Bitoma crenata (Fabricius, 1775)
POLYPHAGA Emery, 1886
TENEBRIONOIDEA Latreille, 1802
COLYDIINAE Billberg, 1820
SYNCHITINI Erichson, 1845
Bitoma Herbst, 1793
The only Bitoma to occur in the U.K. and one of the most widely distributed Palaearctic species; distributed from Spain east throughout Europe including Scandinavia, the Mediterranean Islands and Madeira, to Eastern Russia. It is generally common throughout its range although its status is uncertain in many areas where forestry is active. The species is now locally common throughout the Eastern United States following at least two introductions from Europe. In the U.K. it is generally common throughout England and Wales north to Yorkshire and there is a single modern record from the Carlisle area. The typical habitat is woodland, wooded parkland and gardens etc; around Watford almost anywhere with wood in the right condition. They will generally be found beneath the bark of dead or decaying trees, fallen boughs or logs etc. and, more especially, around the boundary of dead and living bark where there is still some sap. Oak and Beech may be the preferred hosts, and this is certainly the case locally, but they occur on a wide range of broadleaf species including Horse Chestnut, Sycamore, Lime, Birch and Willows. Fowler also quotes Pine and Fir. Adults occur year round and are gregarious, usually in small groups, but we have found hundreds together on the denuded wood and under the bark of a fallen Oak in Cassiobury Park during the winter. Often a fallen bough will host several small colonies. During hot days in the summer and autumn they have the unusual habit, for colydiinae, of being active on the surface of logs during the day; when disturbed they quickly scatter and vanish into cracks in the wood or among debris and remain hidden for some time. They are also active nocturnally and soon become obvious by torchlight. Both adults and larvae predate other subcortical organisms and may specialize on the larvae of various scolytids e.g. Ips accuminatus (Gyllenhal, 1827) and I. sexdentatus (Boerner, 1766). Full grown larvae are around 6mm long, parallel and depressed and white with a reddish tinge to some parts of the body.
Although small, the overall shape and the distinctive colouration of B. crenata will soon become obvious in the field. Teneral specimens, which occur late in the season, are entirely orange or pale brown but they generally occur among typical coloured specimens and the comparison will be obvious.
2.5-3.5mm. Head and pronotum black. Elytra black; each with 2 discreet orange or red macula, one in the basal half and one subapically, these may be expanded so as to unite at the suture. Head with dense, flat tubercles from the base to the front margin of the eyes. Clypeus microsculptured and sparsely punctured anteriorly. Antennae brown; 11-segmented with a 2 segmented club which may appear 3 segmented as segment 9 is expanded towards the apex so forming a transverse ‘cupule’. Insertions hidden beneath a wide canthus in front of the weakly convex eyes. Pronotum transverse; densely and rugosely punctured and with very fine, recumbent pale pubescence. Lateral margin rugose and evenly curved to obtuse hind angles and weakly produced front angles. Anterior and basal margins sinuate. Disc with impressions either side of two raised, longitudinal ridges. Scutellum small and densely punctured. Elytra depressed; lateral margins bordered and weakly sinuate, humerus rounded and produced forward. Alternate interstices raised to fine lines which bear pale, semi-erect setae; between these the striae are represented by two rows of shiny punctures. Legs pale red with the femora and tibiae darker in mature specimens. Tarsi 4,4,4. Claw segment longer than the rest combined. Claws prominent; weakly curved, smooth and without appendages.
BITOMA Herbst, 1793
This is a large genus of more than 100 species, including some known only from historical collections, and there are many more waiting to be described. Many more species are known from the fossil record. They have a worldwide distribution and are particularly diverse in Australia and New Zealand and many Pacific islands have endemic species. At least 14 species occur across North America and Canada. At least some species occur in forested regions throughout the world and a very wide range of host plants are utilized; generally broadleaf trees and conifers but also e.g. Yucca, Agave and cacti in the U.S.A. Some species are found on a wide range of trees while others may be confined to a particular species and this can be a guide to their identification where, outside of Europe, they can be difficult to separate. Many are common where they occur and may be found in large groups. Our U.K. species, B. crenata (Fabricius, 1775), is typical of many in having a red and black patterned dorsal surface. Some species show considerable variation in the dorsal pattern while others are unicolourous. Adults and larvae occur beneath the bark of damaged wood or among damaged plant tissue. They are predatory and many are known to feed on various Scolytid larvae.
Within the limits of the Palaearctic region the genus is generally recognizable. Further afield there is variation in body shape and colouration, and several other genera resemble them e.g. Microprius Fairmaire, 1868 from Europe, Africa and North America etc, Paha Dajoz, 1984 which is widespread, and Lasconotus Erichson, 1845 which is virtually worldwide.
They are generally small beetles, <4mm, and usually depressed but sometimes convex or even cylindrical (B. carinata (LeConte, 1863)). Head transverse or quadrate and narrowed in front of the eyes, variously punctured or with dense flat tubercles. The eyes are usually well developed and convex, rarely reduced (B. granulata (Blatchley, 1910)), have fine or coarse facets and are very finely pubescent. The antennae are inserted on the side of the head in front of the eyes and have a well delimited 2-segmented club although this may appear 3-segmented as the ninth segment may be expanded apically. All segments are very finely pubescent and lack sensory setae. Segment 3 is weakly elongate and slightly longer than 4. The pronotum is usually transverse, or, much more rarely, quadrate. The disc has longitudinal carinae and the lateral margins are to some extent explanate and finely toothed. The elytra are elongate and parallel sided, or nearly so, with longitudinal stria formed of rows of punctures and often with fine carinae in between. Procoxal cavities are open. Tarsi 4,4,4.