Phaedon cochleariae (Fabricius, 1792)
Watercress leaf beetle
This species has suffered a general decline throughout its range since the 1960s, it was formerly abundant in suitable habitats everywhere and was an occasional serious pest of commercially grown water-cress but while it remains locally common in many areas it no longer occurs in anything like its former abundance. Its range is rather patchy but includes most of Europe, from lowlands to about 2000m, north to the UK and middle latitudes of Fennoscandia, North Africa, Asia Minor and east to Siberia and Mongolia, it has also been recorded from Japan and, since its accidental introduction in the early 20th century, it has become established and widespread in the north eastern United states and eastern Canada. Typical habitats include wetland of all kinds and also damp grassland, woodland, moorland and coastal dunes, adults occur year-round and are active over a long season from early spring to late in the autumn, peaking in abundance in late spring and early summer, and overwintering among grass tussocks or under debris or bark etc. Typically the host plants include various Brassicas but especially water-cress (Nasturtium officinale Aiton, W.T., great yellow-cress (Rorippa amphibia (L.)) and garlic mustard (Alliaria petiolata (M. Bieb.), while away from wetlands they also occur on e.g. black mustard (Brassica nigra L.) and shepherd’s purse (Capsella bursa-pastoris (L.)) and have been recorded from a range of cultivated Brassica crops such as turnip (Brassica rapa var. rapa L.) and horseradish (Armoracia rusticana Gaertin, Mey & Scherb. Overwintered adults are usually common by early April, they spend a while feeding on host foliage and sometimes within flowers before mating occurs, this extends over a long season into the summer but the species is thought to be univoltine in northern temperate regions such as the UK, and oviposition also continues into the summer; eggs have been recorded in mid-August. Females chew small cavities into the underside of leaves into which they lay single eggs, and larvae emerge after a week or so and feed openly on the foliage. Larvae develop rapidly, they pass through three instars, each lasting about 14 days and when fully-grown they enter the soil and form a cell in which they pupate, both larvae and pupae have been recorded into September and new generation adults continue to appear into October. All stages may be found throughout summer and into the autumn, adults sometimes occurring in large aggregations where the foliage may be skeletonised and appear blue with the beetles, adults are also good flyers and so new habitats may be quickly colonized although they seem be continuing to decline and so may be absent from sites where they were formerly abundant. Sweeping suitable vegetation is the easiest way to sample adults and they also occur among winter samples of flood-refuse and reed and rush stems etc.
2.6-4.0mm. The size and general appearance are very distinctive among our UK fauna and the only possible confusion might be with our other members of the genus; broadly elongate-oval and entirely bright metallic blue in colour although bronze or purple specimens occasionally occur. Head narrowly visible from above, finely and moderately densely punctured, with relatively large convex eyes and the frontoclypeal suture weak or obliterated medially. Antennae inserted much further apart than the length of the basal segment, dark bluish-black with the underneath of the basal segments at least to some extent pale, segments 8-11 broader, forming a loose elongate club. Pronotum transverse, broadest across the base and evenly narrowed to a rounded anterior margin, the anterior angles hardly visible from above, surface evenly convex, lacking microsculpture and with a mixture of fine and very fine punctures throughout. Scutellum large and triangular and usually more shiny than the pronotum and elytra. Elytra finely bordered and strongly sinuate laterally, each with eight strongly punctured striae and ninth which is more sparsely punctured, the fifth not deepened at the base and often united with the sixth, interstices with scattered fine punctures, the sixth and seventh only weakly, if at all, raised inside the humerus. Legs black, tibiae without an external tooth, basal segment of all tarsi enlarged in the male.