What has made these marks in the wood of this tree found in our woodland this morning? You are looking at the marks in the centre of the picture, with the long horizontal one measuring about 5cm in length and the total width from top to bottom being around 6cm. I'm not going to say the species of tree, as that would give it away! Look out for the answer next week....
These marks are beetle breeding and feeding galleries made by insects underneath the bark of (in our woodland) English elm trees, Ulmus procera. And the beetle here is indeed the smaller European elm bark beetle, or Scolytus multistriatus, which is the vector (carrier organism) of the fungus which causes Dutch Elm Disease (DED).
The long horizontal gallery in the picture is where the female beetle has burrowed underneath the bark and laid her eggs. The smaller galleries radiating from the main central one are the feeding galleries made by the larvae. If you look closely, you can see the larval feeding galleries getting larger as the larvae grow.
In early summer when elm leaves are fully developed, the adult beetles emerge from underneath the bark of the host tree and move to the leaf crotches to feed. It is here that they mate, emitting pheromones (special chemicals) in order to attract the opposite sex. Female beetles then move to weak or stressed elm trees, where they burrow under the bark and lay their eggs in a large central gallery as in the photo.
The disease itself is caused by a fungus (or rather a group of three species of fungi) of the ascomycetes. The first recognised species is known as Ophiostoma ulmi, however now two other species are recognised - Ophiostoma novo-ulmi (a very virulent strain of the disease) and Ophiostoma himal-ulmi (which only occurs in the western Himalayas). The disease is spread when adult beetles emerge from infected trees and move to healthy trees to feed. As they feed, they make small wounds in the wood, where the fungal spores carried on the beetle enter the tree and cause infection.
The original DED causing fungus is a native of Asia and was introduced accidentally to Europe in the 1910's, reaching North America in 1928. The newer virulent strain was first recognised affecting elm trees in Europe in the 1940's.