White grubs are the destructive, immature larval stage of various scarab beetles, such as the Japanese beetle, masked chafers, and June beetles. These pests live just beneath the soil surface where they actively feed on the roots of turfgrass. This root-severing activity prevents the grass from absorbing the water and nutrients it needs, leading to significant patches of dead or dying lawn.
The appearance of irregular brown areas signals that a grub population may be causing extensive damage. Managing these pests requires understanding their life cycle and applying the correct control method at the appropriate time to restore the health of your lawn.
Identifying White Grubs and Optimal Treatment Timing
A white grub infestation is diagnosed by visible turf damage and physical inspection. Patches of grass may turn yellow or brown, and the affected areas can feel spongy underfoot. When feeding is heavy, the root system is severed enough that the sod can be rolled back like a carpet to reveal the pests underneath.
Increased activity from insect-eating animals like skunks, raccoons, or birds also indicates a grub problem, as they dig up the lawn while foraging. To confirm the population level, lift a square foot of turf about three inches deep and count the grubs. Finding ten or more grubs in that area suggests treatment is necessary.
Treatment timing aligns with the grubs’ annual life cycle. Adult beetles emerge, mate, and lay eggs in early to mid-summer. The eggs hatch in mid-to-late summer, and the small grubs immediately begin feeding on grass roots.
The optimal time for intervention is late summer through early fall, typically August to mid-September. At this stage, the grubs are small, actively feeding near the surface, and most vulnerable to controls. As soil temperatures drop in late fall, the grubs move deeper to overwinter, making them harder to target.
Non-Chemical and Biological Control Methods
Non-chemical management starts with cultural practices that make the lawn less appealing to adult beetles. Since scarab beetles prefer moist soil, reducing irrigation during the peak egg-laying period in early to mid-summer can naturally deter them. Maintaining a taller mowing height, ideally between 3 and 4 inches, also promotes a robust root system better able to tolerate some grub feeding damage.
Biological controls use natural organisms to manage the grub population without synthetic chemicals. One effective method involves beneficial nematodes, which are microscopic, worm-like organisms. The species Heterorhabditis bacteriophora is a successful biological control agent against white grubs.
These nematodes seek out grubs and enter their bodies through natural openings. Once inside, they release a symbiotic bacterium that kills the grub, which then serves as a food source for the nematodes to reproduce. For effectiveness, nematodes must be applied in the late afternoon to protect them from sunlight, watered into the soil immediately, and used when soil temperatures are warm, generally in late summer.
Another biological option is Paenibacillus popilliae, commercially known as Milky Spore. This naturally occurring bacterium is ingested by the grub during feeding, causing a fatal disease. The dead grubs then decompose and release more spores into the soil, slowly building up a long-term defense against Japanese beetle grubs. Milky Spore is not toxic to non-target organisms and can provide control for many years, though it may take one to three years to become fully effective.
Insecticide Treatment Options and Application
Chemical control strategies are divided into two approaches: preventative and curative treatments, which are timed according to the grub life cycle. Preventative treatments use long-lasting, systemic insecticides applied before eggs hatch, ensuring the chemical is present in the root zone when young grubs begin feeding. These applications are typically made between mid-June and mid-July.
Preventative products often contain active ingredients such as Imidacloprid, Clothianidin, or Chlorantraniliprole. Chlorantraniliprole is less water-soluble and can be applied earlier, sometimes as early as April or May. Products containing Imidacloprid or Thiamethoxam are best applied in June or July. These treatments are highly effective against the small, newly hatched grubs, often providing a 75% to 100% reduction in population.
Curative treatments are short-lived, fast-acting insecticides used when an active infestation is confirmed in late summer or fall, usually from August to mid-September. These compounds are designed to kill the larger, actively feeding grubs that are already causing visible damage. The most common active ingredients for curative control are Carbaryl or Trichlorfon.
Proper application is essential for success regardless of the insecticide type. Both granular and liquid formulations must be watered into the turf immediately after application with at least a half-inch of irrigation. This step moves the insecticide down through the thatch layer and into the root zone where the grubs are feeding. Safety precautions, such as wearing rubber gloves and avoiding entry to the treated area until the grass is dry, should be followed when handling any chemical product.