17. Scolytus Scolytus so good they named it twice

The Elm bark beetle Scolytus scolytus is probably one of the few beetles that the average joe/joanna can name.

Most people have heard of only a few beetles, Coleoptera do not really feature in the public's consciousness much, I asked members of my family to name as many beetles as they could, these were the 5 answers they gave me:  

Stag beetle,

Water beetle, 

Death watch beetle, 

Ladybird, 

and the notorious Elm bark beetle.

I thought they did really well, being as none of them are remotely interested in beetles at all. I was impressed the Elm bark beetle (called the Large elm beetle, there are more than one elm bark beetle) was on their list. To be fair, only the people over 60 y'o' mentioned the Elm bark beetle. So in honour of one of the (almost) famous UK beetles, this blog is dedicated to Scolytus scolytus.

I did not find an Elm beetle, but I did find evidence of it very easily, picture below;

The pic above left is a line of mostly live elms coming into leaf, on the right you can just see green shoots appearing, so definitely alive English Elm Ulmus procera

This is an older elm tree (around 20 years old) in the same line of young saplings, but this was long dead and the bark had fallen away, the marks under the bark are S. scolytus galleries, they look like boats with oars sticking out either side (Zahradnik. 2000)

Contrary to what you might think, the beetle didn't cause the elm to die directly, S. scolytus is a vector for disease, not a pest that kills trees directly.

S. scolytus has been in Europe for thousands of years, and the elm trees tolerated the burrowing the larvae make beneath the bark. What the trees couldn't tolerate in the 60s was a new fungus the beetle carried Ophiostoma nova ulmi, and it is this fungus that causes the elm tree to clog up its xylem tissue and eventually stops water and nutrients from getting to the tree (above ground).

The tree dies of thirst, not because of the burrowing.

                   

                                                    male                          female

Back in the late 60s the fungus began to make its mark, and over 90% of mature elms are no longer with us, however that is not quite the full story as Ulmus procera has a neat trick, it can sucker. The fungus kills the tree above ground, it doesn't kill the roots. The line of elms on either side of the foot path in my pic' were all young, but when their bark is thick enough to allow the beetle to support an egg laying gallery, the beetles move in, along with the fungus it carries with it, and the trees die (above ground), the trees then sucker up from the root, reach 10-20 years old and the cycle repeats itself.

There are in fact still millions of elm trees in the UK, just not many over 20!

Oliver Rackham looked into pollen counts from archaeological, peat and lake mud samples, around 1000 BC and 4000 BC Elm pollen dropped dramatically in the samples, It was suggested by some, this was human activity, stone age people chopping down trees?

Rackham pointed out (in a lecture I attended back in 1999)  how few hunter gatherers were in Britain 6000 years ago, way too few people to cut down the millions of elms required to drop the pollen count in mud samples to virtually zero? The archaeological pollen results showed a dramatic drop in elm trees over a very short time period (40 years) and a slow recovery (perhaps over 400 years).

He thought it unlikely that <50,000 Neolithic people could cut down and process >100 million trees with stone axes, in just 40 years, logistically it would be impossible, even if that was their (men, women and children) only daily task their entire lives. 

Also why elms? Neolithic people weren't idiots, they would definitely have known the elm's fibrous wood makes them the single hardest tree to cut down with a stone axe? The only part of a wooded ship that is made from Elm is the Keel, precisely because it is resistant to being scrapped on beeches, coral, rock.. and of course stone.

His suggestion was that perhaps fungal infection and recovery was cyclic, every 3000 years or so, this is an ongoing debate still. 

The question being do the remaining elms eventually, by natural selection, take a few hundred years for resistant trees to dominate again. and does the fungus take a few thousand years to alter enough to infect the resistant trees, a fungus and tree genetic arms race taking place over a 3000 year cycle?

Rackham, O. (2006) Woodlands. London. William Collins.

Zahradnik, J. (2000) A field guide in colour to insects. Prague. Polygrafia.