Cutworms are the destructive larvae of night-flying moths that threaten turfgrass by feeding at the soil surface. They emerge at night to chew through grass blades, creating patches of damage that expand quickly if untreated. Managing an infestation requires a targeted strategy, starting with accurate identification, followed by cultural, biological, and chemical control measures.
Identifying Cutworm Damage in Turfgrass
Cutworm feeding damage is often mistaken for turf disease or poor mowing practices. The most noticeable sign is the appearance of small, round, brown spots where grass blades have been clipped off near the crown, resembling ball marks on a golf green. These surface-feeding caterpillars often create small, shallow tunnels or holes in the soil where they hide during the day.
Increased bird activity, such as robins and starlings aggressively pecking at the turf, strongly indicates a significant infestation. To confirm their presence, perform a simple “soap flush test.” Mix one to two tablespoons of liquid dish soap into one to two gallons of water and pour it slowly over a one-square-yard section of damaged turf. The soap irritates the larvae, forcing them to the surface within five to ten minutes for visual confirmation.
Cultural and Physical Removal Methods
Effective long-term control starts by adjusting lawn maintenance to make the environment less favorable for cutworms. Since these pests thrive in moist conditions, adjust irrigation to promote drier soil surfaces. Water deeply but infrequently, allowing the top layer of soil to dry out between applications.
Maintaining a higher mowing height (three to four inches) helps turfgrass tolerate and recover from surface feeding damage. Periodically aerating or dethatching the lawn removes thick thatch layers, which cutworms use for shelter and egg-laying sites. Removing weeds is also helpful, as adult moths frequently lay eggs on tall grasses and weeds.
For small, localized infestations, hand-picking can be effective. Search the damaged area at dusk or dawn, or gently dig in the soil near clipped grass blades. Larvae are often found curled into a C-shape just beneath the surface.
Targeted Biological Control Agents
Biological control agents offer a non-chemical method by introducing natural enemies to manage the cutworm population. One effective option is the bacterium Bacillus thuringiensis subspecies kurstaki (Btk). Btk must be ingested by the cutworm, acting as a stomach poison that causes the larvae to stop feeding and die within a few days. Apply Btk late in the afternoon or early evening, as cutworms are nocturnal feeders, ensuring they consume the product on the grass blades.
Beneficial nematodes, specifically Steinernema carpocapsae (Sc), are another powerful biological tool. These microscopic, soil-dwelling roundworms seek out the larvae and enter their bodies, releasing symbiotic bacteria that kill the pest within forty-eight to seventy-two hours. For optimal performance, Sc nematodes must be applied to moist soil and protected from direct sunlight, making evening application recommended. Soil temperature should ideally be between 72 and 82 degrees Fahrenheit for maximum nematode activity.
Using Chemical Insecticides Safely
For severe or widespread cutworm outbreaks, synthetic chemical insecticides may be necessary to quickly reduce the population and prevent turf loss. Commonly used chemicals include pyrethroids, such as bifenthrin, cyfluthrin, and permethrin, which are effective against surface-feeding pests. Carbaryl is also an option, but requires careful consideration due to its toxicity to beneficial insects like bees and pollinators.
The timing of chemical application is crucial because cutworms hide in the soil and thatch during daylight hours. Apply a liquid formulation of the insecticide in the late afternoon or evening so the chemical is present when the larvae emerge to feed. A light watering is often necessary afterward to wash the chemical down into the thatch layer where the cutworms reside. Avoid excessive irrigation immediately after, as it can dilute the product. Always adhere strictly to the product label instructions, including wearing appropriate protective equipment and keeping children and pets off the treated area until the product has dried.