Host of the month’ is a series of Blogs and PDF’s that highlight a tree host and their associated priority pests and diseases that are best seen and recorded in that month. For October, we're looking at maples, Asian longhorn beetle and Citrus longhorn beetles.

Maples are members of the Sapindaceae family, which also includes Horse chestnut species (Aesculus). There are around 132 species worldwide including field maple (A. campestre) which is native to the British Isles. Sycamore (A. pseudoplatanus) and Norway maple (A. platanoides) are non-native but are widespread and common. Species such as Japanese maple (A. palmatum) and red maple (A. rubrum) just two of the many species which are planted for their vibrant autumn colours.

Most species have the familiar palmately lobed leaves though there are some with trifoliate leaves such as box-elder (A. negundo) or even simple leaves such as hornbeam maple (A. carpinifolium). However, the leaves of all maple species are arranged in opposite pairs on the shoots and they all have winged seeds called samaras.

Asian and Citrus longhorn beetles

Asian longhorn beetle (ALB) and Citrus longhorn beetle (CLB) are native to eastern Asia and have now been reported in the USA and across Europe. After the capture of a single beetle in 2009 an outbreak was discovered at Paddock wood in Kent in 2012. The main host species there was sycamore with one tree at the centre of the outbreak having 498 exit holes! Following the felling of 2229 trees and extensive monitoring the eradication programme was declared successful in 2012. The main entry pathways are wooden packing materials (ALB) and live plants (CLB). Neither are currently known to be present in the British Isles.


Adult ALB and CLB are very similar in appearance, both are 2 - 4 cm long and glossy black with pale yellow or white markings. The segmented antennae are longer than the body and are also glossy black but with light blue-white banding. Adult females chew ‘egg pits’ into the surface of host trees, often near branch junctions. The uniformly pale cream-coloured larvae hatch and tunnel into the stem, eventually growing to 3-6cm in length. As they grow they chew out tunnels which can be 30cm long and 1 cm in diameter, and these disrupt the movement of water and nutrients in the tree. Once mature the larvae pupate, emerging as adults via 1cm diameter perfectly circular exit holes.

Because much of the life cycle of these beetles feed within the tree with few external symptoms until infestations reach very high levels which makes them relatively difficult to detect. However, there are six symptoms that may be seen: crown/branch dieback, the characteristic circular exit holes, egg-pits, frass, bark cracking and adult feeding damage.

Several other insects with wood boring larvae can produce symptoms similar to those caused by ALB and CLB. Goat moth, Leopard moth and Large poplar beetle are all native species that have woodboring larvae that are found in broadleaved trees. The Timberman beetle has extremely long antennae and could be mistaken for ALB/CLB, but the larvae are found only in dead or dying Scots pine trees rather than broadleaves.

For more information on the symptoms of Asian and Citrus longhorn beetles and lookalikes see their Observatree resources pages or the host of the month. You can also test your knowledge with the Host of the month Quiz.


ALB and CLB are notifiable pests so if you find them you must report them. Please report sightings via TreeAlert.