Photo 1. At first, the caterpillars of diamond backmoth, Plutella xylostella, eat from the underside of the leaf to the top layer of wax.
Photo 3. Pupae of diamondback moth, Plutella xylostella, surrounded by their net-like cocoons on the underside of a Chinese cabbage leaf.
Photo 6. Cotesia vestalis inside its silken cocoon, close to the remains of a dead diamondback moth larva.
Worldwide. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from American Samoa, Australia, Fiji, Guam, New Caledonia, New Zealand, Northern Mariana Islands, Palau, Papua New Guinea, Samoa, Solomon Islands, Tonga, and Vanuatu..
Members of the cabbage (brassica) family, e.g., head cabbage, Chinese cabbage, radish, cauliflower and broccoli; Amaranthus and watercress.
The caterpillars do the damage. The first two stages are small and feed by mining the leaf; later, when they are larger 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 (Photo 1). Later, the caterpillars eat through the leaf (Photo 2).
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 grey-green; later, when fully grown, they are dark green and approximately 12 mm long (Photos 1&2). 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 (Photo 3).
The pupa is dark green or light brown, about 10 mm long, and usually stuck to the underside of the leaf. The pupal stage lasts 5-10 days, depending on temperature.
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 4).
The life cycle is complete in less than 1 month (14 days at 25oC), depending on the temperature. The moth is most active at night.
Economic damage is most severe when heading begins. The caterpillars tunnel into the leavesof the forming heads of cabbages. The diamondback moth is the most destructive insect pest of brassica crops throughout the world. Worldwide, the management of the annual damage caused by this insect has been estimated to be US$4-5 billion!
Note: other pests often occur on ball cabbages along with this moth, and the combined damage is considerable. The other pests are cabbage centre grub, Hellula undalis (see Fact Sheet no. 114); cutworm, and cluster caterpillar, Spodoptera litura; or cabbage cluster caterpillar (large cabbage moth, LCM), Crocidolomia pavonana (see Fact Sheet no. 78).
It is very important to monitor the start of diamondback moth infestations; this can be done by:
Look for larvae (caterpillars) that are pale green and slightly tapered at each end. They grow through four stages (to a length of about 12mm), and have a dark head in the first two stages. They wriggle when disturbed, often dropping from the plant on a silken thread. Look for the moth with a diamond pattern on the back.
There are several parasitoids of the different stages: eggs - Trichogrammatoidea bactrae; caterpillars - Cotesia vestalis, Diadegma semiclausum, Microplitis plutellae, Oomyzus sokolowskii; pupae - Diadromus collars. Cotesia is found in the tropical lowlands (Photos 5&6), whereas Diadegma (Photos 7&8) is common above 800 m in the cooler highlands.
If pesticides are used, there is need for careful choice. If one chemical is used all the time, it is likely that the diamondback moth will develop resistance to it. Do the following:
AUTHORS Helen Tsatsia & Grahame Jackson
Photos 1&2 Graham Walker, Plant and Food Research, Auckland, New Zealand. Photo 4 Richard Markham, ACIAR, Canberra. Photo 4 Jack Kelly Clark, US Statewide IPM Project. Photos 5-8 Mike Furlong, University of Queensland, Australia.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
The mobile application is available from the Google Play Store and Apple iTunes.