White grubs are the larval stage of various scarab beetles, such as the destructive Japanese beetle and the common June bug. These C-shaped, cream-colored larvae live just beneath the soil surface, where they consume the fine roots of turfgrass. This feeding activity disrupts the flow of water and nutrients, leading to turf damage. Before treating, confirm their presence and determine the severity of the infestation through a simple physical inspection.
Identifying Visible Signs of Grub Damage
The initial indicators of a grub problem often appear as irregularly shaped brown or dead patches that do not recover even after thorough watering. This wilting and discoloration occurs because the grubs have severed the root system, preventing the grass from absorbing moisture. Grub damage tends to be scattered and follows no distinct pattern, unlike damage from drought or disease.
Another key sign is a change in the turf’s texture, which may feel spongy or soft when walked upon. This sensation is caused by the absence of a secure root structure holding the soil and grass together. Increased activity from secondary pests, like skunks, raccoons, or birds, may also point toward a grub problem, as these animals forage for the larvae and dig small holes. In severe cases, the turf can be easily peeled back by hand, revealing the grubs feeding just below the surface.
Step-by-Step Guide to Physical Sampling
The most accurate method for confirming a grub infestation is to physically sample the soil, which is best performed in late summer or early fall. This timing coincides with the period when the grubs are newly hatched, actively feeding near the surface, and are at their most destructive stage. Focus sampling efforts near the edges of discolored or damaged patches, as the grubs move from dead areas into healthier grass.
To create a sample plug, use a flat-bladed spade or a trowel to cut a patch of turf approximately one square foot in size. The cut should be made on three sides, allowing you to lift the turf like a hinged flap. The necessary depth for this excavation is typically two to four inches, as most grubs feed within this shallow zone just below the thatch layer.
Once the flap is lifted, place it on a hard surface, such as a driveway or sidewalk, for close inspection. Break apart the soil plug and the underside of the sod, looking for the characteristic C-shaped, white larvae with brown heads. Count all grubs found within that one-square-foot area to determine the population density. After inspection, the soil and turf should be replaced and watered thoroughly to encourage root re-establishment.
Determining the Infestation Threshold
The number of grubs counted in your square-foot sample determines if treatment is warranted. Generally, an action threshold of five to ten grubs per square foot is considered high enough to justify intervention. Below this range, the turf can often tolerate the root feeding without showing significant visible stress or decline.
This threshold is not absolute, as it depends heavily on the overall health and type of turfgrass. A well-maintained, healthy lawn with a deep, robust root system may sustain a population of ten to fifteen grubs per square foot or more without substantial damage. Conversely, a stressed lawn suffering from poor irrigation or thin growth may show damage at lower counts. Repeat the sampling process in three or four different locations across the lawn and calculate the average count for an accurate assessment.