Rhizophagus bipustulatus (Fabricius, 1792)
This generally common species occurs throughout the Palaearctic region from Portugal and northwest Africa north to the Arctic Circle in Scandinavia and through the Near East and Siberia to the far east of Russia and China although in some Mediterranean regions e.g. Malta, it has been confused with the closely similar R. unicolor Lucas, P.H., 1849 and is now known not to occur there despite being included in the latest Catalogue of Palaearctic Coleoptera (Jelinek, 2007). In the UK it is widespread and usually the commonest species across England and Wales though more local and sparse in the West Country and further north, and in Scotland there are a few scattered records from the northern highlands. The typical habitat is under damp bark or among decaying wood on a wide range of both broadleaf and coniferous trees in all stages of decay, especially where they have become infected by fungus. Adults have a long season, appearing very early in the year, usually in groups under bark, and persisting into the autumn, they are known to feed on mycelia but are primarily predatory on eggs and larvae of various bark beetle larvae, especially those of Xyloterus domesticus Erichson, 1836, and they have been introduced to New Zealand as a biocontrol agent of Hylastes ater (Paykull, 1800) but have failed to become established. On the continent it is recognized as one of the most important predators of Tomicus piniperda (Linnaeus, 1758) and in Scandinavia it occurs in traps baited with pheromones of Gnatotrichus Eichhoff, 1869 (Scolytinae), but while the adults are thus beneficial to the forestry industry they are also known to transmit various species of Ceratocystis Ellis & Halst (1890), fungal plant pathogens. The larvae develop under bark and will occasionally consume bark beetle eggs and larvae but feed predominantly on mycelia. Adults are easily observed by searching under bark, in our experience they are often abundant under Salix bark in marginal habitats, and they may be active on the surface at night, especially around fungus, and they are frequently recorded from flight-interception traps.
Mature adults are readily recognized in the field by the elytral colour; black with humeral and subapical pale areas. 2.0-3.5mm. Elongate and discontinuous in outline, dark brown to black with pale appendages. Head and pronotum with strong elongate punctures. Head convex and smooth with small but prominent eyes, width across the eyes less than the widest part of the pronotum, antennae inserted laterally; 11-segmented, basal segment quadrate and broad, third segment elongate, remainder transverse, the terminal segments forming a wide and abrupt club. Pronotum elongate and widest in front of the middle, lateral and basal margins strongly bordered, surface with very fine cellular microsculpture. Elytra with prominent shoulders, gently curved laterally and separately rounded apically, each with a sub-humeral oblique depression (which may be difficult to appreciate without manipulating a specimen under strong light), and strongly punctured striae. Elytral interstices impunctate except for the occasional puncture towards the base of the first (sutural) or second interstice, the surface with granular microsculpture a little stronger than that on the pronotum. Legs short and robust; tibiae widened towards the apex, middle and hind tibiae straight externally and with at most a few small spines towards the apex. Internal margin of hind tibia angled in the male, straight in the female.