Clytus arietis (Linnaeus, 1758)
A common species throughout England and Wales but with far fewer records north of the Humber and only a few scatered across Scotland. The beetle infests dead parts of living trees, dead trees or fallen dry branches. A wide range of larval hosts have been recorded including Oak, Sweet Chestnut, Rubus, Sycamore, Pear, Fig, Prunus spp., Hawthorn, Rosa, Hazel, Beech, Birch, Salix, Laurus nobilis and Wisteria. The larvae live under bark until about half grown when they tunnel into the xylem. A pupal gallery is constructed parallel to the wood grain and pupation occurs either in august/september or march/april. The complete life cycle takes two years. There are records of this species emerging from both garden and indoor furniture.

Adults occur from may to july and are active during hot weather on a range of flowers, more particularly umbellifers e.g. Hogweed, Angelica or cow parsley, feeding on pollen and flying readily when disturbed. They are wasp mimics even in their behaviour, with rapid jerky movements on flowerheads.

Most commonly seen around woodland borders and glades but also in parkland and domestic gardens around Watford town centre, a single specimen was found in Watford market (july 2005) some distance from any obvious suitable habitat. They are undoubtedly more common in some years, 2005 and 2007 produced an abundance.

9-13mm. Antennae dark, segments 2-4 orange, 3rd longer than 4th. Head black with yellow scales beside eyes, narrower than thorax. Eyes strongly excised around antennal tubercles, with tiny facets. Temples parallel. Apical segment of labial palpi dilated. Thorax quadrate, without lateral borders, spines or tubercles. Front and hind margins narrowly yellow although sometimes interrupted posteriorly. Head, thorax and basal half of elytra with outstanding yellow pubescence. Eltra black or lighter apically, marked yellow as shown, apices smoothly rounded. Last abdominal tergite with dense yellow scales and often protruding. Legs orange, pubescent throughout. Front and mid femora usually darker, mid and hind femora clavate and all femora without spines. Mid and hind tibiae as long as femora. All tibiae with apical spur on inner side.
A variety lacking the yellow V shaped elytral mark (var. medioniger Allen) is described by Allen (1959)

Description from 2 Watford specimens at X10

Allen, A.A. 1959 Ent.Mon.Mag. 95:116