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Widely distributed throughout North America, the cabbage looper (Trichoplusia ni) adult is a gray moth with a silvery V-shaped spot in the middle of each forewing with a wingspan of 1 ½ to 2 inches. The moths fly at night so most people rarely notice them.
Larvae are easily identified by their unique movement in which they double up or “loop” as they inch along. Also called “inchworms”, cabbage looper larvae are large (about 1 ½ inch), pale green caterpillars with a pair of narrow white lines down their backs and one line along each side. Eggs are light green, dome shaped and generally found on the underside of leaves..
Moths emerge from overwintering pupae in mid-spring and lay their eggs on the lower surface of leaves. Eggs hatch in 3-6 days and the larvae feed for 2-4 weeks, then pupate for about 10 days in thin silky cocoons attached to stems or leaves and the new adults emerge. There are three to four generations per year in most areas.
Cabbage looper are common pests of the brassica family of plants, including cabbage, kale, collards, cauliflower, broccoli, turnips, Brussels sprout, kohlrabi, mustard, watercress and others, yet host plants are not restricted to only cole crops; other plant hosts include tomato, cucumber, potato, beet, bean, celery, lettuce, pea, parsley, spinach; flowers include carnation, and nasturtium.
Damage and Signs of Infestation
Larvae have voracious appetites making an “infestation” mean as few as 2 or 3 worms per plant. Cabbage loopers chew large, irregular holes in the leaves of host plants. As they feed they may bore into the center of heads and contaminate them with their fairly noticeable dark green fecal pellets.
A serious infestation can deprive a plant of photosynthesis owing to the absence of leaves, resulting in the death of the plant. A minor infestation will look unsightly but crops may still be edible.
Organic Control and Prevention of Cabbage Looper
1. Use row covers: Growing crops under floating row covers is an excellent and essential method of preventing cabbage looper adults and other pests from laying their eggs on plants. These row covers create a barrier that allows air, light and moisture through but keep insects out.
2. Scout and handpick: Keep a close lookout for caterpillars, eggs, or signs of damage (chewed leaves, dark green droppings). You can easily keep small populations of cabbage loopers under control by picking them off your plants and dropping them in a bucket of soapy water.
3. Cultural practices: Cleaning up after a harvest, tilling the soil and removing dead plants and garden litter can reduce number of overwintering pests in the garden. Another way to avoid serious infestations is to stagger (earlier or later) your planting dates in a manner to avoid crop susceptibility depending on your growing season.
4. Grow healthy organic plants: Strong plants can handle some cabbage looper damage better than weak, struggling plants which is of course a no brainer. Ensuring that your crops are getting enough sunlight and water and that the soil is well-drained and rich in nutrients and organic matter will help minimize damage caused by pests.
5. Biological insecticides: Bt (Bacillus thuringiensis) has been used for suppressing cabbage loopers for a long time now, and has the advantage of not disrupting beneficial insect population. Bt is a naturally occurring bacterium that is fatal to only caterpillars. Spinosad, a biological pesticide is another bacterium that works as a neurotoxin when ingested by many insects including cabbage looper caterpillars.
6. Attract beneficial insects and natural predators: Planting flowers, such as marigolds, calendula, sunflower, daisy, alyssum, or dill can attract beneficial insects such as native parasitic wasps and trichogramma wasps that parasitize cabbage looper eggs. Birds, spiders and predatory beetles will also help in controlling these pests. If you have chickens or ducks, allow them to forage for cabbage loopers once your plants have been established.
7. Red-leafed varieties of cabbage and kohlrabi are less preferred by cabbage loopers, probably because they provide insufficient camouflage.
8. Organic sprays and soaps: Homemade hot pepper spray and garlic sprays have been known to work to some effect. Organic insecticidal soaps will help kill caterpillars but must be applied extensively to achieve good control.