Asian and citrus longhorn beetles have larvae that feed within the trunks, branches and larger roots of a wide range of deciduous trees and shrubs. Whilst most native wood boring insects only feed within already dead or dying trees these beetles can affect healthy trees. The tunnels created by the larvae can cause substantial damage. They are native to China, Japan and other parts of south-east Asia. Citrus longhorn beetles have been moved around the world in ornamental trees imported from Asia, especially in Japanese maples from China. The main entry route for Asian longhorn beetle has been in timber used for pallets and wooden packing cases sent with goods imported from east Asia. These beetles pose a serious threat to horticulture, forestry and native trees in Britain.  

Citrus longhorn beetles have been intercepted in Britain on imported Japanese maples. An outbreak of Asian longhorn beetle at Paddock Wood, Kent detected in 2012 was declared eradicated in 2019. 

Longhorn beetles belong to the beetle family Cerambycidae, there are about 70 species that are found in Britain. None of these native or naturalised species is considered a problem as the larvae of most feed on dead or already dying trees. Longhorn beetles can be large and colourful and many are in decline. The native species are an important part of biodiversity and many help recycle dead wood. Providing this habitat in gardens can help support some of these declining species. 
Find out more about longhorn beetle at https://www.ukbeetles.co.uk/cerambycidae.   

Gardeners may become aware of the presence of these longhorn beetle either when they notice the adult beetle or find exit holes near the base of infested trunks or exposed roots with citrus longhorn, while exit holes of Asian longhorn can be much higher up a tree in the trunk or branches.

• Citrus longhorn beetle is a large (21-37mm long) black beetle with white markings on its wing cases
• The antennae are particularly distinctive because they are up to twice the length of the body and are black with white/light-blue bands
• The Asian longhorn beetle is very similar in size and appearance 
• Some native (non-pest) longhorn beetles have banded antennae but they are not black with white spots
• The legless larvae are creamy white with pale brown heads and are up to 56mm long

The larvae of citrus longhorn beetle make tunnels in the lower trunk and roots of host plants for two years or more. There may be no external signs until the adult beetle emerges, leaving a circular exit hole 6-11mm in diameter. These are typically found near the base of trunks or on exposed roots. Larvae of Asian longhorn beetle develop higher up in the trunks and branches of many deciduous trees. The tunnels created by the larval feeding make trees susceptible to disease and wind damage. The adults cause limited damage by eating foliage and young bark.

Unlike most native wood boring insects citrus and Asian longhorn beetles can kill trees and shrubs.

Asian and citrus longhorn beetle larvae can cause serious damage to healthy trees, both in gardens and in the wider landscape.

Any findings must be reported to the relevant plant health authority or via Tree Alert, contact details can be found on the UK Plant Health Information Portal. 

Most native wood boring insects only feed on and within already dead and dying trees. Dead wood is a valuable habitat for many species of invertebrate and can be a valuable part of a healthy garden. More informaton on dead wood in gardens.
 

If either of these beetles is discovered, the relevant plant health authority will advise what steps must be taken to deal with them. They are difficult to control because the larvae and pupae are concealed and protected within the trunk, branches or larger roots. Currently, the only effective way of controlling larvae is to fell affected trees and destroy them by chipping, burning or deeply burying trunks and roots.