Grub worms, the larval stage of various scarab beetles like the Japanese beetle or June bug, are destructive pests that target home lawns. While a few grubs are harmless, a large population feeds aggressively beneath the surface, damaging the turf’s foundation. Recognizing the specific visual and physical clues of their feeding activity is the necessary first step to determine if these pests are the cause of lawn decline.
Primary Visual Signs of Grub Damage
The initial sign of a grub problem often appears as patches of grass transitioning from a healthy green to a dull, wilted color. These patches are typically irregular in shape and dispersed across the lawn, unlike the uniform browning seen during a drought. As the grubs continue to feed, the affected areas turn straw-colored and eventually brown, indicating the grass has died from a lack of root structure. These spots often start small but merge over time, creating larger areas of dead turf.
A common misdiagnosis is confusing grub damage with simple drought stress, as the visual symptoms can look similar. The key distinction is that grub-damaged grass does not recover even after deep, thorough watering. This occurs because the grubs have consumed the roots, leaving the plant unable to absorb the necessary moisture and nutrients. If brown patches persist despite proper irrigation, grub feeding should be strongly suspected as the underlying cause.
Physical Confirmation and Secondary Indicators
Beyond the visual browning, the physical condition of the turf provides definitive confirmation of grub presence. Walking on an infested area often reveals a spongy or soft feeling underfoot. This sensation is a direct consequence of the larvae severing the connections that anchor the grass to the soil below, resulting from the damaged root system and loosened soil structure.
The most reliable test is the “tug test,” which involves gently pulling on the edges of a damaged patch of grass. If the turf lifts easily, rolling back like a loose piece of carpet, it confirms that the roots have been eaten away. To confirm the population size, cut and peel back a small section of turf, about one square foot and two to three inches deep. Finding five to ten white, C-shaped grubs in this single sample indicates a population dense enough to cause significant damage.
Another frequent sign of a heavy grub population is secondary damage caused by foraging animals. Animals like raccoons, skunks, and birds actively dig and peel back the turf in search of the protein-rich larvae. Pencil-sized holes, dug-up soil, or small patches of peeled-back sod are strong evidence that wildlife has detected a high concentration of grubs beneath the surface.
Why and When Grub Damage Appears
The visible damage is a direct result of the grubs feeding on the grass roots, which are the plant’s lifeline for water and nutrients. By consuming the roots, the larvae effectively starve the grass plant, leading to rapid wilting and discoloration above ground. While a lawn can tolerate a small amount of root feeding, destruction quickly becomes noticeable once the grubs grow larger and their population expands.
The timing of the most severe damage is closely tied to the life cycle of the beetles. Adult beetles typically lay eggs in the soil during the summer. The eggs hatch, and the resulting grubs begin to feed. They reach their largest size and most aggressive feeding stage in late summer and early autumn. This period is when the most significant patches of brown turf appear, as root destruction is at its peak.