4[7] Plutella xylostella [EIFS

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This leaflet is produced by the Ministry of Agriculture & Livestock, Solomon Islands, with support from IPPSI:
Improved Plant Protection in Solomon Islands, a project financed by ACIAR, the Australian Centre for
International Agricultural Research, Canberra. Authors: Helen Tsatsia, MAL & Grahame Jackson, TerraCircle
Inc.

Extension Fact Sheet 20:

Diamond Back Moth

Common name: Diamond back moth

Scientific name: Plutella xylostella

Hosts: Cruciferous vegetables - head cabbage; Chinese cabbage; radish; cauliflower and
broccoli; non-crucifer plants: Amaranthus and watercress.

Damage

The caterpillars do the damage. The first two stages are small and feed by mining the leaf;
later, when they are too large to mine, they burrow through the leaf. The result is 1-2 cm wide
cavities on the lower leaf surface leaving the waxy layer intact, which gives the appearance
of windows in heavily damaged plants. Economic damage is most severe when heading
begins. The caterpillars tunnel into the heads of cabbages.

Note: other pests often occur on ball cabbages along with this moth, and the combined
damage is considerable. The other pests are

cabbage webworm, Hellula undalis; cutworm,

Spodoptera litura; and cabbage cluster caterpillar, Crocidolomia pavonana

Biology and Life Cycle

The eggs are small (0.4 mm long), cylindrical or oblong, white when laid, changing to
yellowish brown as they mature and ready to hatch. The eggs are mostly laid singly or in
groups of two or three, on the lower leaf surface along major veins. One adult female lays
100-150 eggs in a life span of 3-7 days. The incubation period ranges from 3-8 days
depending on the temperature.

There are four larval or caterpillar stages. At hatching, the caterpillars are light brown; later,
when fully grown, they are dark green. When disturbed, the larvae wriggle backwards, and
may drop from the leaf on a silk thread. The larval period ranges from 14-28 days, after
which they make a silken cocoon and pupate.

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The pupa is dark green or light brown, about 10 mm long, and usually stuck to the underside
of the leaf. Pupation lasts 5-10 days.

After pupation, the adult moth emerges from the cocoon. It is about 10 mm long with a 13
mm wingspan, dark brown with three white diamond-shaped patterns on its back; these give
the moth its common name (photo, left). The adult lives for up to 35 days.

The life cycle is complete in less than 1 month, depending on the temperature. The moth is
most active at night.

Detection and Inspection

The onset of Diamond back moth infestation can be monitored by:

• visiting the field everyday, looking for any adults or larvae on the plants;

• putting yellow sticky traps in the field to catch flying adults;

• putting sticky traps baited with Diamond back moth sex pheromone to trap male adults.

Management

Natural enemies

There are several parasitioids of the different stages: eggs - Trichogrammatoidea bactrae;
caterpillars - Cotesia plutellae, Diadegma semiclausum, Microplitis plutellae, Oomyzus
sokolowskii; pupae - Diadromus collars.

Cultural control:

• Always start with healthy, insect-free seedlings;

•

Hand pick larvae when numbers are low;

•

Destroy crop residues after harvest, and crucifer weeds before planting and during the
cropping period;

•

Grow head cabbages during the cooler times of the year (June-November) to avoid
infestations - done by some farmers on the Guadalcanal Plains;

•

Grow collards,

Brassica oleracea acephala cv. Vates, as a trap crop. Trap crops for the

other species are: radish and green mustard (B juncea) for Hellula undalis); Chinese
cabbage (B pekinensis cv. Tempest) or flowering green mustard for Crocidolomia
pavonana. (Note that the latter was also useful as a trap crop for Halticus tibialis);

•

Where farmers have the resources, grow plants under nets or in green houses;

•

For watercress, grown on rafts in the river, sinking one half for 30 min and then the other
end, to drown the caterpillars is a measure worth trying.

Chemical control:

•

If pesticides are used, there is need for careful choice. If one chemical is used all the time,
it is likely that the Diamond back moth will develop resistance to it;

• Those products that are recommended are plant-derived products, such as chili (with

additional soap), and synthetic products that contain disease-causing organisms, such as
spinosad (Success) and Bt - Bacillus thuringiensis. Spinosad and Bt are sometimes sold in
Honiara.

• Indoxacarb (Steward), a new product with a novel action is not yet available in Honiara.