Chaetocnema Stephens, 1831
This cosmopolitan genus includes about 450 species and is well-represented in all the major biogeographical regions. The Palaearctic fauna is diverse throughout; of the 75 or so species more than 40 are recorded from China while about 40 are known from Europe although here the highest diversity is in the south, nonetheless about 25 occur in central and northern Europe and of these 8 are listed from the UK although one of these, C. aerosa (Letzner, 1846) is now very probably extinct there. More than 75 species occur in the Oriental region and about 150 are known from the Afrotropical region, by comparison the Australian fauna is comparatively poor with about 25 species. About 36 species occur in the Nearctic region while more than 100 are recorded from the Neotropical region. New species are regularly described, especially from the tropics but the eastern Palaearctic region is likely to produce many more. The European fauna is among the most thoroughly studied, at least to date, but the genus is popular and revisions regularly occur from individual countries as well as regions. In this respect there is a wealth of information available on line. To the generic level, at least in northern temperate regions, they are easily identified but species are often very difficult and sometimes a little variable so that dissection is frequently needed for reliable identifications. Fortunately very good diagrams and pictures of both sexes are available most of the European species.
All are small,<3.0 mm, elongate-ovate although some e.g. C. aerosa, are short and very broad. They are glabrous and dark metallic bronze, coppery, bluish or greenish but beyond our UK species many have a dark metallic forebody and bright yellow or red elytra with only the suture or the humeri darkened, and in some the black forebody contrasts with metallic blue or green elytra. They may be distinguished from all other European genera by the form of the middle and hind tibiae; here there is an external blunt tooth or projection before a subapical emargination. Useful features for identification of UK species are the form of the forehead; in C. concinna (Marsham, 1802) and C. picipes Stephens, 1831 there is a raised vertical keel between the antennal insertions while in our
other species this area is flat, the form of the transverse frontal furrow and the extent and form of the punctures across the head are also important. The pronotum is usually simply convex, it lacks a transverse basal impression (their may sometimes be the slightest hint of one in some species), and most lack laterobasal fovea. The elytra are strongly punctured and in some they form regular striae but in many these are confused or interrupted for at least part of the length. The front and middle femora are normally developed while the hind femora are greatly enlarged, and only the hind tibia has an apical spur. Males may be distinguished by the dilated basal front and middle tarsomeres. Because of the form of the tibiae these species are easily recognized and the genus is usually well-represented in collections, but specimens can be ambiguous, especially regarding the form of the elytral striae, and reference material is very useful here.
Host plants include a wide range of species from Poaceae, Cyperaceae, Chenopodiaceae, Convolvulaceae, Malvaceae, Polygonaceae and Combretaceae (and no doubt others in tropical regions) but our UK species are associated with either various Polygonaceae and Amaranthaceae (picipes and concinna), or with grasses, sedges and rushes (the remaining species). Of the UK species only the rather polyphagous C. concinna (or possibly picipes as well) has been classed as a pest, here of Beta vulgaris, hence the occasional vernacular of mangold flea beetle. More widely, pests of commercial horticulture include the Holarctic C. confinis Crotch,1873 (Sweetpotato Flea Beetle), the Nearctic C. denticulata Illiger, 1807 which sometimes attacks corn crops, and the almost cosmopolitan C. pulicaria Melsheimer, 1847 which also attacks corn crops and may be an occasional important pest of clover crops. So far as is known the species are univoltine with adults overwintering and breeding in spring and summer. Larvae develop inside roots or low down in stems and move into the soil to pupate during the summer and new-generation adults appear over a long season during summer and autumn.