Cabbage loopers can strip brassica plants down to their veins in a matter of days, so effective control starts with catching them early and using the right combination of physical barriers, biological treatments, and well-timed sprays. These bright green caterpillars feed on cabbage, broccoli, cauliflower, kale, and other cole crops, chewing large irregular holes in leaves and boring into developing heads. A single female moth lays 300 to 600 eggs over her 10- to 12-day lifespan, so populations escalate quickly in warm weather.
How to Identify Cabbage Loopers
The caterpillars are named for their distinctive “looping” movement, arching their bodies like an inchworm as they crawl. Fully grown larvae reach about 1.5 inches long, with a green body that tapers near the head, a thin white line running along each side, and two white lines down the back. You’ll find them most often on the undersides and inner surfaces of leaves near the midrib.
Eggs are small, white, and laid singly on the lower surfaces of leaves starting in July. The adult moths are grayish-brown and fly at night, so you’re unlikely to spot them. Instead, look for the damage: large ragged holes between leaf veins, greenish-brown droppings (frass) scattered across the foliage, and stunted or aborted heads on young cabbage and cauliflower plants. The frass alone can make heads unmarketable even when structural damage is minor.
Understanding the Life Cycle
Temperature drives how fast cabbage loopers develop, and this matters for timing your control efforts. At around 70°F, the full cycle from egg to adult takes about 25 days. At 90°F, that shrinks to roughly 18 days, meaning you can get multiple overlapping generations in a single growing season during hot summers.
Eggs hatch in two days at 90°F but take nearly 10 days at 59°F. Larvae pass through five growth stages (instars) over about 18 to 21 days, depending on temperature and host plant. The pupal stage lasts 4 days at 90°F and up to 13 days at 68°F. Because eggs hatch on a rolling basis throughout the season, you’ll often find caterpillars of different sizes on the same plant at the same time, which complicates treatment timing.
When to Take Action
Not every looper on a plant warrants spraying. Established thresholds from university extension programs help you avoid unnecessary treatments while protecting your crop at critical stages:
- Before heading: Treat when 35% or more of plants are infested. Plants can tolerate significant leaf loss at this stage without affecting yields.
- During heading to harvest: Treat at 20% infestation for cabbage and broccoli. Damage to forming heads directly reduces quality.
- Leafy greens (kale, collards): Treat at just 10% infestation, since the leaves themselves are the harvested product.
Scout your plants at least twice a week during peak season. Flip leaves over and check near the midrib where loopers prefer to feed. Count the percentage of plants showing active caterpillars or fresh feeding damage.
Row Covers as a First Line of Defense
Floating row covers are the most reliable preventive measure because they physically block moths from reaching your plants to lay eggs. The key is installing them at planting, before moths arrive. If you wait until you see damage, eggs are already on the plants.
For low-growing crops like young cabbage, you can lay the fabric directly over the row with enough slack for the plants to push it up as they grow. Taller crops do better with PVC hoops bent into the soil to create a tunnel. The critical step is burying the edges of the cover completely in soil. Any gap is an entry point for moths.
You’ll need to remove covers periodically to weed underneath, and you should remove them gradually before harvest to let plants acclimate to direct sun. Choose a cloudy day for final removal to avoid sunscald. One important caveat: row covers only work if you practice crop rotation. Planting brassicas in the same spot year after year means pupae may already be in the soil under your cover, defeating the purpose.
Biological Sprays: Bt and Spinosad
Two biological insecticides are the workhorses of cabbage looper control for both organic and conventional growers.
Bt (a naturally occurring soil bacterium) produces proteins that are toxic specifically to caterpillars when they ingest it. It’s most effective against young, small larvae, so timing applications to coincide with recent egg hatches gives the best results. Bt breaks down in sunlight within a few days, which means reapplication is necessary, especially when eggs are hatching on a rolling schedule. It has essentially no toxicity to beneficial insects, pollinators, or humans.
Spinosad, derived from a different soil bacterium, works both on contact and through ingestion. Caterpillars stop feeding within minutes of exposure and typically die within 48 hours. It’s less hazardous to pollinators and beneficials than synthetic options, though it isn’t completely harmless to bees if sprayed directly on them. Apply spinosad in the evening when pollinators are less active. Like Bt, repeat applications are needed as new eggs hatch throughout the season.
Both products work best when sprayed to reach the undersides of leaves and the inner canopy where loopers actually feed. Good spray coverage is more important than higher concentrations.
Beneficial Insects That Target Loopers
Trichogramma wasps are tiny parasitic wasps that lay their own eggs inside cabbage looper eggs, killing them before the caterpillars ever hatch. They’re the most widely released biological control agent in North America and parasitize the eggs of over 200 moth and butterfly species. You can purchase them commercially and release them into your garden or field during peak moth activity.
Other natural enemies include paper wasps, ground beetles, and parasitic flies. Maintaining habitat for these predators, such as flowering borders and undisturbed ground cover nearby, supports their populations throughout the growing season. Avoiding broad-spectrum insecticides preserves these allies.
Chemical Options and Resistance Management
When biological controls and row covers aren’t enough, selective synthetic insecticides can be effective. Products containing chlorantraniliprole or spinetoram are very effective against cabbage loopers, particularly when applied to young caterpillars in their early growth stages. These are considered moderately selective, meaning they cause less harm to beneficial insect populations than older broad-spectrum chemicals.
Resistance is a real concern with cabbage loopers. They have a documented history of developing resistance to multiple insecticide classes. The most important rule is rotation: never use products with the same mode-of-action group number more than twice per season. Insecticide labels include a group number assigned by the Insecticide Resistance Action Committee (IRAC). Alternating between different group numbers forces the pest to contend with different mechanisms of toxicity, slowing the development of resistant populations.
For example, if you use an organophosphate (group 1B) for one application, your next treatment should come from a completely different group. Combining chemical rotation with biological controls like Bt and spinosad gives you more tools to rotate through and reduces selection pressure on any single product.
Putting It All Together
The most effective cabbage looper programs layer multiple strategies. Start with row covers at planting to exclude moths entirely. Scout twice weekly once covers come off or if you’re growing without them. When you hit the action threshold for your crop’s growth stage, apply Bt or spinosad targeted at young larvae, and reapply as new generations hatch. Release Trichogramma wasps during peak moth flight if you’re managing a larger planting. Reserve synthetic insecticides for heavy infestations that biological controls can’t keep up with, and always rotate modes of action.
Cleaning up crop debris after harvest removes overwintering pupae and reduces next year’s starting population. Rotating where you plant brassicas each year further disrupts the looper’s life cycle by separating emerging adults from their preferred host plants.