Baranowskiella ehnstromi Sörensson, 1997
POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
NANOSELLIINAE Barber, 1924
Baranowskiella Sörensson, 1997
As well as being the smallest European beetle this is the only European member of the otherwise diverse and very widespread Nanoselliinae Barber, 1924, a group which includes the smallest of all known beetles, Scydosella musawasensis Hall, 1999 from Nicaragua at about 0.3 mm. First described in 1997 by the Swedish entomologist Mikael Sörensson from specimens collected some nine years previous in Finland and Sweden by Bengt Ehnström, the species soon proved to be widespread in Sweden and across Northern and Eastern Europe generally, the present distribution includes much of Central and Northern Europe, including the UK, and it is very likely that it will spread throughout Europe and Asia according to the distribution of its host fungi. The species was discovered in the UK in 2015 at several sites in East Anglia and is now probably much more widespread although it is thought to have limited dispersal ability and will no doubt be restricted by the occurrence of its host fungi. Here the host is Phellinus conchatus (Pers.) Quél. 1886, a widespread fungus infesting various willows but mostly White Willow, Salix alba L., Crack Willow, S. fragilis L. and Goat Willow, S. caprea L., while on the continent it has also been recorded from P. punctatus (P. Karst) Pilat, 1941 which grows on various broadleaf trees and shrubs. Although willows are often associated with wetland habitats it seems that the only condition necessary for the beetles is the presence of host fungi and so trees in any situation should be examined. Due to their tiny size adult beetles need to be searched for very carefully with a hand lens, they spend much of their time head-first within spore tubes beneath the sporocarps (other members of the subfamily are known to feed on fungal spores and this is probably generally the case) and so may be very difficult to spot but when disturbed by heat or bright lights they will exit the tubes and walk about the surface, they usually occur in numbers and may be restricted to a particular area of the fungus and so very careful searching is needed. Spore tubes of infected sporocarps are often ‘plugged’ at the base, beneath where the adults occur, which suggests that populations may breed and be long-lived on such samples, the fungus is perennial, producing a new layer of spore-tubes each year and so a single specimen may provide a focus for local dispersal although flight has yet to be observed. Adults are present year-round and they always occur in numbers, a good X10 hand lens will be needed to find them but even so they are very easily overlooked, samples may be taken for extraction but this is destructive and so should be avoided.
Because of the small size adults are very difficult to manipulate and examine and diagnostic features tend to be very difficult to appreciate without slide-mounting specimens, on the other hand the small size and distinctive shape, coupled to their association with the host fungus, makes adults straightforward to identify. Length ~0.5 mm, width ~0.15 mm, body long, slender and discontinuous in outline, finely pubescent and entirely pale brown to orange or with the head darker. Head with large convex eyes and clubbed, 10-segmented antennae. Pronotum quadrate or slightly elongate, broadest near rounded posterior angles and narrowed to indistinct anterior angles, surface smooth and only weakly convex. Scutellum visible. Elytra elongate and evenly curved from rounded shoulders to separately rounded apical margins, very finely punctured throughout and without striae. Legs short and slender. The larva has been described by Coray and Siede (2014), who also give information on the ecology and phenology of the species.