Anoplophora nr versteegi
Why is it a problem? Stem-boring larvae can cause tree decline, mostly by ring-barking of younger trees. Pest populations are slow to build up and are relatively easily managed. Pesticides are not needed.
Where and when is it a problem? The beetle is present in all citrus growing regions, but is mainly a problem in poorly managed or neglected orchards.
Adults are 2.5 to 4 cm long with antennae that can be up to 8 cm long. The beetle is ash-grey with transverse rows of small black spots. They can be confused with other beetles that feed on the woody remains of trunk-borer feeding. Eggs are the size of small rice grains. Newly-emerged larvae are small and white with black mandibles and reddish first segments.
Eggs are often laid in the bark from ground level to 2.5 m, but most commonly in the first 0.6 m. After eggs are laid into the bark a little fluid comes out of the bark. Newly emerged larvae start feeding on the layer between bark and wood. This can be seen on the bark surface as tiny wet or swollen spots. This is the stage where most damage is done to the tree, especially if feeding is on young trees as it can ring-bark the tree, killing it. Infested branches can die off and large trees can be killed if attack is prolonged. After some weeks of feeding on the layer between bark and wood, the larvae tunnel into the heart wood. Their tunnels become larger and large quantities of sawdust-like frass come out of the feeding holes. The entrance hole offers easy access to the microorganisms that may damage the tree.
Confusion with other pests: The citrus trunk borer is the only pest in citrus, although other beetles have been recorded. Other trunk-borers are pests of apple trees, walnut trees and other fruit trees.
Lifecycle: Adults are active during the day and feed on citrus leaves and bark of twigs and branches. Between June and August female beetles make tiny holes with their mouthparts in the bark of the trunk and larger branches of citrus trees in which they place 1-5 eggs just under the bark. Larvae hatch and initially feed in the upper sapwood before tunnelling into the centre of the stem. The larvae pupate in the wood and emerge in April-May. The exit holes are round and circular. The life cycle probably takes two years.
Dispersal: Adults are winged.
When can damage be expected? The lifecycle long so populations build up slowly. Most feeding damage is observed during the growing season when larvae are active.
Hosts: The citrus trunk borer appears to attack only citrus species.
Larval damage can be easily seen and managed without the need for pesticides.
Inspect orchard fortnightly between April and July. Look for dead or dying stems, adult beetles and signs of larval feeding: small swellings on the bark on which drops of liquid can be seen (small larvae), and larger holes with sawdust-like frass (excreta) being pushed out (large larvae).
Boring can be in the first half a metre of the trunk. Basins around trees therefore need to be regularly weeded so that signs of early damage are not hidden.
Effect of variety
- The citrus trunk borer is relatively easy to control.
- Concentrate control efforts on finding and killing younger larvae, recognized by tiny wet or swollen spots on the bark. This is the most damaging stage. When signs of presence are found, scrape the bark open with a small knife and kill the small larvae.
- Larval holes with fresh frass can be poked with a wire to kill the larva. Plug holes the holes with some cotton soaked in petrol (or diesel). Close the hole with mud, preferably mixed with cow dung. Treated trees have to be monitored and if fresh frass is still produced this treatment has to be repeated.
- Irrigate and fertilize trees according to best-practice so that they can compensate for trunk borer damage.
- Make sure that the tree only develops one trunk. This will facilitate checking the infestations later.
- Let children collect adult beetles in April to May and reward them, e.g. with a sweet for every beetle caught. This proved effective in Am Shing village under Gomdar Gewog.
- Remove and burn heavily affected, unproductive trees.
The use of pesticides is not needed. Contact NPPC for advice if a serious outbreak occurs that can’t be managed through cultural control.
Image acknowledgements: NPPC.