Trichoplusia ni, commonly known as the cabbage looper, is a moth in the family Noctuidae. Native to North America, it is a polyphagous pest (feeds on lots of different plants) that can cause significant damage in a variety of crops and potentially lead to plant death. While cabbage looper has a large geographical distribution, it does not overwinter in the Midwest. However, they migrate into the region from warmer southern areas from early spring through late fall. While the cabbage looper feeds on a wide range of plant species, it is most associated with cruciferous vegetables, including broccoli, cabbage, cauliflower, radish, turnip, and bok choy. Other host plants can include alfalfa, cotton, soybeans, tomato, and pepper, although less frequently.
Figure 1. Eggs of the cabbage looper. (Photo by John L. Obermeyer).
Figure 2. Early instar larvae of cabbage looper (Photo by John L. Obermeyer).
The cabbage looper has multiple generations per year, depending on weather conditions. Its life cycle consists of four stages (complete metamorphosis): egg, larva, pupa, and adult. A female can lay up to 350 eggs—the eggs, which are green, pale, and oval, hatch after three to six days (Figure 1). Then, the larvae, which are light green with a white stripe on each side, undergo five instars in 15 to 20 days, depending on the temperature (Figure 2). The distinct looping movement of cabbage looper larvae sets them apart from the majority of other caterpillars found in brassica crops. They move by bringing their back end forward to meet the front legs and arching a section of their body (Figure 3). This is the life stage that is most destructive to the crop, chewing large holes and they consume plant tissue and progress through their larval instars (Figure 4). The larger they get, the more they eat! The pupa is found in a thin cocoon, often on the plant tissue, and takes around 14 days to turn into an adult (Figure 5). The cabbage looper adult is a mottled, gray-brown moth, a nocturnal flyer that lays eggs during the night and spends most of the day resting on its host plants (Figure 6). Its body is about 1 inch (2.5 cm) long with a wingspan of 1.5 inches (3.8 cm).
Figure 3. The characteristic loop shape for which it is named. This is a result of the way their body attaches to the plant as they move along the leaf (Photo by John L. Obermeyer).
Figure 4. Damage caused by cabbage looper larvae (Photos by John L. Obermeyer).
Figure 5. Cabbage looper pupa (Photo by John L. Obermeyer).
Figure 6. Cabbage looper adult. (Photo by John L. Obermeyer).
The management of cabbage loopers can be achieved by monitoring pheromone traps to detect the emergence (or immigration) of adults and conducting regular scouting to look for eggs and small larvae on the underside of leaves on host plants. There are two beneficial insects that aid in managing this voracious pest: a parasitic wasp (e.g., Trichogramma pretiosum) and a tachinid fly (e.g., Voria ruralis). Including insectary plants, such as sweet alyssum, can help attract natural enemies that contribute to pest suppression. The biopesticides Bacillus thuringiensis and Spinosad offer good control but may need to be reapplied often. Other control measures can be found in the Midwest Vegetable Production Guide. These will also help manage the other Brassica caterpillars that are often found in conjunction with cabbage loopers. As with all chemical applications, it is important to follow the instructions provided on the product label – the label is the law.