Phaedon armoraciae (Linnaeus, 1758)
Widely distributed and common throughout England, Wales and southern Scotland, more scattered further north but common where found (Cox). Associated with wetland habitats, adults are found on a variety of waterside plants including Marsh Speedwell (Veronica scutellata L.), Brooklime (V.bettabunga L.) which is also the larval foodplant, Watercress (Nasturtium officinale R.Br.) and Mare's tail (Hippuris vulgaris L.) among others, they may also be found among the flowers of various aquatic plants. They occur year round and are often found in Winter samples of grass tussocks, reed litter (Radlett road, Watford) or under debris, active from the first warm days of March and by April are common. They may be swept from vegetation along all the river systems around Watford and are active until late in the year; in 2006 we swept them from pondside vegetation in Cassiobury nature reserve on Christmas day. There is one generation each year, full grown larvae may be found from June onwards, pupation occur in a subterranean earthen cell and adults emerge from July to September.

Often found alongside P.cochleariae Fab., around the disused watercress beds bordering Cassiobury park both are abundant through the summer, they are closely similar even with a hand lens so specimens should be taken for examination.

3.5-4.7mm. although smaller specimens very occasionally occur. Generally bright metallic blue but completely black specimens are sometimes found. Antennae entirely dark and metallic including underside of basal segments (cf. P.cochleariae), segments 7-11 widened and pubescent, insertions much wider apart than length of first segment. Eyes transverse, head with angled impression between antennal insertion, finely punctate throughout. Pronotum transverse with front angle protruding and front margin evenly rounded, finely bordered laterally and evenly punctate over entire surface, about as strongly as head, sometimes a little more coarsely towards sides (cf. P.tumidulus (Germ.)). Scutellum generally impunctate and very shining, contrasting against pronotum and elytra. Elytra with 9 punctate longitudinal striae across middle and fine raised border, interstices finely and randomly punctate (X20). Base of fifth stria deepened inside (usually) strong humeral callus, sixth stria usually shallower than fifth, sixth and seventh transversing callus (cf. P.cochleariae). All legs dark metallic. Terminal abdominal sternite with pale rufous or testaceous margin about as wide as dark basal part (cf. P.cochleariae).

Description from six Watford specimens at X20

See ID Aids for a guide to the genus