White grubs, the larval stage of scarab beetles like the Japanese beetle or June beetle, are a common cause of severe damage to turfgrass. These pests feed voraciously on grass roots from mid-summer through the fall, often leading to large, unsightly brown patches in a previously healthy lawn. The destruction occurs because the grubs sever the root system, preventing the grass from absorbing the necessary water and nutrients. Taking action against these subterranean pests requires a multi-faceted strategy that begins with accurate identification and moves through targeted treatment and long-term prevention.
Identifying Grubs and Assessing Damage
Confirmation of a grub problem begins with recognizing the distinct signs they leave behind. Infested turf often develops irregular patches of brown, dying grass, and the area may feel unusually soft or spongy underfoot. The root system beneath the damaged turf is so severely eaten that the grass can be easily peeled back from the soil surface, much like rolling up a loose piece of carpet.
A clear indication of a significant infestation is the presence of secondary predators, such as skunks, raccoons, or moles, which tear up the lawn while digging for the grubs. A small square-foot section of turf should be cut and peeled back for inspection. The grubs themselves are creamy white, typically one-half to one inch long, and curl into a characteristic “C” shape. If the count exceeds approximately five to ten grubs per square foot, treatment is generally warranted to prevent widespread turf loss.
Biological and Natural Control Methods
Several biological agents offer effective grub control. Beneficial nematodes, specifically Heterorhabditis bacteriophora (Hb), are microscopic roundworms that actively seek out and infect grubs in the soil. The nematodes release a symbiotic bacteria that kills the host grub within 48 hours, and they are effective because they are “cruiser” nematodes that actively burrow to find their prey.
Nematodes are sensitive to heat and ultraviolet light, making application in the early morning or evening on an overcast day preferable. The soil must be kept moist, requiring thorough watering before and immediately after application, and remaining moist for at least two weeks to help the nematodes establish themselves. The ideal timing for this natural treatment is in the late summer or early fall when grubs are small, actively feeding, and closer to the soil surface.
Another biological option, though slower-acting, is the application of Milky Spore powder, which is a bacterium (Paenibacillus popilliae) that causes a disease specific to the grubs of the Japanese beetle. While it is a species-specific treatment and its effectiveness can take several years to become fully established, it provides a very long-term control method once the spores are distributed in the soil. Simple cultural practices can naturally deter grubs; for instance, allowing the top inch of soil to dry out during the peak beetle egg-laying period in mid-summer makes the area less attractive for the adults to lay their eggs.
Chemical Treatment Strategies
Chemical insecticides are often utilized, but their effectiveness depends entirely on the timing and type of product chosen. Preventative products are applied early in the season to stop grubs from causing damage, while curative products are used later to address an existing problem.
Preventative treatments are significantly more effective, often providing 75-100% control, and should be applied before the eggs hatch in mid-summer. Products containing the active ingredient chlorantraniliprole are highly valued because they are less water-soluble and provide a long residual effect, allowing them to be applied as early as April or May for season-long protection. Other preventative ingredients like imidacloprid or clothianidin should be applied in June or July, closer to the time of grub hatching, and require immediate irrigation of at least 0.5 inches of water to move the chemical into the root zone.
Curative, or rescue, treatments are necessary when damage is already visible in the late summer or fall, but they are less effective than preventative measures, often yielding 20-80% control. The active ingredients trichlorfon or carbaryl are the most common options for rapidly killing larger, actively feeding grubs. These products are short-lived and must be watered into the soil immediately after application to reach the grubs before they move deeper into the soil to overwinter.
Long-Term Prevention Through Lawn Care
Shifting the focus from elimination to prevention can make a lawn environment much less hospitable to egg-laying adult beetles. A simple adjustment in mowing height can provide a degree of protection, as maintaining a taller grass height, typically between three and four inches, discourages female beetles from laying their eggs in the turf.
Proper watering techniques also play a significant role in making the lawn less appealing to beetles. While deep, infrequent watering encourages strong root growth that can withstand minor feeding, it is helpful to allow the topsoil to dry out during the peak egg-laying season in mid-summer. A healthy, dense lawn with robust root systems naturally tolerates higher grub populations without showing visible damage. Practices like regular aeration and balanced fertilization promote deep root development, which helps the turf recover more quickly from any minor root damage that may occur.