Lawn grubs are the destructive larval stage of several types of scarab beetles, including the Japanese beetle, masked chafer, and June bug. These soft-bodied pests live just beneath the soil surface, where they consume the roots of turfgrass. The damage they cause is significant because they destroy the grass’s ability to take up water and nutrients, leading to large, unsightly brown patches across the lawn. Addressing a grub infestation is fundamental to maintaining a healthy, vibrant lawn, as unchecked populations can quickly ruin an entire turf area.
Identifying the Pest and Damage
A grub infestation often goes unnoticed until the damage becomes severe, typically in late summer or early fall when the larvae are largest and feeding most aggressively. The grubs themselves are whitish-gray, C-shaped creatures with a distinct brown head capsule and three pairs of legs near the head. They range in size from about one-quarter to one inch long, depending on their species and maturity.
One of the most telling signs of a problem is the appearance of irregular, dead brown patches that do not green up, even after sufficient watering. If you can easily peel back sections of the turf like rolling up a piece of carpet, it indicates the roots have been severed by the grubs feeding below. Secondary signs include damage from animals, such as skunks, raccoons, or birds, which dig into the lawn to feed on the grubs. To confirm the presence of grubs, cut and lift a one-square-foot section of turf in a damaged area and count the grubs in the soil. A population of six to ten grubs per square foot is considered the threshold for treatment.
Understanding the Grub Life Cycle
Effective elimination relies on understanding the grub’s one-year life cycle, as timing determines the success of any control measure. Adult beetles emerge from the soil in late spring to early summer, mate, and then lay their eggs in the turf, typically between June and August. The eggs hatch within a couple of weeks, producing tiny, vulnerable first-instar grubs that immediately begin feeding on the grass roots.
This late-summer period, usually from August to September, is when the grubs are small and near the surface, making them most susceptible to curative treatments. As the weather cools in the late fall, the grubs move deeper into the soil to overwinter, where control measures are ineffective. They return to the root zone to feed briefly in the spring before pupating. By this time, they are much larger, in their third-instar stage, and significantly more difficult to control chemically. The cycle completes when the pupae transform into adult beetles, which then emerge to start the process over.
Chemical Treatment Options
Chemical control of grubs is divided into two primary strategies: prevention and cure, utilizing different synthetic active ingredients applied at specific times of the year. Preventative treatments are designed to kill the newly hatched, small grubs before they can cause noticeable damage. Active ingredients like imidacloprid and chlorantraniliprole are systemic insecticides applied between late April and early July. The product is watered into the soil, where it is absorbed by the grass roots, creating a toxic barrier that kills the grubs when they begin feeding later in the summer.
Chlorantraniliprole can be applied as early as April because it is less water-soluble and remains effective in the soil for an extended duration. Other preventative options, such as those containing imidacloprid, are best applied between mid-June and early August to ensure the chemical is present when the eggs hatch. These preventative treatments are highly effective at reducing grub populations, often showing better results than curative options when timed correctly.
Curative treatments are necessary if a severe infestation is discovered in late summer or early fall when the grubs are actively feeding and damage is visible. These fast-acting, contact insecticides contain active ingredients like trichlorfon or carbaryl. They work quickly to kill the larger, existing grubs that are causing the current destruction.
Timing is crucial for curative applications, which should be made in August or September when the grubs are still feeding near the surface. The product must be immediately watered into the lawn with at least half an inch of irrigation to move the chemical down to the root zone where the grubs are located. While effective for an active infestation, curative products are short-lived in the soil and generally not as effective at long-term population reduction as the preventative options.
Biological and Cultural Controls
Non-chemical methods offer environmentally conscious ways to manage grub populations, focusing on biological agents and long-term turf maintenance practices. Beneficial nematodes are microscopic roundworms that naturally parasitize grubs and other soil-dwelling insects. The species Heterorhabditis bacteriophora is the most effective type for grub control, as it actively seeks out and infects the larvae.
Nematodes are living organisms, requiring specific soil temperature and moisture conditions for successful application, typically when the young grubs are present in late summer. Another biological control option is the naturally occurring bacterium Paenibacillus popilliae, known commercially as Milky Spore. This bacterium infects and kills Japanese beetle grubs, releasing more spores into the soil upon the grub’s death, which can provide long-term control, although its effectiveness can be slow and specific to certain grub species.
Cultural controls involve adjusting lawn care practices to create an environment less favorable for adult beetles to lay eggs and for grubs to thrive. Maintaining a higher mowing height, typically three inches or more, encourages deeper root growth, which allows the grass to better tolerate feeding damage. Reducing irrigation during the peak beetle egg-laying period in mid-summer can make the soil less attractive for egg deposition, as female beetles prefer moist soil. Aerating the soil periodically helps alleviate compaction, promoting a healthier root system that is more resilient to the pests.