Lema cyanella (Linnaeus, 1758)
This is the only European member of the otherwise extensive genus Lema Fabricius, 1798; it is a generally common species extending from Spain to Turkey in the south and from Scandinavia through Siberia to China, Japan and Korea. There have been several attempts to introduce the species as a biocontrol agent against invasive thistle species e.g. in Canada and New Zealand but these have generally been unsuccessful despite the species being very fecund with each female laying up to 1500 eggs. In the UK it is widespread and locally common throughout England and Wales but so far absent further north. Host plants include various thistles e.g. cabbage thistle, Cirsium oleraceum (L.), marsh thistle, C. palustre (L.), spear thistle, C. vulgare (Savi), plumeless thistle, Carduus crispus L., musk thistle C. nutans L. and silver milk thistle Silybum marianum (L.). Adults are present year round, they are active from March to October, peaking in June and August/September, and are likely to be found in a wide variety of habitats where the host plants thrive e.g. wetland margins including river estuaries and salt marshes, woodland, parkland and wasteland, agricultural borders, roadsides and railway embankments etc. Adults appear from their overwintering sites early in the year and feed upon host foliage for a while before mating and commencing oviposition from May onwards. Single eggs, or sometimes pairs, are laid among foliage and larvae hatch after about 10 days, they usually cover themselves with a layer of excrement and debris as protection from predators and parasites and they feed openly above and below the leaves, consuming only the upper epidermis and leaving irregular translucent patches which may become extensive and cover entire leaves, and they are fully developed after about 3 weeks. Fully grown larvae ascend the stems or drop to the ground where they pupate in a cocoon within the soil among the host roots, this stage lasts about 2 weeks and new generation adults appear from late July or August. It is thought that a second generation may sometimes appear in the UK, producing late summer adults which will feed alongside the earlier summer generation to build up reserves before overwintering among litter or tussocks etc, often in marginal environments such as under hedgerows or on agricultural headlands. Adults may be swept from the hosts throughout the milder parts of the year, in the summer often appearing in numbers and sometimes remote from the host as they disperse by flight, through the winter they will occasionally appear among appropriate sieved or extracted samples.
Although very distinctive this species will need to be examined very carefully in the field as some species of Oulema des Gozis, 1886 are superficially similar; Lema is distinguished by the transverse pronotal furrow and lateral constrictions being towards the centre, in Oulema they are towards the base, this will become very obvious with a little experience.
3.5-5.0mm. body entirely dark metallic blue, underside entirely black, appendages black with a faint metallic reflection. Head transverse with very convex eyes, short, basally narrowed temples and vertex with various impressions and fine punctation. Antennae long and robust, the basal segment rounded internally and the second small, remainder elongate. Pronotum coarsely punctured except for a variable medial longitudinal smooth area, laterally rounded from the constriction just behind the middle, anterior margin rounded and posterior angles sharply acute. Elytra much broader than the pronotum at the base, with broad rounded shoulders, sub-parallel lateral margins, and continuously rounded apex. Striae, including an abbreviated scutellary striole, consisting of rows of discrete and in places elongate punctures but otherwise only weakly, if at all, impressed, interstices flat or only weakly convex, finely rugose in places and generally impunctate. Legs long and robust; all tibiae with two spurs on the inner apical angle, and all claws fused at the base.