Grubs are the larvae of various scarab beetles, such as the Japanese beetle and European chafer, and are common garden pests living just beneath the soil surface. They feed voraciously on plant roots, particularly turfgrass, leading to widespread damage that often goes unnoticed. These larvae can turn a healthy lawn into brown, patchy areas or damage the root systems of garden vegetables. Addressing a grub problem requires understanding their life cycle and choosing the appropriate removal method and timing. This guide provides steps for identifying, preventing, and eradicating grub infestations.
Identifying Grubs and Assessing Damage
Confirmation of a grub problem begins with recognizing the visual signs of damage, which often appear as irregular brown patches in the lawn, especially during late summer or early fall. Unlike drought stress, which affects the entire area uniformly, grub damage appears in scattered spots, and the turf may feel spongy underfoot. A simple “tug test” can confirm the issue: if the grass lifts away easily like a loose piece of carpet, it indicates grub damage because the roots have been chewed away.
To directly inspect the population, gardeners can perform a “shovel test” by digging up a square foot of soil to a depth of two to three inches near a damaged area. Grubs are typically creamy white, C-shaped larvae with a distinct brown head capsule and six legs near the head. Finding fewer than five grubs per square foot usually does not warrant immediate treatment, but a count of ten or more per square foot indicates a significant infestation that requires action.
Non-Chemical Prevention Techniques
The first line of defense against grubs involves modifying the garden environment to make it less appealing for adult beetles to lay eggs. Adult beetles prefer to lay eggs in well-irrigated turf, so adjusting watering practices is effective. Watering deeply but infrequently, rather than daily, encourages deeper root growth and allows the upper soil layer to dry out, making it less hospitable for newly hatched grubs.
Maintaining a taller mowing height, typically between 2.5 to 3 inches, also helps deter adult beetles from laying eggs. Longer grass blades provide more shade and a less exposed surface, which adult beetles tend to avoid. Keeping the soil healthy through practices like aeration and dethatching reduces compaction and strengthens the grass. Strong root systems are better equipped to withstand minor grub damage without showing noticeable signs of browning.
Biological Eradication Solutions
Biological controls utilize living organisms to manage the grub population, offering an environmentally conscious alternative to synthetic chemicals. Beneficial nematodes, which are microscopic roundworms, are highly effective against grubs. Specific species like Heterorhabditis bacteriophora (Hb) actively seek out grubs in the soil and infect them, releasing a symbiotic bacterium that kills the host within a few days.
Successful application of nematodes depends on precise timing and conditions, as they are sensitive organisms. The soil must be moist, but not saturated, during application, and the temperature should be above 60°F, but ideally below 82°F. The most effective time to apply is typically in the late summer or early fall (late August through September), coinciding with the presence of small, newly hatched grubs feeding near the soil surface. Applications should be done in the evening or on a cloudy day, as sunlight kills nematodes. The treated area must be watered immediately afterward to wash the organisms into the soil.
Another biological method is the use of Paenibacillus popilliae, commonly known as milky spore, a naturally occurring bacterium. This bacterium is highly specific, primarily targeting the grubs of the Japanese beetle (Popillia japonica). Once ingested, the spore multiplies inside the host, causing a fatal disease that releases billions of new spores into the soil as the grub decomposes. Milky spore is considered a long-term solution; it can take up to three years to establish fully in the soil, but it can provide control for a decade or more once established.
Targeted Chemical Control
Synthetic insecticides are available, but their effectiveness is dependent on timing relative to the grub life cycle. These controls are generally divided into two categories: preventative and curative. Preventative products, which often contain active ingredients like chlorantraniliprole or imidacloprid, are applied early in the season, typically late May or June, to kill the grubs shortly after the eggs hatch. Chlorantraniliprole has a long residual activity and can be applied earlier in the spring, while imidacloprid should be applied closer to the egg-hatch period in June.
Curative treatments, which use active ingredients such as trichlorfon or carbaryl, are designed to kill larger, actively feeding grubs. These are applied later in the season, usually from late August to September, when the grubs are at their most destructive stage and feeding close to the surface. Both preventative and curative chemicals must be watered into the soil immediately after application, typically with about 0.25 inches of water, to ensure the active ingredient reaches the root zone where the grubs are feeding. When using any chemical control, it is important to exercise caution and follow label instructions precisely to minimize impact on beneficial insects, such as bees.