Cabbage moth larvae are small green caterpillars that feed on brassica crops like cabbage, broccoli, cauliflower, and kale. They’re one of the most common garden pests in North America, and the term “cabbage moth larvae” actually covers three closely related species that show up in similar ways: the imported cabbageworm, the cabbage looper, and the diamondback moth caterpillar. All three are green, all three eat holes in your plants, and all three can be managed with the same basic strategies.
Identifying the Three Common Species
The imported cabbageworm is the larva of the white butterfly you see fluttering around your garden on warm days. These caterpillars are velvety green with a faint yellow stripe down the back, and they grow to about 1.2 inches long. They have a fuzzy texture from dense short hairs and move sluggishly. Their bodies are blunt at both ends, making them easy to distinguish once you know what to look for.
Cabbage loopers are pale to dark green with thin white or light yellow stripes. They’re the largest of the three, reaching 1.5 inches, and they move in a distinctive arching motion, forming their body into a loop with each step (like an inchworm). Their heads are tapered and their bodies are smooth with only a few sparse hairs.
Diamondback moth caterpillars are the smallest at just half an inch long. They’re greenish gray, smooth, and tapered at both ends. They have no stripes, and if you disturb one, it wiggles vigorously and may drop off the leaf on a silk thread. This reaction is unique to diamondback larvae and makes identification straightforward.
What the Damage Looks Like
All three species chew irregular holes in leaves, but the pattern varies. Diamondback moth larvae are small enough that they often feed on the underside of leaves, creating “windowpane” damage where they eat everything except the thin upper layer of leaf tissue. Imported cabbageworms and loopers chew all the way through, leaving large ragged holes and depositing dark green droppings (frass) on the leaves and in the folds of developing heads.
The caterpillars blend in remarkably well against green foliage. You’ll often notice the damage, the frass, or the white butterflies flying around your garden before you spot the larvae themselves. On heading crops like cabbage and broccoli, larvae can burrow into the developing head where they’re nearly impossible to reach with sprays, which is why early detection matters.
Life Cycle and Timing
The imported cabbageworm, the most common of the three, overwinters as a pupa attached to host plants or nearby debris. Adults emerge in mid-spring and lay eggs singly on the undersides of outer leaves. The eggs are oblong, white to cream colored, and stand upright on the leaf surface, sometimes described as football-shaped. They turn yellowish before hatching in 4 to 8 days.
Once hatched, larvae feed for two to three weeks before pupating on the host plant. The full cycle from egg to adult takes 3 to 6 weeks, and adults emerge in one to two weeks to start the process again. In most climates, this produces 3 to 5 generations per year, meaning caterpillar pressure builds throughout the growing season. By late summer, overlapping generations can produce heavy infestations if left unchecked.
When to Take Action
Not every caterpillar warrants treatment. University of Maryland Extension recommends checking 50 plants and calculating the average number of larvae per plant. Before heads start forming, treatment is justified when you find more than 0.5 “larval units” per plant. During heading, that threshold drops for cabbage (still 0.5) but rises slightly for broccoli, cauliflower, and Brussels sprouts (1.0 larval unit). The thresholds are weighted by species because cabbage loopers do significantly more damage per caterpillar than the other two. A single large looper counts as a full unit, while an imported cabbageworm or diamondback caterpillar of any size counts as only 0.1.
In practical terms for a home garden: a few small holes in outer leaves aren’t worth worrying about, especially before the plant starts forming a head. Once you’re seeing consistent damage on multiple plants and white butterflies are still actively laying eggs, it’s time to intervene.
Physical Barriers
Floating row cover is the single most effective prevention method. It’s a lightweight spun-bonded fabric that lets light, water, and air through while physically blocking adult moths and butterflies from reaching your plants to lay eggs. It eliminates imported cabbageworms, cabbage loopers, and diamondback moth caterpillars without any sprays.
The key details: row cover must go on at planting or transplanting, before adults have a chance to lay eggs. You need to bury the edges completely in soil, leaving no gaps. For taller crops, support the fabric on wire hoops so it doesn’t rest directly on the leaves, which can cause abrasion on tender plants. Since brassicas don’t need pollination to produce the parts you eat (leaves, heads, florets), the cover can stay on all season.
Biological Sprays
Two organic-approved options are widely used against cabbage moth larvae, and both work well when applied correctly.
Bt (Bacillus thuringiensis)
Bt is a naturally occurring soil bacterium that produces proteins toxic specifically to caterpillars. It works because caterpillars have uniquely alkaline digestive systems that activate the toxin. Other insects, birds, mammals, and beneficial predators are unaffected. After a caterpillar eats treated foliage, it stops feeding within hours but may take a few days to die, so don’t assume it isn’t working if you still see live caterpillars the day after spraying.
Bt breaks down in sunlight and washes off in rain, so it stays effective for about a week under good conditions. Reapply weekly while butterflies are active and damage is visible. In climates with frequent rain or evening showers, you may need to reapply more often.
Spinosad
Spinosad is derived from a soil bacterium and is effective at very low application rates. It has extremely low toxicity to mammals and birds, and it carries only a one-day pre-harvest restriction, meaning you can spray the day before you pick your broccoli. To prevent resistance, limit use to two or three consecutive applications and no more than three times in a 21 to 30 day period.
The important caveat: spinosad is toxic to bees while wet. Apply it in the evening or early morning when pollinators aren’t actively foraging, and it becomes relatively safe once it dries. Adding a silicone-based surfactant can improve how well it sticks to the waxy leaves of brassica crops.
Encouraging Natural Predators
Tiny parasitic wasps in the Trichogramma group are among the most effective natural enemies of cabbage moth larvae. These wasps lay their eggs inside moth and butterfly eggs, killing the pest before it ever hatches into a caterpillar. They parasitize the eggs of over 200 species of moths and butterflies and are the most widely released biological control agent in North America. You can purchase them commercially or attract naturally occurring populations by planting a variety of flowering plants near your brassica beds. Small-flowered herbs like dill, cilantro, fennel, and yarrow provide the pollen and nectar that adult parasitic wasps need to survive and reproduce.
Paper wasps, ground beetles, and spiders also prey on caterpillars. A garden with diverse plantings and minimal broad-spectrum pesticide use will naturally support these predator populations, reducing how much active management you need to do yourself.
Putting It All Together
The most reliable approach combines prevention with targeted treatment. Start with row covers at planting to block egg-laying entirely. If you garden without covers, inspect the undersides of leaves weekly for small yellowish eggs and remove them by hand before they hatch. When caterpillars do appear, handpick them if numbers are low. For heavier infestations, apply Bt weekly during the active season, switching to spinosad if Bt alone isn’t keeping up. Planting flowers nearby to support parasitic wasps gives you a passive layer of control that compounds over multiple seasons as beneficial insect populations build in your garden.