Diamondback moth

Identification

A small, grey-brown moth with a wingspan of about 12–15 mm; in the resting insect the folded wings form a pale, diamond-shaped (patterned) mark along the back, from which the English name 'diamondback moth' is derived. The damage is done by small, green, spindle-shaped caterpillars up to 10 mm long, feeding mainly on the underside of leaves: they scrape away the lower skin, leaving transparent 'windows', while older larvae chew irregular holes. It attacks brassicas — cabbage, broccoli, cauliflower and also radish; in warm, dry summers it produces several generations and can be exceptionally numerous. The moths are active at dusk and at night, resting hidden among the leaves during the day; when disturbed they make short, nervous flights. The female lays yellowish eggs singly or in small groups on the underside of leaves, along the veins. A very useful identifying sign is the behaviour of the caterpillars: when touched they wriggle violently and often drop on a thin thread of silk — a trait rare among other brassica caterpillars. Pupation takes place in a loose, net-like cocoon attached to the leaf. Under favourable conditions the full cycle takes only 2–3 weeks, so 3–5 overlapping generations develop over the season and the moth flight is spread out over time. The pupae overwinter on brassica debris; in mild winters the adults also survive, and in spring the populations are supplemented by an influx of moths migrating from southern Europe. The most serious damage is injury to the heart (growing point) of transplants and to the curds of broccoli and cauliflower; the characteristic 'windowing' is most easily seen by holding the leaf up to the light.

Treatment

The beds are covered with fleece from transplanting onwards, crop rotation is applied, and brassica plant debris on which the pupae overwinter is removed after harvest. The most effective biological measure is spraying with Bacillus thuringiensis var. kurstaki, applied to young caterpillars and repeated, since the moth flight can be spread over time. The species is notorious for rapidly developing resistance to insecticides, so chemicals (if necessary) are used with rotation of active substances and only under heavy pressure. It is worth hanging pheromone traps — they catch the onset and intensity of the moth flight, which makes it possible to time the Btk treatment to the most susceptible, young caterpillar stage; this is precisely what determines the effectiveness of the biopreparation. The spray is directed at the underside of the leaves, where the larvae feed, and given the waxy, water-shedding surface of brassica leaves the addition of a wetting agent helps. It is crucial to spare natural enemies: parasitic wasps (including Diadegma semiclausum, Cotesia) and predators can strongly limit the population, while broad-spectrum insecticides wipe them out, triggering a sharp rebound in the pest's numbers. Site hygiene — crop rotation and removal of brassica debris — however only limits the local source of overwintering pupae and does not protect against the influx of moths migrating from southern Europe, so it remains a supporting practice rather than a standalone way of breaking the life cycle. Since resistance is the rule in this species (to pyrethroids it can be complete), successive sprays must belong to different mode-of-action groups according to the IRAC classification — the same substance must not be repeated, nor should chemicals be used preventively under low pressure, because this speeds up the selection of resistant forms.

How to tell it apart from look-alikes

Feature	Diamondback moth	Large white butterfly
Caterpillar — size and appearance	Small, up to 10 mm, light green, spindle-shaped (tapered at the ends), almost hairless	Large, up to 40 mm, yellow-green with black spots, distinctly hairy
Feeding pattern	Scrapes off the lower skin → transparent 'windows', then small holes, mainly from below	Chews large, irregular holes and strips leaves down to the veins
Reaction when touched	Wriggles violently and drops on a thread of silk	Does not drop, stays on the leaf
Distribution of caterpillars	Singly, scattered, mostly on the underside of leaves	In groups, in whole clusters on a single leaf
Adult	Small, grey-brown, nocturnal; diamond-shaped pattern along the back	Large, white day-flying butterfly with black wing tips

Control — organic and chemical methods

Organic and biological methods

Method	How it works	When to use
Fleece or mesh barrier	A tight cover over the bed, put in place from transplanting onwards, prevents the moths from laying eggs; it must fit closely to the soil along the edges.	From transplanting until harvest
Bacillus thuringiensis var. kurstaki (Btk)	A selective biopreparation; once eaten by young caterpillars it paralyses their gut. Harmless to bees and parasitoids, it requires repeat treatment.	On young caterpillars, repeat every 7–10 days
Crop rotation and removal of brassica debris	Limits the local source of overwintering pupae, but does not prevent the influx of migrating moths — a supporting practice. Do not plant brassicas after brassicas.	After harvest and when planning beds
Protecting natural enemies (parasitoids)	Parasitic wasps (Diadegma, Cotesia) and predators naturally limit the population; avoid broad-spectrum sprays that destroy them. A supporting method.	Continuously, by avoiding non-selective chemicals

Chemical methods

Method	How it works	When to use
Spinosyns	A group with neuronal action, effective against caterpillars and also permitted in organic growing; high risk of resistance — rotate strictly with other groups.	As an intervention, under heavy pressure
Anthranilic diamides	They activate the ryanodine receptor and paralyse the caterpillars' muscles; an effective, modern tool — use in IRAC rotation with Btk and spinosyns.	As an intervention, under heavy pressure

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.