Grubs are the larvae of various beetles, such as Japanese beetles or masked chafers, that pose a significant threat to a healthy lawn. These C-shaped, off-white pests cause damage by feeding voraciously on turfgrass roots. The destruction manifests as irregular brown patches that resist watering and a spongy, loose feeling underfoot. Timing the application of grub killer is the most important factor for effective control, as success depends on hitting the grubs during their most vulnerable life stage.
Understanding the Grub Life Cycle
The life cycle of common annual white grubs follows a predictable one-year pattern, which dictates the best time for intervention. The cycle begins when adult beetles emerge from the soil in late spring to early summer, with peak activity often occurring in July. Females then burrow into the soil to deposit 40 to 60 eggs in a moist, grassy area.
The eggs hatch after about two weeks, typically from late July through early September, producing small, newly hatched larvae. These tiny grubs immediately begin feeding on grass roots near the soil surface. This small larval stage is the most vulnerable point for chemical control. As summer progresses into fall, the grubs grow quickly into larger larvae that cause the most visible turf damage.
When soil temperatures cool in late fall, the grubs burrow deeper, sometimes up to eight inches, to overwinter. They remain dormant until the soil warms again in early spring, usually around mid-April. They then move back toward the surface to feed briefly before entering the pupal stage. The large grubs present in the spring are resistant to most control products because they have completed much of their feeding and are preparing to transform into adult beetles.
The Optimal Window for Preventative Control
The most effective strategy for managing grubs is preventative control, which involves applying a long-lasting insecticide before the eggs hatch. This approach targets the vulnerable, newly hatched larvae when they begin feeding, preventing turf damage. The ideal application window for preventative products is early summer, generally late May through early July, depending on the region and climate.
Applying the treatment during this window ensures the active ingredient is integrated into the soil’s root zone when the eggs hatch in late summer. Systemic insecticides like imidacloprid or chlorantraniliprole are commonly used. Chlorantraniliprole has a wider application window, often effective from April through mid-June, but requires more time to become active in the soil.
The timing for imidacloprid application should be slightly later, closer to June or July, just before the anticipated egg hatch. After applying the granular or spray product, water the lawn immediately with about a quarter to a half inch of water. This irrigation moves the insecticide down into the root zone, activating it where the grubs will feed.
A natural indicator that the timing is right can be observed through local plant life, such as when shrubs like the Rose of Sharon or hydrangeas are in full bloom. Using these biological cues alongside soil temperature readings helps fine-tune the application time. The high success rate of preventative treatment, which can reduce grub numbers by 75 to 100 percent, makes it the preferred method.
Applying Curative Grub Killer
Curative control is a reactive measure used when a grub infestation is present and causing visible damage. This necessity arises when the turf feels spongy, brown patches appear that cannot be explained by drought, or when animals like skunks and raccoons begin digging to feed on the grubs. Curative treatments are applied in late summer or early fall (August or September) when grubs are actively feeding near the surface, or in early spring (April or early May) when overwintering grubs rise to feed again.
Because the grubs are larger at this stage, they are more resilient, and the treatment is less effective than a preventative approach. Curative products, such as those containing trichlorfon or carbaryl, are fast-acting, contact killers designed to provide a quicker knockdown of active feeders. These products require watering immediately after application to carry the active ingredients down to the root zone where the grubs reside.
Curative treatments applied in the fall are often only 20 to 80 percent effective in reducing the grub population. Applying the curative product in the spring after mid-May is not recommended because the grubs stop feeding as they prepare to pupate. Curative application addresses the immediate problem but does not prevent future generations, and the lawn often requires reseeding or repair because the damage has already occurred.