Galeruca tanaceti (Linnaeus, 1758)
This is a generally common species throughout the Palaearctic region from Portugal though Europe and Asia Minor to China and Japan, extending north to the UK and beyond the Arctic Circle in Fennoscandia and south into northwest Africa and Iran. Here it occurs throughout England and the west of Wales; it is most frequent in the southeast and around the Humber and generally rare and sporadic further north to the Scottish Highlands, Orkney and the Outer Hebrides. Adults occasionally occur early in the year but these will have overwintered, they become common from mid-June when the new-generation emerges and remain so into October or November. The typical habitats are open grassland and heathland hosting a variety of vegetation, and the beetles usually appear in numbers over a small area; after emerging they feed for a while on foliage before the majority aestivate in tussocks or under debris during the warmer part of the summer, becoming active again during late August to continue feeding and begin mating. Host plants include cuckooflower, Cardamine pratensis L., common knapweed, Centaurea nigra L., species of Cerastium L. and Veronica L., devil’s-bit scabious, Succisa pratensis Moench, and Yarrow, Achillea millefolium L., adults feed on foliage but may switch when the usual hosts are unavailable e.g. they have been observed feeding on potato foliage, and on the continent they have been reported as a pest of cultivated Oregano. Mating occurs from August and hugely distended gravid females are soon obvious as they roam the host stems looking for oviposition sites; eggs are laid in batches of about seventy in a soft, pale yellow case attached to a grass or host stem, the case soon hardens and turns dark and the eggs enter a diapause, they will remain within the case through the winter and spring and larvae will emerge from early-May. Larval development is rapid; they feed in numbers openly on foliage, pass through four instars and are fully grown within four to five weeks. When fully grown they descend the stems and enter the soil to pupate in a cell beneath the host, this stage is also brief and new-generation adults appear from late May. Adults generally remain active until October, when the majority will die-off, but at least some will go on to overwinter, they disperse by flight and may be attracted to light. Both adults and larvae may be predated by the ground beetle Lebia cruxmajor (Linnaeus, 1758).
At 6-12mm and entirely black this species should be quite distinctive among our fauna. Head transverse with relatively small and weakly convex eyes, strongly and densely punctured throughout, entirely black although the vertex may be brown even in mature specimens. Antennae inserted between the anterior margins of the eyes, and separated by less than the length of the basal antennomere. Pronotum transverse and widest behind the middle, lateral margin constricted before prominent and slightly obtuse anterior angles, posterior angles almost perpendicular and the basal margin straight, surface strongly and quite densely punctured; in places the punctures separated by about their own width. Scutellum large, broadly-triangular and punctured as the pronotum. Elytra very broadly-oval, more so in the female, with rounded shoulders and a continuously-rounded apical margin, lateral margin explanate and bordered from beneath the shoulders almost to the apex. Elytral surface strongly, and in places confluently, punctured; these are mostly random although there is a tendency to form irregular striae and in many specimens there are two or three weakly-raised and almost complete longitudinal keels. Legs entirely black, including any apical spurs or spines. The sexes are usually obvious from the broader form of the female but males differ in having a short spur on the mid-tibiae and the apex of the last sternite deeply emarginate; females lack this spur and have the apical sternite simple rounded.