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Stem girdlers

  • Photograph of a Stem girdler
    Asynonychus cervinus, a weevil pest of citrus similar in appearance to one of the lychee stem-girdlers.
Scientific name

Orthorrhinus klugii, Euthyrrhinus meditabundus and Asynonychus sp

Description of adult

Lychee stem-girdlers are ring-barking weevils about 5 mm long with the elongated snout typical of the weevil family. There are at least three weevil species involved (Orthorrhinus klugii, Euthyrrhinus meditabundus and Asynonychus sp.). They are mottled grey and brown in colour and blend with the background of bark.

Immature stages

The weevil larvae are legless, creamy-white and have a brown head.

Life history

The adult weevil lays eggs under the bark at the distal end of the branch that it has previously ring-barked. Usually the weevil chews two rings about 2-3 mm wide and about 5 mm apart, around the circumference of a branch about 5-10 mm in diameter. After the eggs hatch, the resultant larva feeds as it burrows through the inside of the branch. Ring-barking of the tree prevents carbohydrates produced through photosynthesis in the leaves moving back past the ring barked area. This improves the quality of the nutrients in the branch for the larva. The larva eventually pupates in the burrow from which the adult weevil emerges some time later.

Distribution

In all lychee/longan districts.

Host range

Lychee as well as unknown native hosts.

Damage

Minor and sporadic pests.

Eggs of the stem girdlers are laid under the bark of branches and the larvae bore within the wood. An indication of infestation and of activity is the sudden death of part of a small branch (i.e. distal to the ring-barked area). Affected branches usually remain in position because of their small size but may break off in strong winds. Seldom are more than 1-2 branches per tree affected.

Control options

No satisfactory action level is available for these pests.

Cultural

Remove and destroy affected branches. Make the cut on the plant side of the ring-barked area.

Chemical

Nil.

Chemical registrations and permits

Check the Australian Pesticides & Veterinary Medicines Authority chemical database and permit database for chemicals registered or approved under permit to treat this pest on the target crop in your State/location. Always read the label. Always observe withholding periods.