Dissemination and Vectors

Long range

It is regional or global trade that increases the risk of invasion of new pests (Brasier 2008; Hemery et al. 2010) and pathogens (Rytkönen et al. 2008, 2011; Santini et al. 2013; Müller et al. 2016; Ghelardini et al. 2017; Liebhold et al. 2017). It is possible that the threat to Ulmus spp. has risen due to the transportation of infected elm seedlings, plants or timber, similarly to what has happened in Sweden, Norway, the UK and the USA (La Porta et al. 2008; Brasier and Kirk 2010; Solheim et al. 2011; Menkis et al. 2016b).

Climate change, also unusual fluctuation of temperatures and heavy rains (Roloff et al. 2009) have an impact on trees making them more susceptible to pests and diseases (Hanso and Drenkhan 2013; Bentz and Jönsson 2015; Ramsfield et al. 2016) in forest ecosystems, as well as in urban areas (Sturrock 2012).

Short range

Bark beetles are essential agents in spreading of DED pathogens (Coleoptera: Curculionidae, Scolytinae); however, if they are not in association with fungal pathogens (Wingfield et al. 2016), they are minor pests. Scolytid species are distributed worldwide (Heliövaara and Peltonen 1999), the number of the species is increasing from north to south (Nikulina et al. 2015) depending on suitable tree species (Heliövaara and Peltonen 1999).

DED is spread in North America by three species of bark beetles:

·  The native elm bark beetle, Hylurgopinus rufipes (Jacobi et. al 2013, Webber 2000).

·  The smaller European elm bark beetle, Scolytus multistriatus (Jacobi et. al 2013).

·  The banded elm bark beetle, Scolytus schevyrewi (Jacobi et. al 2013).

In Europe:

·  The smaller European elm bark beetle, S. multistriatus (Webber 2000)

·   Large elm bark beetle, S. scolytus. (Webber 1990, Waller 2013)

·   Other reported DED vectors include Scolytus sulcifrons, S. pygmaeus, S. laevis, Pteleobius vittatus and Р. kraatzi, S. triarmatus, Xyleborinus saxesenii and Xyleborus dispar ((Anderbrant and Schlyter 1987a, Ижевский 2005, Santini and Faccoli 2013, Webber 1990, 2004, Menkis et al. 2016a, Jürisoo et al. 2021b, Jürisoo et al.  2021c).  

It has been argued that Northern Europe is protected from DED because bark beetles do not occur there (Caulton et al. 1998, La Porta et al. 2008, Santini and Faccoli 2013, Martín et al. 2019); however, warmer climate has probably extended the northern range of Scolytus spp., as recorded in Northwest Russia (Selikhovkin et al. 2020).

The life cycle of bark beetles passes mostly in the wood or secondary phloem where female beetles create a tunnel into the bark of dying or dead elm wood and lay their eggs (Kirisits 2007; Sherif et al. 2014). After hatching into larvae, the larvae feed on sapwood and inner bark and after maturing adult elm bark beetles fly to feed on twig crotches and the inner bark of healthy elm trees transferring DED pathogen spores on the surface of their body and in their gut (Webber 1990, 2004; Moser et al. 2010; Bernier et al. 2014) to xylem tissues (Sherif et al. 2017).

Although elm bark beetles are the primary vector of DED pathogen, the fungus can also spread from infected trees to healthy elms through grafted roots (Gibbs 1978), more frequently in the areas where elms are closely spaced (Sherif et al. 2014).