Biology and management of the diamondback moth

Diamondback moth (Plutella xylostella, DBM) is a small, grey-brown moth, a pest that is present worldwide wherever its brassica host plants grow. Find out recommended crop protection and management for this pest.

Diamond-back moth infests crops sporadically throughout the UK.

The larvae can damage the foliage of cabbage, cauliflower, broccoli, Brussels sprout, kale, Chinese cabbage, swede, turnip, oilseed rape and radish.

Damage can be extensive, but losses depend on the effect on plant growth and quality.

Management is particularly important to protect the marketable part of the plant (cabbage, cauliflower, broccoli and Brussels sprout).

Plants with low levels of damage usually survive, but this may affect uniformity within the crop.

• Weather that favours migration from continental Europe increases the risk
• The moth does not presently overwinter in the UK in large numbers
• The moths are relatively poor flyers, but may be transported long distances by wind

The rate of development of diamond-back moth is dependent on temperature. More generations occur in warm locations.

In the UK, depending on when moths arrive, there may be two to three generations per year.

Adult moths are about 6 mm long, brownish, with three light brown-to-white triangular marks on the trailing edge of each forewing. When at rest, the triangular marks meet to form diamond shapes.

The eggs are yellow and laid singly or in small groups, mainly alongside the midrib or leaf veins. Eggs are laid from May to September, with the peak about July.

Eggs may hatch within 2–3 days of being laid and a complete generation takes about 5 weeks at 15°C.

The larva is light green and has a tapering body that is widest in the middle. Larvae wriggle violently when disturbed. Fully grown larvae (15 mm) construct a flimsy cocoon on the leaf surface before developing into a pupa about 9 mm long.

Larvae consume the foliage of most types of brassica crop. They eat almost all leaf material (at any age), except the upper epidermis, creating translucent ‘windows’. Large infestations destroy leaves.

The life cycles of other species may vary from this pattern. For example, Euxoa nigricans (garden dart moth) lays eggs in the late summer, which hatch in the following spring. The larvae feed on beet seedlings, during April and May, before pupating.

Cutworm damage can kill seedlings and young plants. Larvae may move along the rows of crops, such as lettuce or leek and cut plants off, one after another. Similar to slugs, cutworms make cavities in stems, rhizomes, tubers and roots of large plants. Damage to root crops may not be evident until harvest.

Physical and cultural approaches (crop covers using fine mesh netting, companion planting and trap crops) can help, but are not as effective as insecticides.

To date, biological control with predators or parasitoids has not been investigated in the UK. Pesticides based on microbial control agents (Bacillus thuringiensis, Bt) or viruses can be effective.

Catch male moths with pheromone traps.

None established in the UK.

Resistance to pyrethroids has been confirmed in the UK.

Elsewhere in the world, diamond-back moth populations have developed resistance to almost all of the insecticide groups to which they have been repeatedly exposed.