Pest

Diamondback moth

Plutella xylostella

The short answer

The diamondback moth is a small grey-brown moth whose caterpillars feed only on the cabbage family. The caterpillars are green, spindle-shaped and small, under ½ inch fully grown, and they wriggle violently or drop on a silk thread when disturbed. They feed from the underside of the leaf and often leave the upper surface intact as a translucent "windowpane". Heavy feeding strips seedlings and can disrupt head formation in cabbage, broccoli and cauliflower. Its reputation comes from insecticide resistance: pyrethroids began to fail in the 1980s, and some populations now resist Bt too. UF/IFAS says that cutting back on sprays, and pyrethroids above all, can return it to minor-pest status by letting its parasitoids survive.

hero macro · larva and windowpane damage on a leaf underside

A late-instar Plutella xylostella larva beside windowpane feeding on a cabbage leaf underside. Photo: pending.

Identification

Full-grown caterpillars are small: 0.3 inch (UC IPM), 5/16 inch (Kentucky), 3/8 inch (Wisconsin) or 11 mm (UF/IFAS). They are green, taper at both ends, and have a pair of prolegs sticking out at the back that forms a distinctive V. Touch one and it wriggles violently, moves backwards, or drops from the leaf on a silk thread. The pupa sits in a loose, gauzy white silk cocoon on the lower or outer leaves, and in broccoli and cauliflower sometimes in the florets.

The moth is slender, grey-brown and about 6 mm (¼ inch) long, and it rests with its wings folded like a tent. A cream or light brown band along the back forms one or more diamonds when the wings are closed, which gives the moth its name. The RHS gives its wingspan as 15 mm; an older Wisconsin sheet says one inch. It flies weakly and low, usually within 6½ feet (2 m) of the ground, but wind carries it long distances. The eggs are tiny, 0.44 × 0.26 mm, yellow or pale green, and laid singly or in groups of two to eight on the leaves.

The other cabbage caterpillars are all larger. The imported cabbageworm is velvety and sluggish and grows to about 1–1¼ inches. The cabbage looper grows to 1½–2 inches and arches into a loop as it walks. The cross-striped cabbageworm has black bands across its back.

EGGEgg0.44 mm, yellow to pale green, singly or in small groups. Hatches in about 5.6 days.

LARVAYoung caterpillarFirst instar mines inside the leaf; the mines are hard to see.

LARVAFull-grown caterpillarUp to 11 mm, green, forked tail. Four instars, about 17 days in all (UF averages).

PUPAPupa in cocoon7–9 mm, in loose white silk on the leaf. About 8.5 days (5–15).

ADULTMothAbout 6 mm, diamond pattern on the back. Lives 12–16 days; about 150 eggs.

Easily confused with

Imported cabbagewormAlso a pestVelvety green and sluggish, with a faint yellow stripe down the back and four pairs of prolegs in the middle of the body, so it crawls flat rather than looping.
Cabbage looperAlso a pestSmooth, pale green with white lines along the sides, and only two pairs of prolegs toward the rear: it walks by arching its middle into a loop, like an inchworm.
Cross-striped cabbagewormAlso a pestBluish-grey on top with many black bands across the back and a yellow line down each side, and several feed together on one plant.

Damage symptoms

The caterpillars are small, but they can be very numerous. First-instar larvae mine inside the leaf, then emerge and feed on the underside. They chew irregular patches and usually leave the upper surface as a clear film. Heavy feeding can strip everything but the veins, which is especially damaging to seedlings. Larvae also feed at the growing point and in the bud, which can stunt young plants and malform cabbage heads. In broccoli and cauliflower, larvae in the florets can make a crop unsaleable even when little tissue is eaten.

Small holes that leave the upper leaf surface as a translucent "windowpane"
Feeding spread across the foliage, not concentrated in the head
Damaged growing points and stunted young plants
Misshapen cabbage heads after bud feeding
Tiny caterpillars and loose white silk cocoons in broccoli and cauliflower florets
Caterpillars dropping on silk threads when a leaf is moved

Lifecycle & degree days

In temperate areas the adult moth overwinters, but it does not survive cold winters such as most of Canada's. Winds carry fresh moths north again each spring, so northern outbreaks often start with migrants, or with larvae riding on transplants grown in the South. UMass notes that caterpillars have turned up by May in recent years, which suggests some now overwinter in high tunnels. Development from egg to pupa takes 25–30 days on average (17–51). There are four generations a year in cold climates such as southern Canada, three to five in Wisconsin, and perhaps eight to twelve in the South. UC IPM finds it most numerous in spring to early summer and again in autumn, and it can be active all year, especially in coastal areas.

Overwinters: Adult moth, where winters are mild; migrates north each spring
Egg to pupa: 25–30 days average (range 17–51)
Generations / yr: 3–5 Wisconsin; 4 southern Canada; 8–12 in the South
Egg-laying: About 10 days per female; around 150 eggs
Peaks: Spring to early summer, and autumn (UC IPM)

Pressure by month

Host crops

Cabbage Broccoli & cauliflower Collards & kale Brussels sprouts & kohlrabi Mustard, radish & turnip

The diamondback moth eats only plants in the cabbage family, including virtually every cruciferous vegetable and watercress. Moths prefer collards to cabbage when laying eggs, which is why two or more rows of collards or mustard around cabbage can delay or prevent the pest moving into the crop. Mustard, turnip and kohlrabi are among the more resistant crucifers. Glossy varieties, which lack the normal grey waxy bloom, are somewhat resistant, but they tend to carry more cabbage flea beetles. Cruciferous weeds are important hosts early in the season, before crops are available.

Monitoring & action thresholds

Action threshold

No numeric threshold for gardens

UC IPM says no threshold has been set in California and to watch for feeding on growing points. UMass sees no need to treat young plants until 35% or more are infested with caterpillars, then treats heading crops at 15–20%, and kale, collards and mustard at 10–15%. Texas considers up to 0.3 larvae per plant below the treatment level, and Florida and Georgia treat only at one or more holes per plant. UF/IFAS notes that growers who use thresholds instead of trying to prevent all damage need considerably fewer sprays.

Start checking when plants are young, turning over the lower leaves on 25 plants across the planting. Look for small caterpillars coming out of tiny holes on the underside of the leaf, and for windowpane feeding. Check the bud leaves of young plants wherever you see damage. The most important time to look is just before heads form. Pheromone traps catch the moths and may predict larval numbers 11–21 days later, but each field needs its own trap.

Beneficial

Biological control

The diamondback moth's specialist parasitoids are the reason it is often a minor pest where spraying is light. The main one in North America is Diadegma insulare, an ichneumonid wasp. It can parasitise up to 90% of the larvae, and parasitised caterpillars eat 80% less. It needs nectar from wildflowers, and it is highly susceptible to pesticides. The braconid Microplitis plutellae is also important; Oomyzus sokolowskii matters in the Southeast. In a two-year field study in collards (Ghani & Bilbo 2026), parasitism averaged up to 86%. Interplanted sweet alyssum plus Bt sprays gave the lowest diamondback pressure and the highest marketable yield, while plots sprayed with bifenthrin had more diamondback moth. On egg parasitoids the sources disagree. UF/IFAS says none are known, but UC IPM lists Trichogramma pretiosum attacking the eggs, and UMass says the purchased T. brassicae parasitises all the brassica caterpillars.

Cultural controls

Conserving parasitoids and starting with clean transplants are the main ways to prevent outbreaks.

Grow your own transplants, or check bought ones: larvae, and resistant strains, travel north on transplants from the South
Cover small plantings with floating row cover to keep the moths off
Plant sweet alyssum or other nectar flowers near brassicas to feed Diadegma
Use overhead sprinklers in the evening where practical: rain kills many young larvae, and sprinkler-irrigated crops carry fewer than drip-irrigated ones
Border cabbage with two or more rows of collards or mustard to intercept the moths
Dig in crop residues soon after harvest so the moths do not move to the next planting

Chemical, last resort

Resistance is widespread: rotate modes of action, and avoid pyrethroids, which kill the parasitoids and can make outbreaks worse. Spinosad is toxic to bees: UC IPM says not to spray it on or let it drift onto flowering plants, except between sunset and midnight where the label allows. Bt needs no bee precaution beyond the label.Check any product before it goes on. Open the compatibility matrix →

Bt kurstakiREI 4 h✓ Safe
SpinosadREI 4 h! Caution
ChlorantraniliproleREI 4 h! Caution
MethomylREI 48 h× Lethal

Sources

UF/IFAS EDIS EENY-119/IN276, Diamondback Moth, Plutella xylostella (Capinera)

UF/IFAS EDIS EENY-124/IN281, A Wasp Parasitoid, Diadegma insulare (Sourakov & Mitchell)

UC IPM Pest Management Guidelines: Cole Crops — Diamondback Moth (Natwick et al., 12/20)

UC IPM, Relative Toxicities of Insecticides Used in Cole Crops to Natural Enemies

UMass Extension Vegetable Program, Diamondback Moth (2022)

UMass Extension, Caterpillars in Brassica Crops (Hazzard, Scheufele & McKeag, 2022)

UW–Madison Extension XHT1031, Diamondback Moth (Delahaut, 2004)

Univ. of Kentucky ENTFACT-300, Cabbage Pests (Bessin, rev. 1/26)

Ghani & Bilbo, Sweet alyssum and selective insecticides for DBM biocontrol, J. Econ. Entomol. 2026

RHS Advice, Cabbage caterpillars

Bonide Thuricide concentrate label, EPA Reg. 4-226 (2016)

DiPel DF label, EPA Reg. 73049-39 (2022)

UC IPM Pest Management Guidelines: Cole Crops — Imported Cabbageworm; Cabbage Looper

Page status

Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.

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