The cabbage looper (Trichoplusia ni [Noctuidae]) is a destructive pest of crucifers around the globe. In North America, loopers overwinter only in the southernmost states and reinvade most of the US and all of Canada each summer.
Cabbage loopers can be somewhat cyclical in occurrence: they may be abundant one summer and scarce the next, probably due to fluctuations in the numbers of predators and diseases that limit their numbers.
Although their larvae bear similarities and both species can inflict heavy damage on susceptible crops, cabbage loopers should not be confused with the white cabbage butterflies that are seen flitting over gardens every spring and summer.
Cabbage looper adults are drab-colored, mottled moths that are most active during evening and nighttime hours, when they feed on nectar and lay their eggs.
The Cabbage Looper Life Cycle
- Upon arriving in northern gardens each summer—and practically year-round at southern latitudes—adult moths lay their hemispherical, yellowish to greenish eggs on both the upper and lower surfaces of target plants.
- Eggs hatch in two to five days at typical garden temperatures. The larvae are yellowish-white at first, but become pale green once feeding begins. While initially somewhat hairy, the caterpillars become smoother with maturation, and older larvae possess a distinct whitish stripe running longitudinally along each side.
- Seventeen to 20 days after hatching, larvae form cocoons on the undersides of leaves, in plant debris, or in crevices in soil. The cocoons consist of a silky covering that encloses the pupa, which turns from green to dark brown or black within a few days.
- The pupal stage lasts from four to 13 days, depending on ambient temperatures, after which the adults emerge, mate, and begin to lay eggs. Adult moths are distinguished from other noctuid species by prominent silvery-white spots on the forewings.
- Adults live 10 to 15 days, during which females can lay around 500 eggs. The number of generations that can occur during a growing season varies from two or three in northern areas to seven in southern locations. (Shorey H. The biology of Trichoplusia ni [Lepidoptera: Noctuidae]. II. Factors affecting adult fecundity and longevity. Annals of the Entomological Society of America 1963;56: 476-480 and Toba H, et al. Temperature and the development of the cabbage looper. Annals of the Entomological Society of America 1973;66: 965-974)
Crops that are Damaged by Cabbage Loopers
Not all plants that loopers feed upon are ideal for completion of the insect’s life cycle. While the larvae can be found feeding on any number of crops, they survive and develop most efficiently on crucifers, cotton, and a few other plants. (Soo Hoo C, et al. Trichoplusia ni [Lepidoptera: Noctuidae] larval development on wild and cultivated plants. Environmental Entomology 1984;13: 843-846)
Nevertheless, looper damage can occur on all of the following:
- Beans
- Beets
- Broccoli
- Cabbage and Chinese cabbage
- Cantaloupe
- Cauliflower
- Celery
- Collards, kale, mustard, and other greens
- Cotton
- Cucumbers
- Flowers (chrysanthemum, hollyhock, snapdragon, sweetpea)
- Lettuce
- Melons
- Parsnips
- Peas
- Peppers
- Potatoes and sweet potatoes
- Radishes
- Rutabagas
- Spinach
- Squash
- Tobacco
- Tomatoes
- Turnips
Damage Caused by Cabbage Loopers
Crops infested by loopers exhibit a characteristic “moth-eaten” appearance. The larvae are leaf feeders, and they usually chew holes through the foliage without feeding along the leaf margins. Cabbage is sometimes ruined by the caterpillars’ tendency to bore into the developing heads.
Loopers can consume three to four times their weight in plant material daily; because the larvae tend to feed from the bottom sides of leaves—thus avoiding predators—their activity is sometimes first detected by the accumulation of sticky fecal material falling onto the leaves below.
Controlling Cabbage Loopers
- When infestations are relatively light, looper larvae can be hand picked from crops and dropped into a soap solution or destroyed in some other manner.
- Heavy infestations can be avoided by the placement of row covers (these must reach the ground on all sides, else the adult moths will find their way onto the crops).
- Neem, applied via mist or light spray to susceptible crops, acts as a feeding deterrent and larval growth inhibitor. Resistance to other insecticides, including rotenone, has become problematic in many areas.
- Bacillus thuringiensis (Bt) has proven effective in controlling looper infestations, and has the added benefit of specifically targeting caterpillars without affecting beneficial insect populations.
- Mass releases of Trichogramma wasps, whose own larvae parasitize the looper larvae, will help to reduce cabbage looper numbers to manageable levels.
- Some gardeners have learned to recognize loopers infected by a naturally-occurring virus called nuclear polyhedrosis virus. They blend the diseased caterpillars into a mixture that can be reapplied to crops, thereby infecting other loopers (the virus is not dangerous to humans).
Cabbage loopers have voracious appetites and can severely damage plants in a short time. Vigilance and expeditious management will prevent extensive loss of garden crops.
 |
 |
 |
 |
