Caterpillars frequently target lettuce and other leafy greens. The tender, water-rich leaves of the Lactuca sativa plant make it a favorite target for several species of moth and butterfly larvae. For the home gardener, understanding which specific pest is attacking and how to manage it organically is the clearest path to a successful crop. This guide provides the necessary information to identify these common garden invaders and implement effective, non-chemical control strategies.
Identifying the Most Common Lettuce Pests
The majority of caterpillar damage on lettuce is caused by three primary groups: Cabbage Loopers, Imported Cabbage Worms, and Cutworms.
The Cabbage Looper (Trichoplusia ni) is a smooth, pale green caterpillar with white stripes down its back and sides. It gets its common name because it arches its body into a loop as it crawls, lacking the middle pairs of prolegs. The adult is a mottled gray-brown moth that lays tiny, dome-shaped eggs singly on the undersides of leaves.
The Imported Cabbage Worm (Pieris rapae) is a velvety green caterpillar with a faint yellow stripe down its back, reaching about one and a quarter inches at maturity. Unlike the looper, this larva is sluggish and has a full set of prolegs. It is the larval stage of the common white butterfly, which lays distinctive bullet-shaped eggs on the foliage.
Cutworms, the larvae of various night-flying moths, are robust, greasy-looking caterpillars that range from gray to brown or black. They are primarily nocturnal feeders and curl tightly into a “C” shape when disturbed. These pests damage young plants by feeding at or just below the soil line.
Recognizing the Visual Signs of Infestation
Caterpillars are often camouflaged, but the damage they cause provides clear evidence of their presence. The most common sign is irregular, ragged holes chewed into the lettuce leaves. Cabbage Loopers frequently leave behind only the midribs and veins, a form of damage known as skeletonization, especially on younger leaves.
Another sign is the dark, moist droppings, or frass, left near the feeding sites. These greenish-brown pellets accumulate on the leaves or at the base of the plant. For the cutworm, the damage is distinct: a young seedling will be completely severed at the soil line and often left uneaten nearby.
To confirm an infestation, inspect the undersides of the outer, older leaves, where eggs are laid and newly hatched larvae begin feeding. Checking plants in the morning is the best time to find cutworms, as they hide in the soil during the day, usually within a few inches of a damaged plant.
Effective Physical and Organic Management Strategies
Hand-Picking and Physical Barriers
Hand-picking is the most immediate control method for a small garden, effective against larger loopers and cabbage worms. Check plants daily, especially on the underside of leaves, and drop collected caterpillars into soapy water. For cutworms, search the soil around a recently damaged plant to find the pest.
Physical barriers prevent adult moths and butterflies from laying eggs on the lettuce. Lightweight floating row covers, made of spun-bonded material, can be draped over the bed and secured at the edges. This barrier allows water and light through while excluding flying pests, but must be applied immediately after planting or seeding.
Biological Control (Btk)
For widespread infestations, the biological control Bacillus thuringiensis subspecies kurstaki (Btk) is the most effective organic treatment. Btk is a naturally occurring soil bacterium that paralyzes the caterpillar’s digestive system when ingested. The pest stops feeding within hours and dies within a few days. Since Btk breaks down in sunlight, apply it thoroughly to the entire plant, including leaf undersides, in the late afternoon or evening. Reapply after rain or every five to seven days if pests persist.
Protecting Transplants and Encouraging Predators
Protecting young transplants from cutworms requires placing a physical collar around the stem of each plant. A simple barrier made from a cardboard tube or plastic cup should be pushed an inch into the soil and extend two to three inches above the surface. This prevents the cutworm from wrapping around the stem to cut it. Encouraging natural predators like parasitic wasps and ground beetles, by planting a diversity of flowering plants nearby, can help keep caterpillar populations in check.