Phaedon armoraciae (Linnaeus, 1758) 

This is a truly Holarctic species; it occurs commonly throughout Europe and Asia Minor, and east through the Caucasus to Siberia, Kazakhstan, Mongolia, China and Japan. Common and widespread in the United States and Canada. In the U.K. it is common throughout England and Wales, including the islands, and there are records scattered through Scotland and Ireland. Locally in South Hertfordshire it is generally abundant in suitable habitats although there are years of great abundance (e.g. 2012 and 2013) and years when it is almost absent (e.g. 2015). Typical habitats are wetland margins; ponds, reservoirs, and slow moving streams, but they are also occasionally found far from water. Host plants are usually marginals; Rorippa nasturtium-aquaticum (water-cress), Myosotis secunda (creeping forget-me-not), Hippuris vulgaris (mare’s tail), Veronica beccabunga (brooklime), Veronica scutellata (marsh speedwell) as well as other species. During years of abundance the disused water-cress beds at Cassiobury Park can appear blue with the beetles and by June damage caused by the adult’s feeding on the foliage is obvious everywhere. Adults overwinter among grass tussocks, matted vegetation, or under bark and logs, often some distance from water; we have found them in samples taken from various local parkland and woodland sites throughout the winter. They appear in March or April, depending on season, and are soon common on waterside vegetation as well as on flowers e.g. Caltha (marsh marigold) and Ranunculus spp. (crowsfoots etc.) In favourable years they remain active into the winter and we have often found them active on marginal vegetation in December. Oviposition occurs early in the year and larvae may be found from mid April. Full grown (third instar) larvae occur from June and are usually obvious; dark grey or brown and swollen, with several rows of black, spine bearing tubercles along the body. They may be found on the underside of leaves and accumulations or trails of frass are usually evident close to ragged feeding holes starting near to the mid ribs. By July or August the plants display signs of extensive grazing. Full grown larvae descend and burrow into the soil where they construct a cell for pupation. New generation adults appear during July, August and September. They are capable of flight but we have yet to witness this.