White grubs are the larvae of scarab beetles, including the Japanese Beetle, European Chafer, and May/June Beetles. These plump, grayish-white larvae have brown heads and six legs, and they typically curl into a distinct C-shape when disturbed. Grubs feed voraciously on grass roots just beneath the surface, severing the plant’s connection to water and nutrients. This root pruning leads to characteristic irregular brown patches in the lawn. If left untreated, the turf can become so damaged that it can be rolled back like a piece of carpet, requiring prompt identification and treatment.
Confirming a Grub Infestation and Optimal Timing
Effective management begins by confirming that grubs are the cause of the lawn damage, as brown patches can also result from drought or disease. A common indicator of a serious infestation is a spongy feel to the turf underfoot, where roots have been eaten away. Secondary damage from animals like skunks, raccoons, or birds digging for the larvae also suggests a problem. The most reliable way to confirm an infestation is by performing a physical inspection. This involves using a shovel to lift a one-square-foot section of turf about three inches deep to examine the soil for the C-shaped larvae. Finding ten or more grubs per square foot generally indicates a population density that warrants treatment.
Timing is a determining factor for successful control because treatments are most effective when grubs are small and actively feeding near the soil surface. Adult beetles typically lay eggs during the summer. Young grubs hatch and begin feeding from late summer through early fall, approximately August to early October. Curative treatments are most successful during this late summer window because the grubs are in their vulnerable first or second instar stage and are close to the surface. Applying a curative product too late in the fall or in the spring is less effective because the grubs move deeper into the soil to overwinter or pupate.
Fast-Acting Chemical Control Options
Synthetic insecticides offer the quickest relief from an active grub infestation and are categorized as either preventative or curative treatments. Preventative products, which often contain active ingredients like chlorantraniliprole or imidacloprid, are systemic and target newly hatched larvae. These are typically applied in late spring or early summer (June or July) to ensure the chemical is present in the root zone when eggs hatch. Chlorantraniliprole has a longer residual effect and can be applied earlier, sometimes as early as April or May, because it is less water-soluble and takes longer to reach the feeding area.
Curative insecticides are designed to kill larger, actively feeding grubs and are applied once damage is visible, typically in late summer or early fall. Active ingredients like trichlorfon or carbaryl are short-lived, kill-on-contact products that provide a faster knockdown of the pests. For both preventative and curative applications, proper watering is necessary for the treatment to be effective. Applying about a half-inch of irrigation immediately after application helps move the active ingredients down through the thatch layer and into the root zone.
Safety precautions must be followed when using these products, including wearing protective gear like gloves. Application should be avoided when weeds are flowering to minimize risk to beneficial pollinators like bees. Curative treatments may only kill between 20% and 80% of grubs, depending on timing and the active ingredient. Therefore, a follow-up preventive treatment the following season is often recommended. If a curative application is necessary, it is best done when the grubs are near the surface, and the lawn should be mowed immediately before application to remove flowering weeds.
Long-Term Biological Treatments
Biological controls provide a natural method for managing grub populations, though they often act more slowly than synthetic chemicals. One effective biological agent is the beneficial nematode Heterorhabditis bacteriophora (Hb), a microscopic roundworm that parasitizes grubs. These nematodes are “cruiser” species, meaning they actively seek out prey in the soil. They infect the larvae by entering through natural body openings. Once inside the grub, the nematodes release symbiotic bacteria, which cause septicemia and kill the host within a couple of days.
Successful application of Hb nematodes is dependent on specific environmental conditions. They must be applied as a soil drench in cool, overcast conditions, ideally during the early morning or evening. This is because they are quickly neutralized by exposure to direct sunlight. The soil temperature must be warm, generally above 68°F, for the nematodes to be active and search for grubs.
The soil must be kept consistently moist for at least two weeks following application to allow the nematodes to establish and spread effectively. Another long-term biological option is Milky Spore, which contains the bacteria Paenibacillus popilliae. This product specifically targets only Japanese Beetle grubs. Grubs must ingest the spores as they feed, and the bacteria multiply inside, turning the insect’s internal fluids a milky white color, eventually killing the grub. Milky Spore is a slow-acting treatment, but once established, it offers long-term control as the spores can persist in the soil for many years.
Cultural Practices to Prevent Future Infestations
Cultural practices focus on making the lawn environment less hospitable for adult beetles and strengthening the turf’s ability to tolerate or recover from damage. Mowing the grass at a higher setting, typically between 2.5 and 3 inches, helps shade the soil surface. This shaded surface is less attractive to egg-laying beetles. Taller grass also promotes a deeper, more robust root system, allowing the turf to better withstand root damage caused by grubs.
Strategic irrigation is another effective preventative measure, as adult beetles prefer to lay eggs in moist soil. Allowing the topsoil to dry out during the peak egg-laying season in mid-summer can deter adult beetles from choosing the lawn. Core aeration helps to improve soil health and allows essential nutrients and water to reach the roots more effectively.
Overseeding the lawn with new grass seed, particularly after aeration, improves overall turf density. A thick, healthy stand of grass is more resilient and better able to recover from any minor grub feeding that may occur. Reducing excessive thatch, the layer of organic matter between the soil and grass blades, also removes a favorable environment for beetle eggs and newly hatched larvae.