Proven organic strategies for controlling cabbage looper.
Widely distributed throughout North America, the cabbage looper (Trichoplusia ni) is a common and destructive pest most often found on cabbage-family, or cole crops. The larger its larvae grow the more damage they do. In the larval stage, cabbage loopers eat three-times their body weight in plant material a day, doing the most harm during the last few days of their development.
Loopers, commonly known as inch worms, are most easily recognized by their unique method of movement in which they double up or “loop” as they inch along. This characteristic trait comes from the absence of legs at the looping segments. The six-legged looper could use the eight that other leaf worms have.
Larvae are large (1-1/2 inch long), pale green caterpillars with a narrow white stripe along each side and several narrow lines down the back. Adults are night-flying, gray moths (1-1/2 inch wingspan) with a silvery, V-shaped spot in the middle of each dusky forewing.
Discerning cabbage loopers from the larvae of garden webworms, diamond back moths, and other inch worm type larvae is not critical to their mangement. The same integrated pest management practices work on them all.
Pupae spend winter attached to host plants or in nearby garden debris. Moths emerge in the spring and become widely dispersed including north into areas too cold for winter survival. The moths deposit pale green, domed eggs on the plants. The eggs hatch in 3 or 4 days. The destructive larval stage reaches full development in 2-4 weeks. They pupate in thin silk cocoons attached to the stems or undersides of leaves, and adults emerge within 10 days. Loopers as far north as Canada will go through two to three generations in a year and several generations per year are possible in warmer areas.
Cabbage loopers occur as far south as Mexico and are established in commercial fields with insufficient rotation there and in the U.S. They mainly attack cabbage crops including broccoli, cauliflower and kale. Young larvae eat only from the bottom of leaves. Older larvae chew large, irregular holes in the leaves of many plants. Loopers bore into the center of cabbage heads leaving behind masses of wet, slimy fecal matter. Once established, loopers are difficult to get rid of. In addition to cruciferous plants, cabbage loopers will also attack lettuce, spinach, celery, cucumbers, and tomatoes. Each year, large amounts of pesticides are targeted at loopers on commercial fields, causing serious degradation of water and soil.
Cabbage Looper Control
Cabbage looper damage becomes ruinous at some point. Many gardeners tolerate a single tunnel into a cabbage or irregular scars on their Brussel sprouts. This kind of damage may be unavoidable in the organic garden. But it’s important to take action at that point before eggs hatch and a second generation prepares to make slaw of what’s left. And an ounce of prevention? Well, you know what it’s worth.
- Keep a close, frequent eye on your cabbage plants, both outside and under leaves. Pick off the hungry, inching larvae and brush off eggs before they start an outbreak. The larvae can be drowned in a jar of soapy water for easy disposal.
- Cabbage looper larvae are easy, visible targets for predators. Take advantage of its many natural enemies by encouraging birds and beneficial insects to your garden. Certain herbs, including parsley, dill, fennel, coriander and sweet alyssum, attract the kinds of insects and other creatures that prey on worms.
- Use pheromone traps to signal the arrival of moths to your garden.
- Cover plants with floating row covers to keep migrating moths from landing and laying eggs. This can make a significant difference if timed correctly.
- Wasps are the looper larvae’s biggest enemy. Release trichogramma wasps to destroy eggs.
- The natural, soil dwelling bacterium Bacillus thuringiensis or Bt-kurstaki is particularly effective on inch worms of all types. Use easily applied spray to hit worms and protect the leaves at the first signs of damage. BTK sprays do not harm honey bees or birds and are safe for use around pets and children.
- Spinosad, another biological agent derived from fermentation, is also very effective. It’s the active ingredient in Monterey Garden Insect Spray, a product classified as organic by the U.S.D.A. National Organic Program and listed for organic use by the Organic Materials Review Institute.
- Other botanical insecticides, like Safer’s Tomato & Vegetable Insect Killer or Pyrethrin Spray, can be used as a last resort.
- After harvest, bury spent cole crops to destroy cocoons before adults emerge in spring.
Rotating cabbage crops in a family sized garden is a good idea but not necessarily effective in preventing loopers on your plants. The night flying moths disperse widely and will find your vegetables no matter how far you’ve moved them. That’s why floating row covers to keep moths from landing on leaves and depositing eggs, are a good idea.
Photo Credit: Phil Sloderbeck, Kansas State University