Lawn grubs are the larval stage of various scarab beetles, including the Japanese Beetle and June Bug. These pests reside beneath the soil surface, feeding primarily on the fibrous root system of turfgrass. Root destruction disconnects the grass from water and nutrients, leading to a visible decline in the lawn’s health. Accurately diagnosing a grub problem requires a focused inspection, as the damage they cause can often be mistaken for other common lawn issues like drought stress or disease.
Recognizing the Visible Symptoms of Grub Damage
The initial signs of a grub infestation often manifest as irregular patches of brown or yellowing turf that appear in late summer or early autumn. Unlike drought-stressed grass, these discolored areas typically will not recover even after receiving sufficient water. This lack of response to irrigation is a strong indicator that the root system supporting the grass has been compromised.
As the grubs continue to feed, the affected turf begins to feel spongy or bouncy when walked upon. In more severe infestations, the grass may be easily pulled up from the soil, often rolling back like a loose piece of carpet. This separation occurs because the grubs have effectively severed the roots connecting the sod layer to the underlying soil.
Increased activity from animals that feed on the larvae is another telltale sign. Nocturnal predators like raccoons and skunks, as well as birds, will tear up sections of the lawn while searching for the grubs, creating secondary damage. The presence of these digging marks, particularly near the edges of discolored patches, suggests a concentrated food source beneath the surface.
The Physical Inspection: Digging for Confirmation
While surface symptoms provide strong evidence, a physical inspection is necessary to confirm the presence of grubs. To perform this check, select an area that shows visible damage or feels spongy, focusing on the perimeter where the dying grass meets the healthy turf. Grubs tend to congregate and actively feed where the roots are still intact.
Using a spade or trowel, cut a square section of turf measuring approximately one square foot. The cut should penetrate the soil to a depth of about two to four inches, which is typically the root zone where the grubs are feeding. Carefully lift and peel back this section of sod to expose the soil and the underside of the grass roots.
Once the sod is lifted, closely examine the exposed soil and the root mass for the larvae. Grubs are identifiable by their off-white bodies, brown heads, and characteristic C-shape when curled up. Counting the number of larvae provides the data necessary to determine if an intervention is warranted. After the inspection, the square of sod should be replaced and watered to encourage re-rooting.
Determining the Infestation Threshold
The decision to treat a lawn for grubs is based on the number of larvae counted in the square-foot sample. The accepted threshold that warrants intervention is a count of five to ten grubs per square foot. Finding fewer than five grubs is considered a normal, tolerable population that healthy turf can withstand without showing significant damage.
This numerical threshold can fluctuate based on the specific type of grass and the lawn’s overall health. A highly stressed lawn, perhaps due to inadequate watering or poor soil conditions, may show visible damage with a lower grub count. Conversely, a well-maintained lawn with a deep, dense root system might be able to tolerate upwards of ten grubs per square foot before experiencing noticeable thinning or die-off. Sampling several locations across the lawn, rather than just one spot, will provide a more accurate assessment of the average population density.