Hermaeophaga mercurialis (Fabricius, 1793)

The majority of records of this species are from Southern England and south Wales below a line joining the wash and Carmarthen. Welch (1972) provides data suggesting a close association with chalk, tertiary, greensand and oolitic limestone deposits of the southeast, as well as providing a map which correlates the insect's distribution with these deposits, evidence is given suggesting that larvae require dry, preferably calcareous soils and that the species is unable to overwinter in 'cold, wet, heavy soils'. Cox (2007) provides a few more northerly records to south Cumbria. Both works record Welch's bizarre 1965 find from the Isle of Harris, still apparently the only record from north of the border. These factors must be borne in mind when searching for this species eg Easton(1946) describes an example of a locally discontinuous distribution of Hermaeophagus despite an abundance of the foodplant, Mercurialis perrenis (Dogs Mercury). This may well be the case in our own area; it occurs abundantly through Whippendell wood, and sweeping Mercurialis anywhere near the carpark at Rousebarn lane through the spring and summer will produce hundreds, if not thousands, of specimens (2004-8) but sweeping this plant below Cassiobury park, only 1.5 Km away but essentially wetland, has so far provided only occasional specimens through the summer. Mercurialis perrenis occurs throughout our area, generally in shaded and wooded situations, but we have thus far (July, 2008) recorded the beetle only from Whippendell wood/Cassiobury park complex, Oxhey park and a solitary specimen from the very wet south side of Common moor. It must be said though that being otherwise rather unproductive we do not very often sample Mercurialis.

There is a single generation each year, adults emerge from pupae in earthen cells within the soil during August and September and hibernate in grass tussocks and leaf litter, mature adults have been recorded from February (Cox). According to Welch it is thought that most of the population overwinters as final instar larvae or pupae with adults emerging during February but remaining in the cell for a month or more. Adults are active from March or April and their presence may be detected by the small feeding holes they produce on the leaves of the host. In feeding experiments conducted by Welch (ibid) adults were offered leaves from a variety of common ground species eg Glechoma hederacea but were not observed to feed on any of them. Egg laying occurs from April to June (Cox) and larvae are present from late April to August. Larvae feed on the roots of Mercurialis, the second and third instars bore into the centre as feeding continues.

Detailed line drawings of the first instar larva and pupa as well as much interesting information is given in the 1972 paper by R.C.Welch.

2.2-3mm. Globular oval and glabrous, dark shining metallic blue or almost black. Head impunctate, very shining with strong frontal furrows and depressed between the eyes. Eyes convex and protruding, coarsely faceted and touching front margin of pronotum in normal setting. Antennae black with four basal segments lighter, often testaceous, inserted on forehead in front of eyes and separated by about the length of the first segment. Pronotum very convex and transverse (about 3:2), bordered laterally and basally, lateral margin evenly curved and broadest near middle. Front angles protruding. With a short longitudinal impressed line either side at base and a transverse line, or at least an impression, connecting these. Entire surface finely punctate. Elytra slightly elongate (7:6), evenly rounded and broadest at middle. Without striae, entire surface punctate; punctures more dense and stronger than those on pronotum. Legs black or obscurely lighter at joints, tarsi brown. Hind femora enlarged; adapted for jumping.

Description from 20 Watford specimens at X20

Easton,A.M., 1947. The Malacodermata and Phytophaga of Bookham commom. London Naturalist (1946) 26:87-93
Welch,R.C., 1972. The biology of Hermaeophaga mercurialis. Entomologists Gazette 23:153-166