D-Day for invasive tree-killing beetle threat to Scottish forests

Plant health experts gathered at Dawyck Botanic Garden in the Scottish Borders to fell and examine a century-old weeping spruce – the latest casualty of a Britain-wide infestation of the virulent pest, Dendroctonus micans (D-micans), more commonly known as the great spruce bark beetle.

The beetle tunnels into the bark of living trees to lay eggs and the larvae feed on the living woody material. This forms cavities which can weaken the tree and eventually kill it.

However, the fight-back has begun with the release of a specially bred biocontrol beetle – the rhizophagus grandis – which feeds solely on the D-micans.

The grandis, the result of 30 years of work by Forest Research, has an extraordinary ability to locate its prey without posing any threat to trees or native beetles.

Dr Matt Elliot, a plant health and biosecurity scientist at the Royal Botanic Garden Edinburgh said: “The D-micans beetle spread across Europe over the last century and has now entered the UK.

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“It is one of the top issues in forestry in Europe and now in parts of the UK. The population can build up to quite high levels and infest live trees and once that happens it can seriously hinder timber production.

Larvae of D-Micans (Image: NQ)

“The species is very hardy and can survive in the Arctic Circle, their only restriction is they need a temperature of above twenty degrees in order to fly.”

The D-micans lifecycle is not seasonal and it can take several years to build up a population. Once it arrives on a site it can take around three years before its presence is noticed and depending on the type of tree it could be two to three years before the tree dies.

There are a number of indicators of D-micans infestation. When conifers – including cedars, firs, pines and spruces – are attacked by the beetles, the trees flood the attacked area with sap from the trunk as a defence system.

The beetles, when they are coming out of the tree, go through the resin and form tubes which are a good indication you have these beetles present. As it progresses, the tree will start to defoliate and begin to look increasingly bare.

The beetles are specially bred by Forest Research and the adult grandis, also referred to as the bio-control species, hunts out the pest beetle. When they find the pest beetle larvae, it lays its own larvae close to it and when bio-control beetles hatch they eat the pest species.

It is estimated their use could reduce losses in a forestry setting to between 1-2%.

Dr Elliot said: “It works really well in forestry. With any bio-control, you need the numbers to increase so they always have something to feed on, so in a plantation of all the same species of tree, both the pest and the predator build up a population where they are self-sustaining and it’s very effective."

The century-old tree is felled (Image: NQ)

Anyone who sees signs of D-micans infestation can use the Tree Alert app, which is a pest reporting tool. If the sighting is positive, the bio-control species can be released to deal with it.

Despite gaining the name ‘great’, adult spruce bark beetles are only 6-8 mm long and 2.5-3mm wide. While the beetle is a common pest throughout mainland Europe, it is not native to Britain. It is suspected that D-micans was accidentally introduced with a consignment of imported timber in the late 1960s or early 1970s.

Once an invasive species, such as D-micans, enters a new region, it has a disproportional impact because the predators that would normally control it in its native range are not present.

The bark is removed from Picea breweriana showing evidence of the presence of Dendroctonus micans (Image: NQ)

This leads to population explosions. Examples of this are now commonplace. Emerald ash borer and bronze birch borer beetles have reached epidemic proportions in North America, and Asian longhorn beetle is having an impact in parts of Europe.

Dr Elliot said: “The key is spotting these things early so it’s really important that people make themselves familiar with the pests and diseases of trees before infestations get completely out of hand and it’s too late.

“Forest Research have a great website with loads of information on pests and diseases and regular checks on trees are something people should be doing,” he added.