Green looper caterpillars are light to dark green larvae that move with a distinctive “looping” motion, arching their bodies as they crawl. They belong to the moth family Noctuidae and are common pests of vegetables, soybeans, and other crops. The name “green looper” is used broadly and can refer to several closely related species, including the cabbage looper and the soybean looper (Chrysodeixis includens), which share similar appearances, feeding habits, and control methods.
How to Identify a Green Looper
Green loopers are easy to spot once you know what to look for. The body is predominantly green with white stripes running along the back and sides. It tapers toward the head, with the rear end noticeably thicker. Fully grown larvae reach about 1.3 inches (3 cm) in length.
The key feature that separates loopers from other green caterpillars is the number of fleshy leg-like structures (prolegs) on the abdomen. Green loopers have only two pairs of abdominal prolegs, which is why they “loop” when they move, pulling their rear end up to meet the front like an inchworm. Many similar-looking caterpillars, like the velvetbean caterpillar, have three or more pairs and crawl in a smooth, straight line instead. If you’re trying to tell a soybean looper from a cabbage looper specifically, check the small legs near the head: the soybean looper can have black or green thoracic legs, while the cabbage looper’s are always green.
Life Cycle and Development
After hatching, a green looper caterpillar feeds for two to four weeks before pupating. During that time it passes through about six growth stages (instars), shedding its skin between each one. The first stage lasts three to four days, most middle stages take two to three days, and the final stage stretches to five or six days as the caterpillar does the bulk of its eating and growing. After the caterpillar spins a loose cocoon and pupates, the adult moth emerges roughly two weeks later.
The adult moths are mottled brown or gray with a distinctive silvery figure-eight marking on each forewing. They’re nocturnal, so you’ll rarely see them. Females lay pale, dome-shaped eggs on the undersides of leaves, often singly or in small clusters. In warm climates, multiple generations can cycle through in a single growing season, which is why infestations can escalate quickly.
What They Eat and the Damage They Cause
Green loopers are generalist feeders. Cabbage loopers target brassicas like cabbage, broccoli, kale, cauliflower, and Brussels sprouts. Soybean loopers prefer soybeans but also feed on cotton, tomatoes, peppers, lettuce, and other broadleaf crops. In home gardens, you’re most likely dealing with cabbage loopers on your cole crops.
The caterpillars chew ragged, irregular holes in leaves, starting on the undersides where they’re harder to spot. Young larvae create small “windowpane” holes by eating the leaf tissue from one side, leaving a thin translucent layer. Older, larger caterpillars eat straight through the leaf and can consume large amounts of foliage in just a few days. On heading crops like cabbage, they’ll bore into the head itself, contaminating it with frass (droppings). A heavy infestation can strip a plant down to its veins and midribs.
When to Take Action
Not every caterpillar you find warrants treatment. A few small larvae on large, healthy plants won’t cause meaningful damage. University of Minnesota Extension recommends treating when about 10% of your plants have one or more medium-to-large looper larvae or eggs. For seedbed-stage plants, that same 10% threshold applies, since young plants are far more vulnerable to defoliation.
Timing matters more than the product you choose. Small caterpillars are dramatically easier to kill than large ones. Once they reach their final instar, they’re tougher, eat the fastest, and respond poorly to most treatments. Inspect the undersides of leaves weekly during the growing season, and act as soon as you notice early-stage larvae.
Biological and Natural Controls
Green loopers have several natural enemies that can keep populations in check without any intervention from you. Tiny parasitoid wasps in the Trichogramma genus attack looper eggs before they even hatch. These wasps are commercially available for release in gardens, and they naturally occur in landscapes that include flowering plants. Planting herbs, wildflowers, or other nectar sources near your vegetables attracts these beneficial wasps and gives them a reason to stick around.
Bacillus thuringiensis (Bt), a naturally occurring soil bacterium, is one of the most effective and widely recommended treatments. It’s sold as a spray (often labeled as Btk) and works by producing a protein that destroys the caterpillar’s gut lining after it feeds on treated leaves. Bt is highly specific to caterpillars and leaves parasitoid wasps, bees, and other beneficial insects unharmed. It works best on small, actively feeding larvae. Apply it to the undersides of leaves where the caterpillars actually feed, and reapply after rain.
Other Control Options
For small gardens with light infestations, handpicking is surprisingly effective. Check plants in the early morning or evening, pluck caterpillars off, and drop them into a bucket of soapy water. It’s tedious but works well if you stay consistent.
If you need something beyond handpicking or Bt, several low-risk pesticide options exist. Pyrethrins, derived from chrysanthemum flowers, kill on contact but must be sprayed directly onto the caterpillars to work. Neem, another plant-based product, doesn’t kill caterpillars outright. Instead, it causes them to stop feeding, and they die over the following days. Both options have less impact on beneficial insects and pollinators than broad-spectrum chemical insecticides, though pyrethrins can still harm bees if applied while they’re actively foraging. Spraying in the evening reduces this risk.
Limiting the use of broad-spectrum insecticides is important for long-term management. These products kill the parasitoid wasps and predatory insects that naturally suppress looper populations. Wiping out those natural enemies often leads to worse infestations later in the season.
Prevention Strategies
Physical barriers are the most reliable way to prevent green looper damage entirely. Lightweight insect exclusion netting or row covers placed over crops block adult moths from laying eggs on your plants. Install them at planting time and secure the edges to the ground, since moths will find any gap.
Plowing or tilling the soil after harvest destroys overwintering pupae and reduces the population that emerges the following season. Removing crop debris and nearby weeds also eliminates alternate host plants where loopers can survive between growing seasons.
For larger-scale growers, pheromone traps help monitor adult moth activity so you know when egg-laying is happening. These traps use synthetic versions of the chemical signals female moths produce to attract males. Place traps along field edges, at least 40 to 60 feet apart from traps targeting different species, and check them two to three times per week. Lures should be swapped out every two weeks to stay effective, and stored frozen or refrigerated until use. A spike in trap catches tells you that egg-laying is underway and that you should start scouting for tiny larvae within the following week.