Phloeonomus Heer, 1839
This is a large Holarctic and Old World genus of about 40 species included in 2 subgenera; 2 occur in the Nearctic zone and 9 are listed from the Palaearctic and of these 4 are widespread in Europe including 2 which extend to the UK. P. minimus Erichson, 1839, the only European member of the subgenus Phloeonomodes Smetana, 1964, is widespread in central and southern areas while P. sjøbergi Strand, 1937 is widespread in Northern Europe, and both might be expected to occur in the UK in the future. Both our UK species are very widespread across Europe; P. punctipennis Thomson, 1867 extending south into Asia Minor, and P. pusillus (Gravenhorst, 1806) extending into North Africa, both reach high latitudes in Fennoscandia and P. pusillus extends beyond the Arctic Circle. In the UK P. pusillus is widespread and locally common across the south of England and Wales, becoming more local and rare further north to the Scottish Highlands, P. punctipennis is the more common species across Wales and Southern England but north of the Wash it is very local and it seems to be absent from Scotland. The distribution of both species is to some extent uncertain because P. punctipennis was formerly regarded as a variation of P. pusillus but they are sufficiently distinct to have been keyed separately by Tottenham in the 1954 RES handbook.
All members of the genus are saproxylic with larval development taking place under bark and adults in general associated with a range of either deciduous or coniferous trees. P. pusillus is associated with various conifers and both larvae and adults occur under bark, usually in numbers and often where the tree is infested with bark beetles; on the continent it is often found in association with Tomicus piniperda (Linnaeus, 1758) (Scolytinae, a widespread UK species). P. punctipennis occurs on a range of deciduous trees, especially oaks and beeches but also poplar, birch and hornbeam etc, all stages occur under bark and adults may also be found among decaying xylem and, occasionally, among arboreal fungi. They will tolerate a wide range of conditions from closely fitting and sappy bark on recently fallen timber to lose bark filled with debris and frass on old and well-decayed logs, they often occur in numbers near galleries of bark beetles and we have found them on several occasions in numbers among populations of Siagonum quadricorne Kirby, 1815 under damaged bark on fallen oak trunks and old decaying Aspen logs. Adults occur year-round but seem to be most common from January until August. The larva is described HERE.
Adults are small, 1.5-2.0mm, and rather nondescript insects; elongate and parallel-sided with the head narrower than the pronotum and the pronotum narrower than the elytra, drab dark to pale brown with pale appendages and very finely punctured and microsculptured dorsally. Head broadest across convex and protruding eyes, vertex with distinct depressions anterior to well-developed and obvious ocelli, temples sharply angled immediately behind the eyes then strongly converging to a wide neck, clypeus produced and narrowed in front of the eyes. Terminal segment of all palps long and narrow, at most only slightly longer than the penultimate segment. Antennomeres 1-3 elongate, 4-10 short and progressively wider to a rounded terminal segment; 6-10 often abruptly more transverse than 4 and 5. Pronotum transverse, widest at or in front of the middle and sinuate to sharp posterior angles, anterior angles rounded. Surface rather flat, with a wide and shallow depression either side on the disc but without a median depression or line. Elytra quadrate to slightly elongate, with rounded shoulders and posterior angles, very finely and randomly punctured and without any trace of striae. Abdominal tergites strongly bordered and finely punctured and pubescent, punctation generally denser towards the lateral margins, usually 6 tergites visible beyond the elytral apex. Tarsi much shorter than the tibiae, terminal posterior tarsomere longer than 1-4 combined.
The small size coupled with the form of the pronotum and tarsi, and the very fine dorsal punctation will identify the genus among our rove beetle fauna, and the following key, adapted from Tottenham, should separate out species. Because they have distinct host preferences a knowledge of where a specimen was taken can be a great advantage.
-Elytral punctures distinct; shallow, wide and evenly distributed across the disc, visible at X40. Pronotal depressions deeper, wider and typically extending onto the anterior half.
-Elytral punctures much finer and variously only just visible among the microsculpture at X40. Pronotal depressions less extensive and deep; often, or usually, confined to the posterior half.