Why is it a problem? Larvae bore into stems causing twig dieback. This is most damaging in seedlings where the main leading shoot can be destroyed. Outbreaks are very easily managed without pesticides, provided trees are monitored.
Where and when is it a problem? Outbreaks are quite localised and rare. They have been reported in Paro and Thimphu valleys. Bumthang is considered to cool.
Adult beetles are about 2 cm long and are easily recognisable. They are reddish orange with long (13-15 mm), black antennae and two black spots on the prothorax (the segment between the head and main body). Larvae are orange with a dark head and reach 25 mm when fully grown.
When females are laying the egg they ring-bark the stem, causing the terminal end to die. The dead or dying leaves on the infested twigs make them very easy to spot. Larvae then enter the shoot before making a narrow tunnel downwards through the centre. Occasional holes are made to the outside through which frass (remains of feeding) is pushed out.
Confusion with other pests: Make sure that stem dieback is the result of a twig-borer, rather than something else such as boron deficiency. Damage cannot be confused with other forms of twig dieback, but in case of doubt identification can be confirmed by the feeding tunnel in the twig. Usually the larva can also be found by cutting the twig open lengthwise.
Lifecycle: In April/May the female bites a small part of the bark of the twigs of grown-up trees, or the main stem of seedlings, and lays her eggs in the cuts. The bark of the selected twig is further fed upon until it is ring-barked, causing the terminal section of the twig to die. Pupation occurs within the tunnel and adults emerge from the twigs in April/May. The full life cycle probably takes two years.
Dispersal: Adults can fly.
When can damage be expected? This species has a long lifecycle so populations are likely to take a long time to build up.
Hosts: It has only been reported from apple trees.
This pest is easily found and managed without the use of chemicals.
Inspect trees for twig dieback during winter-pruning and other periods of orchard management.
Effect of variety
- If noticed, then cut off the infested shoots and either burn or bury them as soon as possible to kill the larva.
- During winter-pruning make sure beetle-affected stems are cut off and destroyed.
Chemical spray is not effective as the larvae are well protected inside the woody shoots.
Image acknowledgements: NPPC.