Scolytus multistriatus (Marsham, 1802)
Along with several other members of the genus, this is one of the principle vectors of the fungus Ophiostoma ulmi (Buisman) Melin & Nannf. (1934) (Ascomycota, Ophiostomataceae), and its various strains, which is better known as Dutch elm disease and which has been the cause of a catastrophic reduction in the number of mature elm trees throughout the beetle’s range. The beetle is native to the western Palaearctic region and is generally common throughout Europe except for the far north, it extends sporadically east as far as China and Mongolia and is otherwise widespread in the Old World, having been recorded from North Africa, India, Australasia and the Oriental region, there are a few scattered records from south and central America and it is widespread and common in North America where it was first recorded in 1909 and since that time has been responsible for the death of more than 40 million elm trees. In the UK it is locally common in south and central England as far north as Nottingham, very local and scarce in Ireland and Wales and absent from the West Country; it is likely to occur wherever elms are present although it can also infect various other broadleaf trees. All three of our main native elm species, English elm and smooth leaved elm (Ulmus minor Mill. and cultivars), and wych elm (U. glabra Huds.), are attacked by the beetle and larger trees are soon killed off by the introduced fungus but the beetles ignore saplings and small trees with insufficient girth to accommodate the larvae galleries, hence our elms may thrive in hedgerows and woodland borders, despite being susceptible to the disease, but they do not grow beyond a few metres in height. The disease first spread across the UK in the 1920s and destroyed about 40% of our elms but this invasion attenuated (probably) because many of our native trees were resistant to the species of fungus responsible, however a much more aggressive species was introduced in the late 1960s and was rapidly spread by Scolytus beetles, large elms died off in large numbers during the 1970s and by the early 1980s most of our larger trees had died. Without their larger host trees the beetles began to die out and by the 1990s they were scarce, but the root systems of many trees survived and produced abundant sucker growth so that in the late 1980s and 1990s our elms were common and often abundant and served to maintain beetle populations as they increased in size, a situation that persists into the 2020s. Because the present species can fly well and disperse over several kilometres it tends to occur in a wide range of habitats, and because it is smaller than the other main vector of the disease, S. scolytus (Fabricius, 1775), it
Scolytus multistriatus 1
© Lech Borowiec http://www.cassidae.uni.wroc.pl/Colpolon/index.htm
Scolytus multistriatus 2
© Lech Borowiec http://www.cassidae.uni.wroc.pl/Colpolon/index.htm
can utilize smaller branches and is often the first to infest expanding sucker growth in new situations. Despite the fact that the fungus is by far the most destructive disease of our native elms there is no doubt that, climate change allowing, it will persist in the UK for many years to come and that it will be maintained by our native tree stock, but large elms may once again feature in our countryside as resistant cultivars are being developed. Adult beetles have been recorded throughout the year; they emerge from bark early in the spring before leaves appear on the trees and fly to weakened trees or to smaller branches on healthy trees to begin maturation feeding, this usually lasts about ten days and consists of boring into healthy bark and producing tunnels a few cm long before they emerge and disperse to find suitable breeding sites. Mature females usually initiate an attack by releasing aggregation pheromones which attract both sexes to a potential breeding sites and mating begins soon afterwards, in the UK this proceeds into the summer so that new generation adults occur over a long season during spring and summer. Mated females bore short oviposition galleries into bark and deposit eggs in closely spaced lateral chambers along their length, this process takes up to three weeks, after which they emerge through the bark and fly off to find further host material. Larvae emerge within week or so and begin to feed on the inner bark and the outer layer of xylem, producing long wavy feeding galleries radiating perpendicular from the oviposition gallery, these galleries widen as the larvae increase in size and may be up to 150mm in length. Larval development is rapid; they feed on sap from phloem tubes and pass through five instars in about thirty days and when fully grown they bore galleries into the xylem in which to pupate. The pupal stage lasts about two weeks and fresh adults emerge from the bark after a short period of development and pigmentation. In favourable temperate climates there are usually two generations each year, the early generation begins in early spring to produce summer adults, and the second starts in the autumn with larvae overwintering within galleries and pupating in the spring. Both sexes introduce fungal spores into maturation feeding galleries and so are very efficient vectors of the disease, females also introduce them into oviposition galleries where they infest larval feeding galleries and are carried into pupal chambers so that emerging beetles of both sexes are infected before they emerge. Lifting old elm bark will usually expose the characteristic Scolytus galleries but by this time the adults are usually long-gone, although occasional dead specimens within old pupal chambers are not uncommon, and various forms of trapping are usually employed to sample them; they come to light and are common in suitably placed flight-interception traps, fermenting fruit etc can be a powerful attractant and may produce large numbers of specimens and pheromone traps are used for academic study, the very best way to see them is to search about the lower branches of old fallen trees at night as we did in our local park during the summer of 2019 when we saw large numbers of specimens among the bark.
This is the easiest of our UK species to identify due to the form of the second abdominal ventrite. 2.0-3.4mm. Members of Scolytus may be distinguished among our scolytids by the form of the front tibiae; the external margin is smooth and produced into a curved apical hook, and the present species is unique in having a projecting process on the apical margin of the second ventrite, this is obvious in lateral view and can be readily appreciated in the field. Entirely shiny dark brown, usually with the forebody darker than the elytra, appendages reddish-brown and the ventral surface dark brown to black. Head clearly visible from above, vertex and frons almost flat, finely punctured and with long creamy pubescence which is much denser in the male, eyes widely transverse, weakly convex and only narrowly visible from above, antennae inserted laterally in front of the eyes; 11-segmented with a 7-segmented funiculus and large and well-defined club. Pronotum quadrate or nearly so, almost parallel-sided in the basal half and curved to a slight constriction before the apical margin, basal margin curved and posterior angles rounded, lateral margins finely bordered throughout, surface finely and moderately densely punctured, more strongly and densely so towards the margins. Scutellum triangular, depressed across the base and raised towards the apex, the surface strongly punctured. Elytra narrower across the base than the base of the pronotum, widest about the middle and gradually narrowed to separately-rounded apical margins, striae deep and strongly punctured and interstices with rows of fine punctures. Apex of middle and hind tibiae produced externally, tarsi without widely lobed segments.