“Drones join battle against eight-toothed beetle threatening forests”

So wrote the BBC a couple of days ago. As an ecologist with a special interest in crunchy invertebrates the size of peppercorns, this was a disappointing headline. Replace “eight-toothed” with “bark” and you have a better start. [It does have eight teeth, but not for chewing; see below.]

This was always going to have a climate angle – but as usual our friends at the BBC miss the important background, and leave us hardly any better informed than when we started. This is a shame, because the story would have been a great opportunity to educate the public about the relationship between the beetles and the trees. Of course, the BBC writers are not specialists, and do not have time to dig beneath what they are being told by press release. Nevertheless, it’s a chance missed, and another case of reader beware: Gell-Mann Amnesia awaits on the next story you read.

The tale was of how a forest pest was colonising south-east Britain, and the arsenal of weapons being used against it, including…

…sniffer dogs, drones and nuclear waste models

It is said that the beetles have been eradicated from the south east thanks to these strenuous efforts.

The climate angle:

But climate change could make the job even harder in the future.

You already know why: drier summers, milder winters.

The spruce bark beetle, or Ips typographus, has been munching its way through the conifer trees of Europe for decades, leaving behind a trail of destruction.

Try hundreds, or thousands, of years, not decades. “Trail of destruction” is a nonsensical way of putting the results, suggesting that they leave a barren wasteland in their wake. Of course, they don’t. Think of it this way: if you destroy a human structure, it is permanently destroyed unless a human rebuilds it. If you destroy a tree, or a thousand trees, the forest grows back.

When trees are infested with a few thousand beetles they can cope, using resin to flush the beetles out.

Upon infestation in these numbers, the tree is dead.

“Their populations can build to a point where they can overcome the tree defences – there are millions, billions of beetles,” explained Dr Max Blake, head of tree health at the UK government’s Forest Research.

Billions of brown boring beetles! But he means in a forest, not an individual tree.

Since the beetle took hold in Norway over a decade ago…

It has been present since the last Ice Age relented. There just happens to be a recent outbreak.

I’m going to stop putting in quotes there, but rest assured, there are another dozen that could be quibbled with.

The Actual Facts

The European Spruce Bark Beetle, Ips typographus, as its name suggests, comes from Europe, and lives under spruce bark. It has an interesting way of overcoming the natural defences of a host tree. Trees exude resin when damaged, and this kills beetles. Under stressed conditions, the resin production is insufficient, and the beetles get a foothold. They release pheromones, which nearby beetles home in on. Emphasis here on nearby: they are not flying from the Ardennes to Kent on the promise of a 2 a.m. kebab. The beetles infect the tree with fungus. When the bark is effectively “ringed,” the tree has had it. By concentrating on one or few trees, the beetles can overcome the tree’s defences; if they all attacked different trees, maybe none would be successful.

The tree involved is Sitka spruce. As its name suggests, it is not a native of Kent, or even of Caledonian forest. It is native of the Pacific coast, and the fog belt there. A particular stronghold is the Olympic Peninsula of Washington State. It was introduced to the UK in 1831, as a timber tree, because of its incredibly rapid growth. [You would be unwise to use it as a Christmas tree, as the needles are quite pointy.]

In fact, there are no native spruces in the UK. The Norway spruce was native here before the last glaciation, but never made it back across the land bridge. It was introduced here in about 1500, first for novelty reasons, and then for forestry.

The upshot is that Ips typographus is not native here either. So we are talking about an alien-on-alien war, which we only care about because one of the aliens is a forest crop, and we don’t want it to die before we have time to kill it ourselves.

Forest Research’s entomologists were shocked to find spruce in Kent infested with Ips in 2018, despite checks at the port to intercept infested timber. The discovery meant a meticulous search of 4500 ha of forestry, the use of sniffer dogs, drones, nuclear fallout models, AI-scanned beetle traps, etc.

Now, I want you to conjure two images in your mind: one vision is of Kent, and the other is of the fog belt of the Pacific Coast. One is the misty home of Sitka spruce that reach 80 m, and the other is one of many places it has been planted by hopeful foresters.

Here’s how Hopkinson described the forest of the Queen Charlotte Islands after his visit in 1931:

The luxuriant growth of vegetation in the Queen Charlotte Islands is most striking. The forests are a jungle and to traverse them it is necessary in many places to cut one’s way through an undergrowth composed of such plants as red huckleberry, tall bilberry, thimbleberry, salmonberry, salal, and devil’s club growing eight or ten feet in height, not to mention young Sitka spruce and hemlock, springing up densely all around. Moss covers the ground and fallen trees. Beautiful ferns abound. Lichen hangs from the trees. On every side there is evidence of abundant moisture and a mild climate.

Hopkinson 1931, p.29

Now, I would like you to contrast that with your mental image of a uniform block of Sitka spruce poles with nothing under them but a brown carpet of needles, in one of the warmest and driest parts of the UK. Where would Sitka spruce thrive best?

This is how the Slovakian forestpests.eu describe the most susceptible trees:

Most at risk are spruce stands aged 60 plus, affected by wind and snowbreak damage or weakened by drought.

Of course, these are mostly referring to the European spruces that evolved with I. typographus, not Sitka. But the inference is obvious: Sitka spruce is not suited for Kent, as it’s too dry there. Drought-stressed trees cannot produce enough resin to fight off the bark beetles, and end up succumbing. With or without climate change, Kent is not the place to grow Sitka spruce.

You can go back a long way and find stories about Ips. In Sweden, there was a serious outbreak in the early 1930s following three severe storms in 1931 and 1932:

The catastrophe which laid waste the pine and spruce forests of central and southern Sweden was effected by three storms which swept that country in July and December 1931 and February 1932.

Chrystal, 1935

After the storms, the region’s spruces were attacked by Ips. A population survey of one area in Uppland found 1.3 million Ips per hectare. In other words: this stuff is not new, even if it is new to the UK. Is it new to the UK? Old beetle books by the eminences of the day, usually clergymen, make reference to records of I. typographus in Britain. It is stated to be rare, and in very old works is found under a different genus (Tomicus). Perhaps these records are of chance introductions, and the species was never established.

Naturally it is more likely to find fertile ground when vast quantities of planting are undertaken from 1900 onwards. A useful analogy is that of a fire, which burns out if there is no flammable material nearby. When a forest pest is surrounded by stands of its favourite food, it often thrives.

So there we have it: a tree threatened by climate change, and needing drones and dogs and human surveyors to protect it? Or a tree planted in the wrong place, and growing under semi-permanent stress?

Sitka spruce will do well in the wetter parts of the UK: Monumental Trees reports on one by Loch Awe, with a height of 60 m and a girth of 4 m, achieved in 110 years of growth. [I saw these trees thirty years ago, and they were mighty then. I have not seen the original giants on the Pacific coast, which I would really like to do.]

Oh, the eight teeth? As mentioned, they are not used for chewing. Rather, they adorn the elytral declivity (the back end, for non-aficionados. I presume they are used to backwards-push frass out of the borings). Here is page from a 100-year-old monograph on bark beetles “barkborrar” by Paul Spessivtseff:

Spot the eight-toothed bark beetle.

References:

Chrystal, R. Neil (1935). Bark-beetle outbreaks and their control: a review of some recent literature. Forestry IX, 124-131 [Archive.org]

A. D. Hopkinson (1931). A visit to Queen Charlotte Islands. Empire Forestry Journal, Vol. 10, No. 1, 20-36. [JStor]

Spessivtseff, P. (1922). Bestämningstabell över svenska barkborrar. Meddelanden från Statens skogsförsöksanstalt. [Google Scholar]

The featured image is from Rev. Canon Fowler’s 1891 masterpiece of beetles, The Coleoptera of the British Islands, Volume 5, Plate 179. Ips typographus is number 10, if the numbers are readable. If you really want to read it, you can find all 5 volumes on Archive.org.