early fall (August or September) to kill the larger, actively feeding grubs. Both preventative and curative treatments require immediate watering (0.25 to 0.5 inches) after application to move the active ingredients into the root zone where the grubs are feeding.