Grubs are the larval stage of various scarab beetles, most commonly the Japanese Beetle, European Chafer, and Masked Chafer. These C-shaped, off-white larvae reside in the soil, where they consume the tender roots of turfgrass. The resulting damage appears as irregular brown patches of grass that can be rolled back like a piece of carpet because the root system has been severed. Determining the “best” treatment is highly dependent on when the infestation is discovered and the severity of the damage.
Understanding the Grub Life Cycle and Application Timing
Adult beetles typically emerge in early summer, mate, and lay their eggs in the soil between June and August. These eggs hatch into tiny, first-stage grubs, known as instars, in mid-to-late summer.
This period, from late July through early fall, is when the grubs actively feed near the surface and inflict the most severe damage to the lawn. As soil temperatures drop in late fall, the grubs burrow deeper to overwinter, making them unreachable and less susceptible to treatment. They briefly return to the root zone to feed in the spring before pupating into adult beetles, restarting the cycle. The timing of any application, whether preventative or curative, is more important than the product itself because chemicals work best on young, actively feeding larvae.
The Best Preventative Chemical Controls
Preventative treatments are systemic insecticides that must be applied before the eggs hatch, typically between mid-June and mid-July. This ensures the active ingredient is in place when the young grubs begin to feed. The most common active ingredients used for prevention are the neonicotinoids, such as imidacloprid, clothianidin, and thiamethoxam.
These compounds are absorbed by the grass roots, making the plant tissue toxic to feeding insects. When the newly hatched grubs ingest the treated roots, the insecticide targets their central nervous system, causing paralysis and death. Another highly effective preventative compound is chlorantraniliprole, which is less water-soluble and offers a wider application window, sometimes as early as April or May.
All preventative products require immediate watering—at least 0.5 inches of irrigation—to move the insecticide from the grass blades down into the root zone. To protect pollinators, these systemic products should not be applied to lawns that have flowering weeds, such as clover, which attract foraging bees.
Curative and Immediate Grub Killers
Curative treatments are necessary when a homeowner misses the preventative window and discovers visible lawn damage in late summer or early fall. The primary active ingredients for a curative response are trichlorfon and carbaryl.
Trichlorfon requires five days or more to show results, and carbaryl may take a few weeks to reach full efficacy. These insecticides are short-lived in the soil and act by direct contact or ingestion, making it imperative that they are applied when grubs are feeding near the surface in late summer or in the spring before May 15th. Curative applications are less effective than preventative measures, often achieving a control rate between 20% and 80% depending on the timing and grub size.
The treated area must be watered thoroughly immediately following application to move the product into the root zone. If the grubs have already burrowed deep for the winter, or if the soil is too cold, the curative treatment will have little to no effect. They do not offer the season-long residual protection of the preventative compounds.
Biological and Natural Treatment Methods
Beneficial nematodes, specifically the species Heterorhabditis bacteriophora, parasitize grubs. These nematodes enter the grub’s body through natural openings and release symbiotic bacteria, which multiply and cause fatal septicemia within a few days.
Effective use of nematodes requires specific conditions, including a soil temperature between 55°F and 70°F and consistently moist soil. Applications must be done during the cooler parts of the day, such as evening or on an overcast day, because ultraviolet light can neutralize the live organisms. Milky Spore is a powder containing the bacterium Paenibacillus popilliae.
Milky Spore only targets the larvae of the Japanese Beetle. When a grub ingests the spores, it develops a fatal disease, and the decomposing grub releases billions of new spores into the soil. Milky Spore is a slow-acting treatment that can take two to three years to build up effective concentrations, but once established, it can provide control for a decade or more.