‘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 December we’re looking at spruce (Picea species) and great spruce bark beetle.

Spruce are distinguished from other members of the pine family principally by their foliage which is needle-like and attached singly to the shoots via a short woody ‘peg’. The individual needles can be the same colour all over like Norway or Colorado spruce, or have distinctly differently coloured surfaces like Sitka and Serbian. Like other members of the pine family (Pinaceae) the male and female cones are separate but occur on the same tree. In all spruce species the female spruce cones are initially upright but after pollination they turn and hang down below the shoots. In common with all other Pinaceae each cone scale houses a pair of winged seeds, in spruce the wings are readily detached from the seeds.

There are 35 species of spruce around the northern hemisphere, but none are native to the UK. Norway spruce, native in mountainous areas of continental Europe, was growing here by 1500, but it’s Sitka spruce which arrived in 1831 which has become the most economically important conifer in commercial forestry. Figures from the National Forest Inventory in 2018 show that it accounted for 59% of production forestry in Wales, 58% in Scotland, and 26% in England.

Priority pest – great spruce bark beetle
Great spruce bark beetle (GSBB) is native to mainland northern Eurasia and has been expanding its range for the last 100 years or so. It was first found causing damage to spruce trees in the UK in 1982 in Shropshire and from there it spread first to Wales and western England and has now reached southern Scotland. It will feed on spruce, pines, and some other conifers but in the UK it appears to prefer spruce.

Mature beetles are 6-8mm long, and have a dark brown and black body with a covering of characteristic orange hairs. Female beetles tunnel into the bark of live trees to lay their eggs beneath the bark and when they emerge the larvae feed communally on the outer layers of live wood. The mature larvae pupate beneath the bark, eventually emerging through circular holes and dispersing. In the UK the complete lifecycle can take 18-25 months which leads to extensive overlap of generations and consequently any life-cycle stage can be found at any time of year.


From a distance one of the signs of GSBB infestation is dying and dead foliage on single branches or the very top of the tree as the tunnelling actions of larvae cut off water and nutrient flow. As numbers of GSBB increase the dieback spreads, eventually killing the whole tree. Up close the tunnelling action of the female beetles causes resin to ooze from the tunnelling site, much of it spattering down the tree in white streaks. However, directly around the tunnel entrance the resin forms volcano-like resin tubes and these are distinctive of GSBB presence.

For more information check the Observatree resource pages for great spruce bark beetle, or the Host of the Month for December. You can also test your knowledge with the Host of the month Quiz 



December is an ideal time to seek out spruce trees and see if you can identify any signs and symptoms of great spruce bark beetle. GSBB is a priority pest so please report possible sightings via TreeAlert. Healthy tree data is equally important so please do report those too.