Cabbage moth

Identification

A nocturnal moth with a wingspan of about 35-45 mm, with brown-grey forewings bearing a characteristic pale kidney-shaped spot. The caterpillars are large (up to 40 mm), at first green, later grey-green to brown, very voracious and polyphagous: they chew irregular holes in the leaves and, in cabbage, bore into the interior of the head, contaminating it with frass and opening the way to rot. Besides brassicas it feeds on beet, lettuce, onion and many other vegetable and ornamental plants; usually two generations develop per season, of which the summer-autumn one is the most damaging. The females lay eggs in characteristic, closely packed clusters of several dozen on the underside of the leaves; the eggs are hemispherical, at first pale, yellowish-green. The youngest caterpillars feed gregariously on the underside of the leaf blade, scraping it away and leaving a thin, transparent skin (so-called windowing), and in time they disperse and move on to chewing holes and boring deep into the plants. Older caterpillars are active at night and by day hide at the base of the plant, among the leaves or shallowly in the soil, which makes them hard to detect and means the damage is often noticed only after it has been done. The key symptom distinguishing the cabbage moth from caterpillars that feed only on the surface is the contamination of the interior of the head with moist, dark frass - this, alongside the tunnels bored in the head, is what makes the crop unmarketable and opens the way to rot-causing bacteria. The species overwinters as a pupa in the soil; the first moths usually fly from May to June, and the second, more numerous and more damaging generation in July and August.

Treatment

In small plantings, inspecting the plants and hand-picking the caterpillars is effective, especially in the evening when they come out to feed, as is destroying the egg clusters on the underside of leaves. Moth flight is monitored with pheromone traps, which allows the timing of control against young caterpillars with Bacillus thuringiensis var. kurstaki to be set accurately (older larvae, bored into the heads, are already hard to control). Crop rotation, autumn cultivation that destroys the overwintering pupae, and covering crops with fleece all help. Spraying is effective only when it hits young caterpillars still feeding on the leaf surface - once they have bored into the head, plant-protection products no longer reach them, which is why the timing is set by pheromone monitoring and inspection of the egg clusters, rather than by the sight of damage. Besides Bacillus thuringiensis, the population is kept down by natural enemies: parasitic wasps and predatory ground beetles, while the eggs are parasitised by Trichogramma wasps - introducing parasitoids and avoiding unnecessary broad-spectrum sprays protects these allies. When the threat is high, chemical products are used on young caterpillars: contact pyrethroids or selective stomach-acting products from the spinosyn or anthranilic diamide group, which are worth rotating between groups with different modes of action to limit the development of resistance. Protection is also supported by removing crop residues and weeds from the brassica family, on which the pest can feed and continue to develop.

How to tell it apart from look-alikes

Control — organic and chemical methods

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.