Cassida viridis Linnaeus, 1758
This is the only member of the subgenus Odontionycha Weise, 1891. A transpalaearctic native distributed from Portugal through North Africa and Asia Minor to Japan, and recently discovered to have been introduced into Canada as early as 1974. Here it is locally common throughout England and Wales although sparse in the north and there are a very few records from Scotland. Adults occur year-round, overwintering in tussocks and moss etc. and becoming active from April to October or November. Host plants include various species of Mentha (mints) as well as a range of Lamiaceae including Nepeta (cat mint), Lycopus (gypsywort), Stachys spp. (woundworts), Galeopsis spp. (Hemp nettles) and Sativa spp. (clarys). Typical habitats include wetland and marginal environments where the hosts thrive, and here they may be swept from low vegetation in numbers. Adults spend a few weeks feeding upon host foliage and possibly also pollen before mating in April and May and ovipositing from May to July. Between 1 and 10 eggs laid in oöthecae which are stuck to stems or under lower leaves and covered with frass and leaf fragments, they hatch within 6 to 10 days and the larvae initially feed below the leaves, moving to the upper surface as they grow, they pass through 5 instars and development is rapid; they are fully grown within 4 to 6 weeks. The larvae are very distinctive; broadly oval and only weakly convex, greyish-green with darker margins and a row of 16 pale long branched spines along each lateral margin and relatively short and broad urogomphi. This species does not carry the ‘faecal shield’ used by other Cassida species for disguise. Pupation occurs from June to September, fully grown larvae move to stems and petioles and become attached by a secretion before they pupate, and this stage is also brief, generally lasting about a week, and new generation adults eclose from July to October.
At 7-10mm this is the largest of our tortoise beetles although smaller specimens overlap with several others that may reach 8mm. Only 2 of our species possess toothed claws; the present species and C. hemispherica Herbst, 1799 which is smaller <5.5mm and has the explanate elytral margins bordered with a line of distinct punctures.
A very distinct species which is hardly likely to be confused with any other; elongate-oval with the base of the elytra much wider than the base of the pronotum. Entirely matt green, fading to greyish-yellow in dry preserved specimens, the underside, including the head, is black but for pale abdominal borders. The lags are pale and the antennae are dark with sharply-defined pale basal segments. The extent of the dark underside colouration varies; in ab. nigriceps Fairmaire, 1851 it is extensively pale, and in ab. flaviceps Marseul, 1876 it is entirely pale, but U.K. specimens are typically mostly dark. On the continent the dorsal surface varies from typical green to dark brown and some specimens have the femora darkened, U.K. specimens are typical. The pronotum is transverse with rounded posterior angles and a weakly sinuate basal margin which is flattened in front of the scutellum; the punctation is random; rather fine and dense on the disc becoming larger and confluent laterally. Elytra smoothly rounded with wide explanate margins and rounded, projecting shoulders which are distinctly wider than the pronotum. The suture and the lateral margins are strongly bordered, and the punctation random, dense and strong.