Finding brown patches and damaged turf often leads homeowners to assume they have a “worm” infestation, but the actual culprits are rarely true earthworms. Earthworms are beneficial soil inhabitants whose tunneling and castings improve soil structure and nutrient availability. The organisms responsible for lawn damage are almost always the larval stages of insects, commonly referred to as grubs or caterpillars, which feed aggressively on grass roots or blades. Addressing the problem effectively begins with correctly identifying the specific pest causing the destruction.
Identifying the Culprit
The distinction between a helpful organism and a turf pest lies in their physical characteristics and feeding habits. Earthworms are long, cylindrical, and legless, moving through the soil to consume organic matter. In contrast, the most common damaging pests are distinct insect larvae, each causing a different pattern of lawn damage.
White grubs are the larvae of beetles like the Japanese beetle or June bug. They are pale white, C-shaped, and possess a dark head and six small legs near the front of their body. These pests feed directly on grass roots, severing the connection between the turf and the soil. A sign of a severe grub infestation is turf that feels spongy underfoot and can be rolled back like a carpet because the roots have been eaten away.
Cutworms and armyworms are the larval stages of various moths, appearing as caterpillars up to two inches long, often with stripes or patterns. Cutworms are solitary night feeders that chew grass off near the soil surface, sometimes leaving small, circular dead spots. Armyworms are known for moving and feeding in large groups, rapidly consuming grass blades and leaving behind areas that appear ragged or mown down to the crown. Observing the exact feeding pattern or finding the larvae is necessary to select the appropriate control method.
Non-Chemical Management Strategies
Shifting the turf environment to be less hospitable to pests avoids the use of chemical treatments. Adjusting the mowing height is an effective cultural control. Setting the mower blade to one of the highest settings encourages deeper root growth and helps the grass tolerate minor root-feeding damage. Taller grass also provides shade to the soil surface, which can deter some adult beetles from laying eggs.
Proper water management is a tool against many turf pests, particularly grubs and armyworms that prefer moist conditions. Watering deeply but infrequently promotes a strong, deep root system that can withstand root-feeding activity. Avoiding overwatering helps dry out the upper layer of the soil, making it less attractive for egg-laying and larval development.
Physical disruption of the soil can help manage shallow-dwelling pests. Dethatching removes the dense layer of organic matter above the soil, eliminating a protective habitat for cutworms and armyworms. Aeration, which involves removing small plugs of soil, helps disrupt the tunnels and feeding areas of shallow-burrowing larvae. For surface feeders, a drench test using soapy water poured onto a small area of the lawn will often force cutworms or armyworms to the surface for manual removal.
Targeted Biological and Insecticidal Treatments
Active eradication involves the use of targeted biological agents or carefully timed insecticides. Biological controls utilize naturally occurring enemies of the pests and are safer for non-target organisms like earthworms and pollinators. Predatory nematodes, specifically Heterorhabditis bacteriophora, are microscopic roundworms that hunt and parasitize white grubs and other soil-dwelling larvae. They are applied to the lawn with water and require the soil to be moist for optimal movement and effectiveness.
Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium effective against caterpillars like cutworms and armyworms. The variety kurstaki (Btk) must be ingested by the feeding larvae to be effective, making it potent when the pests are young and actively feeding. For Japanese beetle grubs, the bacterium Paenibacillus popilliae, known as milky spore, causes a fatal disease in the larvae after they ingest the spores. This treatment is specific to the Japanese beetle grub and can take two to three years of consistent application to build up in the soil. However, it can provide control for many years afterward.
Chemical treatments are categorized as either preventative or curative, and precise timing is necessary for success. Preventative insecticides, which often contain imidacloprid or chlorantraniliprole, are applied before the grubs hatch, typically from June to July. These products remain in the root zone to kill the newly hatched larvae while they are small and vulnerable. Curative insecticides, such as those containing trichlorfon or carbaryl, are fast-acting and are used when an active infestation of larger grubs is confirmed, usually in late summer or early fall. All chemical applications must be followed immediately by a half-inch of water to move the active ingredient into the root zone where the pests are feeding.
Long-Term Prevention Through Lawn Health
The defense against future worm and grub infestations is maintaining a dense, healthy turf ecosystem. A thick lawn naturally outcompetes pests and supports the root system, making it more tolerant of minor feeding activity. Overseeding thin areas, particularly in the fall, helps establish a strong, protective layer of grass that is less appealing to egg-laying adults.
Understanding the soil composition is foundational to long-term health; a soil test provides data on nutrient levels and pH. Applying fertilizer based on a soil test, rather than a generic schedule, ensures the grass receives a balanced diet, which strengthens its natural defenses. Avoiding excessive nitrogen application is important, as this can lead to rapid, succulent growth that is attractive and vulnerable to surface-feeding pests. Focusing on deep roots and dense growth helps the lawn become an environment where destructive larvae cannot thrive.