Damaged turf often leads homeowners to suspect a “worm” infestation, but this general term includes both destructive pests and beneficial organisms. Effective pest management relies entirely on correctly identifying the creature causing the problem. Earthworms are soil allies, while beetle larvae, commonly known as grubs, are the true culprits of root-feeding damage. Knowing the difference is the first step before applying any control measures.
Identifying the Culprit
The most common beneficial “worm” is the earthworm, which has a smooth, cylindrical, reddish-brown body with clear segmentation and no legs. Earthworms improve soil health through aeration and nutrient cycling. The only visible sign of their presence is small, coiled soil deposits called castings on the surface. These organisms should not be targeted for control.
The most common destructive pest is the white grub, the larva of various beetles like the Japanese beetle or masked chafer. Grubs are easily identified by their creamy white, soft, C-shaped bodies, dark heads, and three pairs of small legs. They feed directly on grass roots, causing the lawn to feel spongy and allowing large patches of turf to be rolled back like carpet.
Surface feeders, such as armyworms and sod webworms, are types of caterpillars. These pests feed on grass blades, leaving behind chewed or ragged edges, thinning grass, and brown patches that often resemble drought stress. Armyworms are dark greenish-brown with longitudinal stripes and an inverted “Y” shape on their heads.
Modifying the Yard Environment
Adjusting cultural practices makes the lawn less hospitable to pests and more resilient to damage. Mowing height plays a role in turf strength; keeping the grass slightly longer (typically 2.5 to 3.5 inches) encourages deeper root growth. A deeper root system helps the turf better withstand minor damage from root-feeding grubs.
Watering habits directly influence pest populations, particularly grubs and sod webworms, which thrive in moist soil. Reducing irrigation frequency, while still providing a deep soaking of one to one-and-a-half inches when the turf shows stress, promotes deeper root growth and stresses the pest larvae. This practice also discourages egg-laying by adult beetles.
Cultivation practices like aeration and dethatching help reduce pest habitat. Thatch layers exceeding one-half inch can harbor insect eggs and larvae; mechanical dethatching removes this protective layer. Core aeration alleviates soil compaction, improving drainage and allowing the grass to develop a robust root system better equipped to recover from pest feeding.
Targeted Biological and Chemical Solutions
Targeted solutions for existing infestations are divided into biological and chemical methods. Biological control utilizes naturally occurring organisms to manage pests, offering a selective approach.
Beneficial nematodes, specifically Heterorhabditis bacteriophora, are microscopic roundworms that seek out and infect grubs in the soil. They must be applied when grubs are actively feeding, typically in late summer to early fall when larvae are small and close to the surface.
Nematode Application Requirements
- Soil temperature must be between 50°F and 82°F.
- Apply in the evening or on a cloudy day, as nematodes are sensitive to ultraviolet light.
- The treated area must be kept moist for at least two weeks afterward.
Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium effective against surface-feeding caterpillars, such as armyworms and webworms. The kurstaki strain (Btk) works as a stomach poison; the caterpillar must ingest the treated grass blades for the toxin to paralyze its digestive system. Bt is most effective when applied to young larvae and is harmless to beneficial insects, pets, and humans.
Chemical control involves using synthetic insecticides, categorized by their timing of application.
Preventative Treatments
These treatments, containing active ingredients like chlorantraniliprole or imidacloprid, are applied early in the season (typically late May through early July) before the beetle eggs hatch. Chlorantraniliprole can be applied earlier in the spring for season-long control. Imidacloprid must be timed closer to the egg-laying period.
Curative Treatments
Curative treatments are necessary when significant grub damage is already visible, usually in late summer or early fall, and act quickly to kill larger, actively feeding larvae. Active ingredients such as trichlorfon or carbaryl are used for this immediate knockdown effect. All chemical applications, especially curatives, require immediate irrigation with at least one-half inch of water to wash the product into the root zone where the grubs are feeding.
Following all label instructions precisely is necessary when applying chemical treatments, including wearing protective gear and avoiding application near water sources. Improper timing or dosage can render the treatment ineffective, harm non-target organisms, and pose risks to the environment. Choosing the correct chemical type depends on the stage of the grub life cycle and the severity of the infestation.