Diamondback moth: cabbage pest in Indonesia and Western Australia

Page last updated: Wednesday, 9 May 2018 - 11:08am

Diamondback moth is a major pest of cabbage crops in both Indonesia and Western Australia.

The Department of Primary Industries and Regional Development, Western Australia has worked with the Indonesian potato industry to increase the productivity of  crops planted with WA seed potatoes.

Improved productivity requires accurate identification of pests in potatoes and also in the cabbage rotation crop.

As well as helping Indonesian farmers this information will assist Western Australian seed potato exporters understand the challenges their Indonesian customers face.

Identification

Diamondback moth’s Indonesian name is "ulat kubis" and its scientific name is Plutella xylostella.

Diamondback moth adults are about 8 mm long with a diamond pattern on the wings
Diamondback moth adult

Diamondback moth adults are about 8mm long with a diamond pattern on the wings when at rest. Moths are weak fliers but their dispersal may be assisted by wind. They can infest crops from early stages through to harvest.

Adult female moths produce a pheromone, or scent, which attracts males. Synthetic pheromone lures can help monitor the timing and intensity of insect infestation.

Diamondback moth eggs are about 0.5mm long and yellow when first laid.
Newly-laid diamondback moth eggs

Diamondback moth eggs darken just before hatching. Egg on left has hatched
Advanced diamondback moth eggs with hatched egg on left

Eggs laid by diamondback moths are about 0.5mm long and are yellow when first laid, but darken when close to hatching.

After hatching, transparent egg shells remain. Eggs are laid singly or in small groups near veins mainly on the upper surface of cabbage leaves.

Newly emerged diamondback moth larvae burrow into leaves causing wiggly mines
Leaf mines from newly-emerged diamondback moth larvae

Near mature diamondback moth larvae are green and chew holes in leaves
Near mature diamondback moth larva

Newly emerged diamondback moth larvae burrow into leaves causing wiggly mines. Larvae are cream when young and grey/green to green as they grow larger. When disturbed, larvae wriggle forwards and backwards quickly. Older larvae chew holes in leaves of brassica plants.

Plants on left damaged by diamondback moth larvae in an untreated plot
Plants on left damaged by diamondback moth larvae

Feeding by diamondback moth larvae on the cauliflower curds makes them unmarketable
Cauliflower curd damage by diamondback moth larvae

Heavy infestations of diamondback moth larvae can retard plant growth. Large larvae can feed on the curd of cauliflowers making them unmarketable.

Diamondback moth cocoon within a silk sleeve produced by the larva. Photo courtesy Mike Furlong

When fully fed, larvae of diamondback moth spin a silk case and form a cocoon within it. Cocoons are pale when young but become brown as they get older and before the moth emerges.

Management

Feeding by diamondback moth larvae can slow the growth of young plants and makes older plants unsaleable. Infestations are most destructive when plants are young.

Diamondback moth larva and pupae killed by a fungal disease
Diamondback moth larva and pupae killed by a fungal disease

Diamondback moth eggs and larvae are attacked by generalist predators such as spiders, ground-dwelling beetles, ladybeetles and lacewings, and insect-killing fungal diseases.

Diadegma wasp laying an egg inside a diamondback moth larva. Photo courtesy Mike Furlong
Diadegma wasp parasitising a diamondback moth larva. Photo courtesy Mike Furlong

The introduced parasitic wasps Diadegma semiclausum and D. collaris have established in highland regions of Indonesia. Diadegma also occurs in Western Australia.

These insects help control the pest but they are killed when broad-spectrum insecticides are used. 

Diamondback moth has developed resistance to all insecticides, including Bacillus thuringiensis (Bt). Not all diamondback moths in all regions are resistant to insecticides but if any insecticide is used repeatedly resistance may develop. The most effective approach to managing diamondback moth is integrated pest management where key natural enemies such as Diadegma are conserved. When insecticides are required, select products that have a range of active ingredients to apply during a crop cycle to slow or avoid the development of insecticide resistance. An example of this is the strategy recommended to vegetable brassicas growers in Western Australia.

Monitor for natural enemies by holding diamondback infested cabbage leaves in jars with gauze covers for three weeks. Adult wasps like those pictured will emerge.  The relative presence of wasp parasites and diamondback moth adults shows how many wasps are present in the cabbage crop.

Monitor crops weekly for diamondback moth by checking 50 plants across the crop.

Checking for eggs provides a good indication of the potential pressure from larvae. Only plants with larvae should be recorded as infested, because insecticides do not kill eggs.

Do not attempt to control moths with insecticide.

Spray when more than 10% plants are infested with diamondback moth larvae. This would be five of 50 plants infested.

Destroy crop residues as soon as possible after harvest.

Acknowledgment

Funding for this work to support Indonesian potato farmers and WA seed potato exports was provided by ACIAR (the Australian Centre for International Agricultural Research) and the Department of Primary Industries and Regional Development, Western Australia.

Contact information

Stewart Learmonth
+61 (0)8 9777 0167

Diamondback moth: cabbage pest in Indonesia and Western Australia

Author

Stewart Learmonth