Cutworms are the larval stage of several species of night-flying moths, posing a significant threat to young garden plants. These pests are notorious for severing the stems of tender seedlings right at the soil line, often causing sudden and complete loss overnight. This guide provides actionable methods to protect your garden from cutworm damage using physical, cultural, and biological controls.
Identifying Cutworms and Their Damage
Cutworms typically appear as plump, soft-bodied larvae, ranging in color from gray or brown to black. When disturbed, these caterpillars instinctively curl into a tight “C” shape, which is a helpful identifier. They can grow up to two inches long and are most often found just beneath the soil surface near damaged plants.
The most common culprits are surface cutworms, which cause the characteristic damage visible above ground. This damage is typically a clean cut through the stem of a newly transplanted seedling or sprout. Damage often happens during the night or very early morning hours, particularly in the cool, moist conditions of early spring.
Subterranean and climbing cutworms exist, but surface feeders cause the most widespread destruction to newly established garden beds. Gardeners should look for stems that have been cleanly severed and dropped, as the cutworm usually only feeds on the base of the plant. Finding this specific type of damage signals the immediate need for preventative action.
Physical Barriers for Seedling Protection
The simplest and most effective defense against surface cutworms involves creating a physical barrier around the plant stem. Protective collars prevent the larva from wrapping its body around the stem to feed. These barriers can be fashioned from everyday materials like cardboard, plastic cups with the bottom removed, or aluminum foil.
Proper placement of the collar is paramount for effectiveness. The barrier must be pushed into the soil approximately one inch deep to prevent the cutworm from burrowing underneath. The collar should also extend at least two to three inches above the soil line, creating a sufficient vertical obstacle. Once the plant stem has hardened, the collar can typically be removed after a few weeks.
Other physical deterrents can be applied directly to the soil surface surrounding the plant base. A ring of diatomaceous earth (DE) can be spread in a thin circle around the stem. While DE is an organic insecticide, its primary function here is to act as a physical irritant and barrier, preventing the cutworm from crossing the boundary.
Fine mesh screens or netting placed over entire rows of seedlings can also serve as a barrier, preventing the adult moth from laying eggs nearby. This method is particularly useful in the early season when the first generation of moths is active. These physical exclusions break the pest’s life cycle before it can begin.
Soil Preparation and Cultural Practices
Managing the soil environment is a proactive long-term strategy for prevention. Tilling the garden soil in late fall or early spring exposes overwintering pupae and larvae to the elements and natural predators like birds. This mechanical disruption significantly reduces the population before the planting season. Maintaining a clean garden free of weeds, grass clippings, and plant debris is also helpful, as these materials provide shelter for the larvae and egg-laying sites.
Cutworms prefer moist soil surfaces, so adjusting watering practices can make the area less hospitable. Watering deeply but less frequently, allowing the soil surface to dry out between applications, discourages the larvae from moving across the ground. Gardeners can also delay the planting of vulnerable seedlings until two weeks after the first observed cutworm damage.
Introducing sacrificial “trap crops” can help divert the pests’ attention away from valuable plants. Highly attractive crops, such as lettuce or mustard greens, can be planted around the perimeter of the main garden to lure cutworms away. This concentration allows for easier daily inspection and removal of the pests by hand.
Handpicking is most effective when conducted at night using a flashlight, which is when the cutworms are actively feeding. Searching the soil one to two inches deep around any freshly damaged plant will often reveal the larva curled up. Consistent nightly inspection for the first few weeks after planting provides excellent control and is a highly targeted method of population reduction.
Targeted Organic Control Methods
When prevention methods are not enough, targeted organic treatments can manage persistent infestations. The biological insecticide Bacillus thuringiensis subspecies kurstaki (Bt-k) is highly effective against cutworms and other caterpillars. This naturally occurring bacterium must be ingested by the cutworm, causing the larva to stop feeding and eventually die.
Application of Bt-k should be timed to coincide with the cutworms’ nocturnal feeding schedule. Spraying the product directly onto the stems and leaves of vulnerable plants just before dusk ensures the highest rate of ingestion. Since Bt-k degrades quickly in sunlight, repeat applications every few days may be necessary for adequate control.
For severe or widespread infestations, beneficial nematodes offer an alternative biological control method. Specific species, such as Heterorhabditis bacteriophora, are microscopic roundworms that naturally occur in the soil. These nematodes actively seek out and parasitize cutworm larvae, introducing a lethal bacterium. Applying nematodes to moist soil in the evening when temperatures are between 60 and 80 degrees Fahrenheit provides the best results.