Lawn grubs, the soft-bodied, C-shaped larvae of various beetles (such as the Japanese beetle and masked chafer), are a common and destructive turf pest. These insects spend their immature stage beneath the soil surface, feeding voraciously on grass roots. This root pruning quickly turns a healthy lawn into dead, brown patches, raising the question of whether control measures are necessary. This guide explains how to identify a grub problem, determine the need for action, and select the appropriate treatment strategy.
Recognizing the Signs of Grub Damage
The most common visual indicator of grub activity is the appearance of irregular, brown patches of turf, often in late summer or early fall. This browning occurs because the grubs have severed the grass’s root system, preventing the turf from absorbing necessary water and nutrients. These damaged areas may look similar to drought stress, but unlike dry grass, the turf will not recover even with sufficient irrigation.
A more telling physical sign is a change in the turf’s texture, which may feel spongy or loose underfoot due to the compromised root structure. In severe infestations, the grass can be lifted easily, often rolling back like a piece of carpet, because the grubs have consumed nearly all the anchoring roots. This physical inspection is the clearest way to distinguish grub damage from other lawn diseases.
Secondary damage caused by animals is a strong clue that grubs are present. Raccoons, skunks, and birds actively forage for grubs, a protein-rich food source, leading to patches of dug-up soil or small holes across the lawn. While these animals also eat earthworms, excessive digging activity in late summer or fall, combined with other turf symptoms, suggests a grub population is active near the surface.
Determining If Treatment Is Necessary
A visual assessment of damage is often insufficient to mandate treatment, as healthy lawns can tolerate a low population of grubs. Determining the severity of an infestation requires a physical inspection of the soil to quantify the grub count. This involves cutting and lifting a one-square-foot section of turf, typically in an affected or adjacent healthy area, to examine the soil and root zone beneath.
The removed turf plug should be about two to four inches deep, as this is the zone where grubs actively feed during late summer and early fall. Once the section is lifted, carefully break apart the soil and count the number of grubs found within that one-square-foot area. Count the grubs in several spots across the lawn, not just a single area, to get a representative sample of the overall population.
The decision to treat is based on a specific quantitative threshold, typically between five and ten grubs per square foot for most turfgrass species. A healthy, well-maintained lawn may tolerate up to ten grubs per square foot without showing damage. Conversely, a lawn under stress from drought or poor maintenance may show significant damage with as few as five to seven grubs per square foot. If the count is below this established range, control measures are not necessary, and the turf can often recover with proper care.
Choosing the Right Control Method
Once an infestation exceeding the damage threshold is confirmed, two main approaches are available for control: chemical and biological. Chemical control options are categorized as either preventative or curative, based on when they are applied and how they work. Preventative products, which often contain active ingredients like imidacloprid or clothianidin, are applied before the grub eggs hatch to provide season-long protection.
Curative treatments, such as those containing trichlorfon or carbaryl, are fast-acting insecticides used to treat an active infestation once damage is visible. These are applied later in the season when the grubs are actively feeding, but they are less effective against larger, mature grubs than preventative options. Both chemical types must be watered into the soil immediately after application to reach the grubs in the root zone.
Biological controls offer an alternative to synthetic chemicals. Beneficial nematodes, which are microscopic roundworms, can be applied to the lawn to seek out and kill grubs. Unlike chemical treatments, nematodes are environmentally friendly and pose no risk to people, pets, or beneficial insects. Another biological option is the bacterium Paenibacillus popilliae, known as Milky Spore, which infects and kills Japanese beetle grubs, though its effectiveness is slow to establish and often limited to specific grub species.
Optimal Timing for Grub Treatment
Effective grub control relies on precise timing, dictated by the beetle’s annual life cycle. Preventative treatments are the most effective strategy and should be applied in early summer, typically June through early July, before the new generation of eggs hatches. Applying the product at this time ensures the insecticide is present in the soil to kill the newly hatched grubs, which are the easiest to control.
Curative treatments are applied later in the season, usually from mid-August through September, when the grubs are larger and the damage has become apparent. While these treatments can halt a current infestation, they are less efficient because the grubs are more mature and harder to kill. Waiting too long, such as applying treatments in late fall or early spring, is often ineffective because the grubs have either moved deeper into the soil for winter or are about to pupate.
Applying any grub control product outside of these recommended windows is largely a wasted effort, as the active ingredient may degrade before the grubs are present or may not reach them when they are active. Aligning the treatment with the grub’s most vulnerable life stage—the newly hatched larvae—is the most responsible and successful approach to protecting the turf.