|Polydrusus marginatus Stephens, 1831 Vulnerable|
||This species has long been regarded as scarce eg Joy
(who keys the species as
Metallites marginatus) gives 'England south to Leicester; very local', and a more recent assessment in
Hyman and Parsons classes it as 'vulnerable' although
Hammond (1994) makes a case for downgrading this to simple 'rare'. Hyman and Parsons list eight counties from which
records are known; Hants, Sussex, Kent, Herts, Berks, Salop, Leics and Rutland (pre 1970) and Hammond quotes an old Dorset
record and one from Chalfont St. Giles , Bucks (1943) 12 km west of us. The only recent record (post 1990) record on the
NBN map (May 2009) is from east Berkshire. Morris quotes the
species as 'extremely local... though often abundant where it occurs' and this is borne out both by Hammond's account of
its occurence at Burnham Beeches (NNR), Bucks (18KM SW of us) and our own experiences. We have records from 2004 but
recently the species has been locally abundant; during April and May 2009 we recorded them in numbers from six (widespresd)
sites throughout Whippendell and adjacent woodlands. In early April two specimens (in cop) were extracted from a sample of
moss and fungi taken from the surface of decaying Betula (Birch) logs lying among Mercurialis perrenis
(Dog's Mercury) and Rebus (Blackberry), several more adults were swept from the surrounding vegetation during light
rain but none were found by sweeping the overhanging birch which was just coming into leaf. Hammond notes that adults are
flightless and likely to spend much of their time on the ground, moving up into trees or shrubs when suitable, ie young,
foliage becomes available. Specimens may thus appear in pitfall or interception traps. From early May we have swept them
from Birch (several pairs in cop), Quercus (Oak), Fagus (Beech) and Crataegus (Hawthorn) throughout
the woods; they are particularly common on birch along the western margin of West Herts golf course. Typical habitat is
broadleaved woodland and wooded pasture. The species is polyphagus having been recorded from a wide range of woody plants
including Ameanchier, Cersus, Corylus, Crataegus, Fagus, Fraxinus, Malus, Prunus, Quercus, Sorbus and Ulnus, several
reports have been from Pinus and Cytissus but these are atypical and not necessarily feeding sites,
Fowler also quotes Juniper, again atypical.
Hammond makes the interesting observation that the species is often in areas of woodland inhabited by the Wood Ant (Formica rufa L.) and states that there may be an association. We have not found Wood ants in Whippendell wood and it is very likely, given their striking appearance and overt behaviour, that they do not occur.
Adults persist into July and, in our experience, may be swept from tree and shrub foliage during hot sunny weather. It may be that at certain times and for whatever reasons they are primarily terrestrial in habit and so likely to be overlooked if foliage sampling is the only method being used.
This Polydrusus is atypical in that the elytra are not covered in metallic scales, this difference was sufficient for Joy to have maintained the Genus Metallites and separated it Polydrusus on this character:
-El. glabrous or with outstanding pubescence (ant. and legs reddish yellow)....Metallites
-El. with scales, or more or less metallic scale like pubescence......Polydrusus
The elytral pubescence is pale brown to grey, slightly metallic and arranged longitudinally along the interstices, that along the sutural interstice being more dense, and the immediate appearance, especially in the field, is of a small Sitona (Which, conveniently, do not have hooked femora).
3-6-5mm Cuticle of head, thorax and elytra shiny black. Head and pronotum quite strongly and deeply punctured, head with a longitudinal impression between eyes (sometimes obscure; strength varies). Eyes strongly convex. Temples broadened to base so that the base of the head is the same width as the front margin of the pronotum. Rostrum more or less quadrate (varies a little). Scrobes angled down in front of eyes, from above only the anterior parts are visible. Antennae inserted at front of scrobes, base of scape not visible from above. Scape curved and gradually thickened, short; equal to or slightly longer than distance between eyes (in our specimens this varies between 0.05 and 0.1mm longer), segments 2 and 3 elongate, 4-8 quadrate or slightly transverse and 9-11 forming a pointed club. Pronotum quadrate or slightly transverse, broadest behind middle and not bordered. Pubescence to head and pronotum sparse, underlying puncturation clearly visible. Elytra narrower and subparallel in male; in our specimens on average 5/3, in the female the lateral margins are more rounded, broadest a little behind middle, on average 11/7. Striae strongly puntured, interstices impuncate but finely transversely rugose. Legs pale, clothed with pale pubscence. All femora toothed beneath, these are more prominent in the male. Claws fused at base.
Description from 8 Watford specimens
Hammond, P.M. 1994 The status of Polydrusus marginatus St.in Britain. Coleopterist 3:40-42