Longitarsus dorsalis (Fabricius, 1781)
A widespread European species though common mostly in the south, from Portugal east to Ukraine, more local and sporadic north to the Netherlands and the UK but apparently spreading and increasing in abundance, e.g. formerly classified as highly endangered in Germany but now locally common and no longer considered at risk, it is also widespread in north-western Africa and the Canary Islands. In the UK it is widespread though of local occurrence across southern and central England north to Nottingham but absent from the West Country and most of Wales and very rare further north, and here as in northern Europe it appears to be spreading. Adults are present year-round, they are active from March until September with peaks of activity in the spring and autumn, they occur in open sunny situations with plenty of low vegetation, especially in chalky or sandy areas; grassland, scrub, pasture, roadsides, dunes, open woodland and wooded borders; around Watford they are not unusual in domestic gardens and other areas used by people. Host plants include various ragworts, especially common ragwort, Jacobaea vulgaris Gaertn., but also groundsel, Senecio vernalis Waldst & Kit, marsh ragwort, Jacobaea aquatica (Hill) and hoary ragwort, Jacobaea erucifolia (Guss.) as well as various other Asteraceae, adult beetles feed on the foliage, producing small holes while larvae develop in the roots. Mating occurs early in the year, usually on host flowers, and females oviposit over a long season, from March to July, laying eggs into the soil around the base of the host. Larvae develop within growing roots and become fully-grown by late summer, they pupate within the roots or in a subterranean cell and new-generation adults appear from September onwards; later adults coming from pupae that are still developing in October. Most adults have fully developed wings and can fly well and so may occur remote from suitable breeding habitats e.g. in parks or gardens, especially early in the year when they disperse and when they may be sampled in flight interception traps. Sampling is otherwise a straightforward case of sweeping the host plants although the beetles are active and vanish instantly by jumping, sweeping at night will also produce adults but here they are much less prone to jump and will sit on the net until encouraged to leave.
Among our UK fauna this species is very distinctive and should be obvious even in the field. 2-3mm. Head, appendages and underside black, pronotum orange or yellowish, often with one or several diffuse dark markings, elytra dark with broad pale lateral margins. Head smoothly convex, densely microsculptured though shiny and with a group of punctures beside each eye; eyes circular and convex. Pronotum transverse with finely bordered lateral and basal margins, surface with strong isodiametric microsculpture and fine punctures. Elytra with sloping shoulders and evenly curved to separately rounded apices, surface strongly and randomly punctured and with fine microsculpture, much finer than that on the head and pronotum so they appear shinier. The lateral pale border varies from about a third to a half the width of the elytra and includes the reflexed epipleura.