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Elm bark beetles (Scolytus sp.), along with the globalizing human action, are the responsible for the transmission of the disease on a large scale. They coexisted in balance with the elms, but the appearance of the pathogen established a new insect-pathogen combination in which both were winners and led to the loss of around one billion elms.
Seven species of elm bark beetles are present in the Iberian Peninsula, normally colonizing the phloem of weakened elms and wintering as larvae in galleries. In the spring, they become adults and emerge from the pupal chamber. If the elm was infected with the pathogen, these insects are carriers of pathogen spores in their body. After emergence, adult insects look for appropriate trees for breeding, but before they usually fly to the twigs of healthy elms for feeding. The spores are therefore placed within feeding grooves where germinate and grow into the vascular system of the tree. By expanding through the xylem of the tree, the fungus causes embolisms of xylem vessels.
There are differences in host preference by elm bark beetles, being Ulmus glabra and U. laevis less preferred than U. minor and U. pumila. Some organic compounds produced in the phloem of the first two elm species may be responsible for the deterrent effect to bark beetles.
Male of Scolytus scolytus
Male of Scolytus multistriatus
S. scolytus in pupal chamber
S. multistriatus; detail of the tegument