The cabbage looper (Trichoplusia ni) is a common insect that poses a significant threat to agricultural crops. While a moth, its larval stage (the caterpillar) causes considerable damage. This pest impacts a broad spectrum of plants across North America, particularly cole crops such as cabbage, broccoli, cauliflower, and kale. Unmanaged, it can lead to substantial crop loss.
Identification and Life Cycle
The cabbage looper has four distinct life stages: egg, larva, pupa, and adult moth. Adult female moths lay small, dome-shaped eggs, typically pale green or white, individually on the undersides of host plant leaves. These eggs hatch within three to seven days.
Upon hatching, the larval stage begins. Cabbage looper larvae are light green caterpillars with a faint white stripe along each side. They possess three pairs of true legs near their head and three pairs of fleshy prolegs at the rear, lacking the prolegs in the middle of their body, which causes their distinctive “looping” movement as they crawl. This looping motion, where they arch their backs, is a key identification characteristic.
After two to four weeks, the larva enters the pupal stage. It spins a loose, white cocoon, where it transforms over one to two weeks. The adult moth emerges from this cocoon, characterized by mottled brown forewings with a small, silvery-white spot near the center of each wing. These nocturnal moths have a wingspan of about 1 to 1.5 inches and are strong fliers.
Signs of Infestation and Damage
Cabbage looper infestation is indicated by the damage caused by feeding larvae. These caterpillars chew large, irregular holes in plant leaves. They often begin feeding from the underside of the leaves, creating ragged perforations through the leaf blade.
Extensive feeding can lead to defoliation of younger plants. On cole crops like cabbage and cauliflower, the caterpillars may bore directly into developing heads, rendering the produce unmarketable. Another clear sign of their presence is the accumulation of greenish-brown, pellet-like droppings, known as frass, found on leaves beneath their feeding sites.
Management and Control Strategies
Effective management of cabbage loopers involves several control strategies. Cultural and physical methods are initial defenses. Using floating row covers early in the growing season can physically prevent adult moths from laying eggs on susceptible plants.
Hand-picking larvae from plants is another effective physical control for smaller garden infestations. This method is most successful when performed regularly, especially in the early morning or late evening when caterpillars are more active. Maintaining healthy soil and plant vigor also helps plants withstand minor pest damage.
Biological controls utilize natural enemies of the cabbage looper. Parasitic wasps, such as Copidosoma truncatellum, lay their eggs inside looper larvae, eventually killing the host. Another effective biological agent is Bacillus thuringiensis (Bt), a naturally occurring bacterium. When ingested by looper larvae, Bt produces toxins that disrupt their digestive system, causing them to stop feeding and die within a few days.
Bt is available as a spray and is specific to caterpillars, posing no harm to beneficial insects, pets, or humans. For severe infestations, chemical controls may be considered as a last resort. When using insecticides, it is important to select products labeled specifically for cabbage looper control and to strictly follow all label instructions regarding application rates and safety precautions.