Diamondback moth

The diamondback moth (Plutella xylostella) is the most destructive insect pest of brassica crops across the world. Its host plants include:

• brassica vegetable and forage crops
• cruciferous weeds
• ornamental crucifers

Reliance on chemicals as a control measure for the diamondback moth has resulted in the moth developing resistance to many insecticides. Resistance to synthetic pyrethroid insecticides has been detected in populations of diamondback moth in all Australian states.

Since 1993, brassica growers in Victoria have had difficulty in controlling the caterpillars of diamondback moth and have experienced insecticide control failures. In extreme cases, damaged crops have been ploughed in and produce has been unmarketable (Figure 1).

Description

The adult is a small moth about 10mm to 12mm long (Figure 2). The male moth is dark brown with 3 white diamond-shaped patterns aligned on its back. The female moth is tan coloured and its diamond patterns are less distinct than those of the male.

Eggs are pale yellow and 0.5mm long.

There are 4 caterpillar growth stages:

• In the first 3 stages, caterpillars are grey-green with a dark head
• In the last growth stage, they're green with a green-brown head

Caterpillars grow to approximately 12mm in length (Figure 3).

Behaviour and life cycle

The adults are active at dusk and throughout the night. During the day, moths will fly from their resting places if plants are disturbed.

Female moths lay eggs singly or in small clusters on stems and both sides of the leaves. A female moth can lay more than 150 eggs during her lifetime.

The first and some second growth-stage caterpillars are leaf miners tunnelling inside the leaf. Subsequent growth stages feed on the underside of leaves or tunnel into the plant. Caterpillars that are disturbed from feeding will wriggle backwards rapidly across the leaf surface and may drop to the ground on silken threads.

Pupation occurs in an open-mesh cocoon (Figure 4).

The time taken to complete the life cycle depends on temperature. In Victoria, the life cycle of the moth will be completed in about 1 month in summer. During winter, the pest develops much more slowly and causes little damage. As the temperature increases in spring and summer, the moth goes through its life cycle more quickly and pest numbers build up.

In one year, the moth will complete 6 to 7 life cycles. Generations overlap throughout the warmer months of the year.

Damage

Caterpillars eat many small holes in the leaves of the host plants, often leaving the leaf epidermis (outermost layer of cells) intact, making a 'feeding window'.

Most damage is caused by the caterpillars tunnelling into the heads of plants such as cabbage and brussels sprouts. They also cause contamination of produce by pupating inside broccoli florets and cauliflower curds.

Seedlings of cruciferous forage crops and rapeseed may be destroyed by this pest and severe defoliation or pod grazing may reduce rapeseed yield.

Control

Chemical control

Chemical control is often one of the methods available for plant pests as part of an integrated pest management program. More information is available from:

• your local nursery
• cropping consultants
• chemical resellers
• the pesticide manufacturer.

For information on currently registered and or permitted chemicals, check the Australian Pesticide and Veterinary Medicine Authority (APVMA) website.

Always consult the label and Safety Data Sheet before using any chemical product.

It is difficult to get insecticide contact with the diamondback caterpillar because its feeding sites are under leaves and within the plant tissue. This inaccessibility, known insecticide resistance issues, means that a single chemical solution is not possible.

Biological and cultural control

Management of diamondback moth requires an integrated approach.

Biological control agents such as predators and wasp parasitoids of this pest are present in crops in low numbers and can provide some level of control.

The following integrated pest management methods should be used to control the pest:

• Start off with clean, healthy transplants.
• Understand the life cycle of the moth and recognise the caterpillar stages.
• Check brassica crops at least once a week during the warmer months of the year. Small caterpillars (dark head stage) are easiest to control.
• Control summer and autumn weeds (wild radish and Lincoln weed) to reduce survival of larval populations.

More information

• cesar PestNotes: Diamondback Moth
• GRDC Diamondback moth fact sheet