You pull back a cabbage leaf and there it is: a velvety green caterpillar the exact colour of the leaf it was eating, next to a hole the size of a quarter. Cabbage worms are the most common brassica pest in American gardens.
Three species do almost all the damage. They are easy to tell apart, and the most effective control costs about $25 and goes on once, before a single egg is laid.
"Cabbage worm" covers three different caterpillars feeding on the same crops. The imported cabbageworm (Pieris rapae) is the larva of the small white butterfly you see over the beds all afternoon. The cabbage looper (Trichoplusia ni) is the larva of a mottled grey-brown night-flying moth. The diamondback moth (Plutella xylostella) is the smallest and the most troublesome.
They matter separately because they feed differently. Imported cabbageworm chews large ragged holes on the upper leaf surface near the midrib and works toward the centre of the plant as it grows, according to University of Wisconsin Horticulture Extension. Diamondback larvae feed on the underside and leave the upper skin intact, producing a translucent "windowpane" that University of Maryland Extension uses as its main field diagnostic.
Here is what this guide covers:
Key Takeaway
Exclusion beats treatment. A row cover installed at transplant, sealed at the edges, stops all three species before they lay an egg. Everything else in this guide is what you do when that first line has already been breached.
Look at the gait first, then the stripes. A caterpillar that arches its middle into a loop is a cabbage looper. A small pale-green one that thrashes and drops on a silk thread when touched is a diamondback larva. Anything velvety and sluggish with a faint yellow line down its back is an imported cabbageworm. A pale winding tunnel inside the leaf is not a caterpillar at all but a leaf miner.
| Feature | Imported cabbageworm | Cabbage looper | Diamondback moth |
| Larva length | 1 in. (25 mm) | 1.5 in. (38 mm) | 0.4 in. (10 mm) |
| Movement | Slow crawl | Loops, arches back | Wriggles, drops on silk |
| Eggs | Yellow-orange, laid singly | White, ridged, leaf underside | Pale, in groups of 2 to 8 |
| Damage | Large holes, upper surface | Ragged stripping, heavy frass | Windowpaning, underside |
| Adult | White day-flying butterfly | Grey moth, silver forewing spot | Small brown moth, diamond pattern |
Sources: University of Wisconsin Horticulture Extension, University of Kentucky Veggie Scout, University of Florida IFAS.
Learn the eggs too. Diamondback eggs measure roughly 0.44 by 0.26 mm and hatch in about 5.6 days on average, per University of Florida IFAS. Imported cabbageworm eggs are larger, yellow-orange and laid one at a time. You can crush them with a thumb during your weekly walk-through.
Not every caterpillar needs a response. Extension services set action thresholds as the percentage of plants carrying at least one caterpillar, and those thresholds move with crop stage. Many brassicas take up to 30% defoliation with no measurable yield reduction, according to University of Minnesota Extension. Once a head forms, tolerance collapses, because one larva inside a broccoli crown ruins the whole thing.
| Crop stage | Cabbage, broccoli, cauliflower | Kale, collards |
| Seedling, 0 to 9 true leaves | Treat at 10% of plants infested | Treat at 10% |
| Vegetative to cupping | Treat at 20% to 30% | Treat at 10% to 15% |
| Head or curd forming | Treat at 5% | Treat at 10% to 15% |
| Heading to harvest | Treat at 5% to 10% | Treat at 10% to 15% |
Sources: University of Maryland Extension, University of Wisconsin Horticulture Extension, UConn Vegetable IPM, August 2025.
Kale and collards sit lower all season because the leaf is the crop. Scout weekly, check the innermost leaves and the undersides of the lower ones, and look for frass at the base of the head. Frass usually shows up before you spot the caterpillar making it.
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Send Me the ChartWork in this order. Each step is cheaper and less disruptive than the one after it, and most gardens never need to reach step four.
Hand-pick, twice a week, in the morning
Ten minutes across a 4 by 8 ft (1.2 by 2.4 m) bed. Drop larvae into soapy water and crush eggs as you find them. In a home planting this alone often holds numbers under threshold.
Spray Bt kurstaki on small larvae only
Bt has to be eaten, and it works best on caterpillars under 0.25 in. (6 mm). Cover leaf undersides. Reapply every 5 to 7 days, because ultraviolet light degrades it within about a week. A 16 oz concentrate runs roughly $15 and lasts a season.
Rotate the mode of action if pressure persists
Diamondback moth develops resistance faster than almost any insect on earth. University of Florida IFAS advises alternating Bt strains, kurstaki and aizawai, rather than hammering one product all season.
Use spinosad sparingly, and never on open flowers
Spinosad is effective on larger larvae, but it is toxic to bees while the spray is still wet. Apply at dusk on non-flowering plants only, and expect it to break down in sunlight within one to three days.
The Mistake That Wastes Most Bt Applications
Spraying big caterpillars. Past about a quarter inch a larva eats faster than the toxin kills it, and feeding continues for days while you conclude the product failed. Bt is a timing tool, not a rescue tool. If the caterpillars are fat, hand-pick and save the spray for the next hatch.
Insect netting, installed the day you transplant. Mesh of roughly 0.8 to 1.0 mm excludes all three adults while letting light and rain through. Ohio State Extension Master Gardeners and multiple other extension programmes describe physical exclusion as the most reliable organic option, and 2026 research on mesotunnels in HortTechnology reports near-complete exclusion of caterpillar pests in properly built systems.
Three things go wrong. Gardeners cover plants that are already infested, sealing the pest in with its food. They leave a gap at the soil line, and the butterflies walk in. And they lift the cover on a hot afternoon and forget to close it. Bury the edges and use hoops.
Rotation matters too. Rutgers NJAES advises against planting brassicas in the same bed in sequence, since imported cabbageworm overwinters as a pupa in crop debris. Clear the residue at the end of the season and put it through a hot compost pile rather than leaving it on the surface.
Why This Works: Conserving the Predator Layer
Bt and netting share one quality broad-spectrum sprays lack: they leave the parasitoid wasps alive. Cotesia rubecula, released across New England, kills young imported cabbageworms, and molecular screening published in 2026 found Diadegma parasitism of late-season diamondback larvae above 80% in New York fields. That is stacking functions: you keep the free labour that suppresses the next generation. Pyrethroids remove it in one pass, which is why gardens that spray hardest often have the worst August, and why Colorado potato beetle has outrun nearly every spray aimed at it.
Partly, and less than the internet claims. The strongest evidence sits with yellow rocket (Barbarea vulgaris) as a trap crop for diamondback moth. Rutgers reports a study in which moths laid 110 times more eggs on yellow rocket than on adjacent cabbage, and no larvae survived on the trap plant. That is a dead-end trap, not a repellent, and it is the only brassica-pest companion tactic with a measured effect size that large.
Nasturtiums are widely recommended as a trap for cabbage whites, and Santa Clara County Master Gardeners describe butterflies laying on nasturtium in preference to broccoli. No peer-reviewed trial has yet quantified how much that reduces larval density on the brassicas themselves. Clover living mulch and aromatic herbs sit in the same category: plausible, good for the wider system, unproven as direct suppression.
Treat companion planting as habitat work rather than a shield. It feeds the parasitoids and predators that do the actual suppression. For which pairings hold up under testing, see our companion planting chart and the guide to what to plant with broccoli and brassicas.
Key Takeaway
Spend your money on netting and your time on scouting. Bt is the backup. Companion planting builds the beneficial-insect population that carries you through the seasons you forget to cover.
Start by hand-picking twice a week and crushing any eggs you find, which in a home-sized bed often keeps numbers below the treatment threshold. If damage keeps climbing, spray Bacillus thuringiensis kurstaki on the leaf undersides while the caterpillars are still small, under about a quarter inch, and repeat every five to seven days. Bt has to be eaten to work and sunlight breaks it down within roughly a week. Save spinosad for larger larvae, apply it at dusk, and never on flowering plants, because it is toxic to bees while wet.
Cover the bed with fine insect netting or floating row cover the day you transplant, before any adults have laid eggs. A mesh of around 0.8 to 1.0 mm blocks the cabbage white butterfly, the looper moth and the diamondback moth while letting in light and water. Bury or weight the edges so there is no gap at soil level, use hoops so the mesh does not rest on the foliage, and never cover plants that are already infested. Rotate brassicas out of the bed the following season and clear old crop debris, since imported cabbageworm overwinters as a pupa in it.
It depends which one you have. The imported cabbageworm becomes the small white butterfly with one or two black spots on each forewing, the one you see flying over the garden in daylight. The cabbage looper becomes a mottled grey-brown moth with a silvery mark on each forewing, and it flies at night. The diamondback larva becomes a small brown moth that shows a diamond pattern along its back when the wings are folded. All three pupate near or on the plant, which is why clearing debris at the end of the season matters.
No. Cabbage worms carry no human parasites and are not toxic. If one ends up in a cooked broccoli floret, it is unpleasant rather than dangerous. The practical fix is a saltwater soak: submerge cut heads in cold water with a couple of tablespoons of salt for fifteen to twenty minutes and the larvae float free. The reason extension services drop treatment thresholds to five percent once heads form is contamination and marketability, not food safety.
Imported cabbageworm overwinters as a pupa tucked into brassica debris and old plant stems, and the adults emerge in spring to lay on cruciferous weeds and early cole crops. University of Wisconsin Extension reports about three generations a season there, with the mid-July generation doing most of the damage. Cabbage loopers migrate north from subtropical areas each spring and can run up to seven generations a year in south Georgia. Diamondback moth manages three to four generations in cold climates and far more in warm ones.
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