Grubs are the white, C-shaped larvae of various beetles, such as Japanese beetles and June bugs, that live beneath the soil surface. These insects feed on the roots of turfgrass, cutting the plant off from the water and nutrients it needs to survive. The resulting damage often becomes noticeable in late summer and early fall when the grubs are largest and feeding most actively. Identifying an infestation requires looking for specific visual signs and performing a physical test to confirm root destruction.
Surface Indicators of Grub Infestation
A yard with a grub problem often displays distinct visual and tactile symptoms pointing toward root damage. One of the first signs is the appearance of irregularly shaped, brown patches of grass that begin to wilt and die. Unlike the uniform browning caused by drought, grub damage typically presents as scattered, non-symmetrical patches that do not green up even with consistent watering.
Walking across an affected area may reveal a spongy or soft feeling underfoot, similar to walking on a thick sponge. This sensation occurs because the grubs have consumed the root system, detaching the turf from the underlying soil. Increased activity from animals is another strong indicator, as skunks, raccoons, moles, and birds will actively dig small holes and tear up the lawn while hunting for the larvae. This secondary damage from foraging predators can often make the initial grub damage look much worse.
Physical Confirmation and Severity Assessment
To definitively confirm a grub infestation, the first step is a “tug test.” Gently grabbing the affected brown grass and pulling upward reveals the health of the roots. If the turf lifts easily, rolling back like a piece of loose carpet, it confirms that the root structure has been severely compromised or completely eaten away.
A more precise assessment involves the “dig test,” which requires cutting a small sample of the turf. Using a spade, cut and remove a square foot section of sod, approximately two to three inches deep, at the border between healthy and damaged grass. Peeling back this section allows for a visual inspection of the soil and the exposed root zone for the presence of the white, C-shaped larvae.
Counting the number of grubs found within that one square foot sample determines the severity of the problem. Finding only one or two grubs is generally not cause for concern, as most lawns tolerate low populations. However, a count exceeding five to ten grubs per square foot indicates a damaging infestation that will likely require treatment.
Distinguishing Grub Damage from Other Lawn Problems
It is important to differentiate grub damage from other common causes of brown patches, like drought stress or fungal diseases. Grub-damaged spots remain brown despite adequate irrigation because the lack of roots prevents the grass from absorbing water. In contrast, drought-stressed grass often shows a more uniform browning across the lawn and will improve within a few days of deep watering.
Fungal diseases, such as brown patch, frequently present as circular or crescent-shaped patterns with defined borders, and the turf feels firm underfoot. Unlike grub damage, which causes the grass to detach easily, a drought-stressed or diseased lawn remains firmly anchored when pulled. The distinct combination of irregular browning, spongy turf, and easy detachment is unique to a grub infestation.