February Pest of the Month: Diamond back moth

Name: Diamond back moth – Plutella xylostella

Hosts of the diamondback moth are buk choy, cabbage and other cruciferous vegetables.

Larvae feed on the leaves, stems, flowers and seed pods. Damage is seen as holes in the leaves, or ‘windows’ (‘see-through’ leaf with an unbroken upper leaf layer) where larvae have tunnelled into the leaf. When infestations are high, leaves may be skeletonised.

Eggs: Pale yellow, flattened and ovalshaped, and 0.5 mm in length.

Immatures: Larvae are pale-green or grey-green with a dark head and a darker green stripe running down the body. They are up to 12 mm in length.

Adults: Grey-brown with a white diamond-like pattern along the back where the folded wings meet (viewed when the adult is at rest). Adults are 10 to 12 mm in length.

Eggs are laid either in clusters of two or three, or singly on leaves and stems. More than 150 eggs can be produced by the female moth in a lifetime. There are four larval stages, the first two stages tunnel inside the leaf. While feeding in the ‘leaf mines’, the young larvae are protected from natural enemies and some pesticides. As they mature they feed on the undersides of the leaves. When disturbed the larva will wriggle backwards and sometimes drop off the leaf, as it falls, it spins a silken thread from which it becomes suspended.

Larvae are favoured by warm and dry conditions. In the wet season, mortality will be higher due to larvae being washed off the leaves as well as being affected by diseases associated with wet or humid weather. At maturity the larva pupates in a green cocoon which eventually turns brown and is surrounded by lacy white silk.

Development from egg to adult usually takes 14 to 21 days. Adults have the ability to migrate and disperse over long distances.

For further information please refer to the Pest ID Tool at: https://pestid.com.au/pest-insect/diamondback-moth-cabbage-moth

Note that the Pest ID tool is now available with free access to industry.

An initiative of the Nursery Levy funded National Nursery Industry Biosecurity Program (NY15004) and ‘Building the resilience and on-farm biosecurity capacity of the Australian production nursery industry’ Project (NY15002).