White grubs are the larvae of various scarab beetles (e.g., Japanese beetles, May/June beetles), representing one of the most destructive pests to turfgrass. These C-shaped larvae live just beneath the soil surface, where they aggressively feed on grass roots. This feeding leads to tell-tale signs of damage, such as irregular brown patches that mimic drought stress. A severe infestation causes the grass to feel spongy, allowing the turf to be rolled back easily because the roots have been completely severed. Eliminating these pests requires a strategic approach combining timing, cultural practices, biological solutions, and chemical treatments.
Identifying Grubs and Their Vulnerable Stage
Confirming a white grub infestation involves physically inspecting the soil beneath damaged areas. Lift a section of sod approximately one square foot and two to three inches deep to expose the root zone. The presence of five to ten grubs per square foot often warrants control measures, though this threshold varies based on grass species and turf health.
Effective treatment relies on understanding the grub’s life cycle. Adult beetles emerge in late spring or early summer to mate, laying eggs in the soil from June through August. Eggs hatch into small, first-instar larvae in late summer (July or August), which immediately begin actively feeding on grass roots.
This late summer and early fall period is the most vulnerable stage for treatment, as the grubs are small and near the surface. In the fall, grubs mature into larger larvae and burrow deep to overwinter, making them difficult to reach. Treating large, mature grubs in the spring is generally ineffective, as they cease significant feeding before turning into adult beetles.
Cultural Practices for Prevention
Preventing an infestation begins with making the lawn an unattractive place for adult beetles to lay their eggs. Altering watering practices during the peak egg-laying period in mid-summer (typically July and August) is effective. Adult beetles prefer moist soil, so reducing irrigation during this time can discourage females from choosing your lawn.
Maintaining a higher mowing height also contributes to prevention. Longer grass blades promote deeper root growth, which allows the turf to tolerate minor grub feeding without showing significant signs of damage. Practices that improve soil health, such as aeration and dethatching, allow the grass to recover more quickly from any root damage. Promoting a healthy, vigorous turfgrass through proper fertilization and maintenance is the foundation of long-term grub management.
Biological Control Solutions
For those seeking an environmentally focused approach, biological controls utilize living organisms to target the grubs without synthetic chemicals. Beneficial nematodes are microscopic, soil-dwelling roundworms that actively seek out and parasitize white grub larvae. The most effective species for grub control is Heterorhabditis bacteriophora (Hb), which is known as a “cruiser” because it actively hunts for prey deep in the soil.
Once the nematode finds a grub, it enters the pest through a body opening and releases symbiotic bacteria that quickly kill the host, typically within 48 to 72 hours. Optimal application requires the soil temperature to be above 60°F and the soil to be moist, but not saturated, before and after application. Since UV light is lethal to nematodes, they should be applied in the early morning, late evening, or on an overcast day, and then immediately watered into the soil to reach the root zone.
Milky Spore
Another biological option is Milky Spore, a commercial product containing the spores of the bacterium Paenibacillus popilliae. This bacterium specifically targets Japanese beetle grubs, causing a fatal disease that gives the larvae a characteristic milky appearance. When the infected grub dies, billions of new spores are released into the soil, slowly establishing a permanent control population that can last for many years.
Milky Spore requires ingestion by the feeding grubs to be effective. While it is highly specific and harmless to other organisms, its efficacy is often slow and sometimes inconsistent against species other than the Japanese beetle. Applications are best made in late summer or early fall when grubs are actively feeding, and the product may require multiple applications over two to three years to establish a sufficiently high concentration in the soil.
Chemical Treatment Options
Chemical solutions for white grubs are categorized based on their timing and mode of action: preventative or curative.
Preventative Treatments
Preventative treatments are systemic and designed to be applied early in the season, typically from mid-June to mid-July, before eggs hatch. These products contain active ingredients, often neonicotinoids like Imidacloprid or Clothianidin, that are absorbed by the grass roots and remain in the plant tissue. When the newly hatched, first-instar grubs begin feeding on the roots in late summer, they ingest the insecticide and are killed. Chlorantraniliprole is a newer preventative option that offers a wider application window, sometimes as early as April or May, due to its longer residual activity. Preventative treatments are highly effective because they target the small, vulnerable grubs before damage occurs.
Curative Treatments
Curative treatments are contact insecticides used later in the season, from late July through September, when damage is already visible and grubs are actively feeding. These are short-lived chemicals, such as Trichlorfon or Carbaryl, that must move quickly into the soil to contact the grubs directly. Curative applications are most successful when the grubs are still small, in early to mid-August, as they become progressively harder to kill as they mature.
For both preventative and curative products, watering the lawn immediately after application is necessary to wash the chemical down to the root zone where the grubs are feeding. A minimum of a half-inch of water is often recommended to ensure the active ingredient reaches the target depth. Products containing neonicotinoids should be used with caution due to their potential risk to pollinators, and all chemical labels must be followed exactly to ensure safety.