BAGOINAE Thomson, C. G., 1859
All British species are rare and generally only encountered by chance, as well as being very difficult to identify. The majority occur in wetland habitats.
This small and very distinct subfamily, formerly included as a tribe of the Molytinae or within the Erirhininae or Curculioninae, includes about 400 described species in 11 genera but the synonymy is extensive and many more genera will be found in the literature, the generic and, especially, specific diversity is very likely to change significantly in the future as the faunas of many tropical areas as well as China are yet to be thoroughly studied. The majority of species, >360, are included in the cosmopolitan genus Bagous Germar, 1817 and the remaining genera are small and of restricted distribution; Hydronomidius Faust, 1819 (2 spp. India), Hydronoplus Fairmaire, 1898 (7spp. India, tropical Africa), Neoephimeropus Hustache, 1936 (monotypic, tropical Africa), Picia Tournier, 1895 (3 spp. Asia Minor, Middle East), Pnigodes Le Conte, 1876 (monotypic, Nearctic), Pseudobagous Sharp, 1917 (monotypic, tropical Africa), Scleropus Faust, 1889 (monotypic, New Caledonia), Azollaebagous Caldera, O’Brien & Meregalli, 2017 (3 spp. east Asia, Oriental), Hydronomus Schönherr, 1825 (monotypic, Palaearctic) and Memptorrhynchus Iablokoff-Khnzorian, 1960 (2spp. Asia). Species of Bagous form groups based mostly on fine structures of the male genitalia and while some groups are widespread, many are restricted to certain areas e.g. the western Palaearctic region includes 24 such groups and 16 appear to be endemic, 8 occur in Japan of which 4 are endemic and of the 7 Australian groups 2 are endemic. The European fauna includes the very widespread Palaearctic Hydronomus alismatis (Marsham, 1802), which extends north to the UK, and more than 60 species of Bagous, 20 of which have been recorded from the UK, many have restricted or discontinuous distributions in Europe and some are endemic to particular countries or Mediterranean islands, none are particularly common and most are very local and rare. Many species have been in decline over recent decades due to changes in their habitat; many are associated with wetland environments and are semi-aquatic, some are truly aquatic and most of these are sensitive to the changes inflicted by human water management schemes.
All are small to medium sized weevils, 1.2-9.0mm although all UK species are <6mm, most are elongate and cylindrical or flattened dorsally and only a few are broadly rounded, most are drab grey or brown or with the head or forebody contrasting, usually darker, and many have patterns of darker or paler scales to the pronotum and/or elytra, in many there are pale sub-apical maculae or darker streaks in the second, third or fourth interstices. The dorsal surface is usually granulate or punctured, usually finely so, and densely clothed with small scales which are often obscured by a varnish-like translucent coating which dries and wears off in life but is usually evident around the granules. The head is very variable but usually convex, transverse and narrower than the pronotum, in many the vertex and/or frons is impressed or foveate, often medially sulcate, and raised or carinate inside the eyes. The eyes are small, weakly convex and usually rounded or almost so. In most the rostrum is shorter than the pronotum, cylindrical, curved and densely scaled, at least towards the base, with deep lateral scrobes level with the eyes. Antennal scape long and usually thickened towards the apex, funiculus 7-segmented (only occasionally 6-segmented), club 3-segmented, usually densely pubescent but in some the basal segment is glabrous, in most the antennal-insertions are towards the apex of the rostrum but this is variable. In most the pronotum is transverse but in some species quadrate or elongate, usually flattened dorsally and with a longitudinal median groove which may weak or interrupted by various impressions towards the base, the surface may have scattered semi-erect setae as well as the usual dense covering of scales, but these are usually very fine and not usually visible. The lateral margins are very variable, from straight and sub-parallel to broadly rounded, but in most there is a distinct, and sometimes very strong, sub-apical constriction which gives the impression of a collar before the anterior margin; in extreme cases the surface is raised behind this constriction producing a hunchbacked appearance, and the anterior and posterior margins are straight or simply curved. Prosternum usually with a distinct, though sometimes shallow, rostral groove which may extend between the front coxae but does not continue onto the mesosternum-this feature will distinguish the present group from superficially-similar species of Erirhinini-this feature is rarely difficult to appreciate but in a few species e.g. Hydronomus alismatis (Marsham, 1802) it can be indistinct, and the anterio-lateral margin with variably developed post-ocular lobes. Scutellum small, rounded and usually clothed with dense hydrofuge scales and secretions. Abdomen with 5 free ventrites; 1 and 2 long; the basal ventrite variously produced between the metacoxae, 3 and 4 short or very short and the fifth variable but usually long, rounded apically and with a pair of subapical setae or small groups of setae. In most the basal ventrite is medially impressed in the male and flat to convex in the female. Elytra usually with broadly-rounded shoulders which are wider than the pronotum and straight or weakly curved lateral margins before a sub-apical constriction, apical margin continuously-rounded and often distinctly produced. Surface with 10 distinct and variously-punctured striae although in females of some species the sixth and seventh are united at the base, interstices usually flat to weakly convex but alternate interstices may be more strongly convex or even raised and the odd-numbered interstices often with a row of small semi-erect setae, interstices 3 and/or 5 often tuberculate or with raised areas before or above the apical declivity. Legs usually long and robust, coxae rounded and projecting, often strongly so, the front coxae closely approximated and strongly convex, middle coxae more widely separated and less convex, hind coxae widely separated and flattened , all coxae and trochanters with a single erect setae. Femora usually to some extent clavate, especially the middle and hind femora, and never with a ventral tooth, tibiae usually long, slender and sinuate or bisinuate, usually incurved apically and sometimes very strongly so, inner margin often denticulate; the teeth small and stout and each with an erect basal setae which may be very long in swimming species, apical margin with a comb of short setae and a pair of longer setae but without terminal spurs. Tarsi very variable, the segments short to elongate and linear to broadly cordate, especially the third which may be dilated and emarginate but not bilobed or cleft apically and hardly wider than the second, and ventrally with dense pubescence, especially the third segment. Claws usually long, fine and simple, always free and diverging from the base.
The majority of species are semi-aquatic or truly aquatic, a small number are largely terrestrial and only very few are associated with plants in arid habitats. The majority of species are either monophagous or oligophagous with a clear preference of host plants and polyphagous species are rare. Most Bagous species are associated with wetland monocotyledons, a wide range of host families have been recorded, particularly among the Alismatales and Poales. A few species of Bagous have been trialled as biocontrol agents outside their normal ranges, thus several species from Australia, India, and the wider Palaearctic and Oriental regions have been released in the United States to help control the invasive aquatic Hydrilla verticillata (L, f.) and the equally invasive aquatic Myriophyllum spicatum L, a widespread Old-World species that has proved to be very difficult to control in the United States. Many species are well-adapted to an aquatic lifestyle e.g. B. subcarinatus Gyllenhal, 1836, which feeds on hornworts (Ceratophyllum L.) in slow-moving waters, can obtain its oxygen entirely from the water or from plants and so live a completely submerged as an adult, it can also swim very well under water and has been observed to do so in numbers in bright sun. Larvae develop within leaves, stems or root nodules and pupation occurs either within plant tissue or in surrounding substrate. Adults may be diurnal or nocturnal; they are generally active over a well-defined season during the spring and summer although some display two peaks of activity, one early and one late in the year, so far as is known they are univoltine in temperate regions and winter is generally passed in the adult stage, less often as a larva (large larvae of some species occur in early spring), either among tussocks or in the ground. Adults may sometimes be sampled by pitfalling or by sweeping marginal or aquatic vegetation, a good method is to spend some time working through or sieving marginal soil and litter, thus we found B. collignensis (Herbst, 1797) in a Watford reed bed (July, 2010), and B. limosus (Gyllenhal, 1827) among flood refuse on an arable headland in North Somerset (June, 2014).
Of the 21 species listed as British none are common; some are widespread but most are local and rare or very rare and four, B. binodulus (Herbst, 1795), B. diglyptus Boheman, 1845, B. petro (Herbst, 1795) and B. robustus Brisout de Barneville, 1863, are thought to be long extinct. The remaining species are infrequently recorded and so the majority are probably rare or very rare but they are difficult to record and more careful sampling will probably produce many more records, on the other hand most are dependent on stable wetland habitats which are becoming increasingly rare in the UK. Many species are known from widely scattered records and some are known from only a few very local areas e.g. B. tubulus Caldara & O’Brien, 1994 is known from South East England and South Wales. B. brevis Gyllenhal, 1836 and B. czwalinai Seidlitz, 1891 occur only in South Hampshire while B. Longitarsus C.G. Thomson, 1868 and B. puncticollis Boheman, 1845 are very local and rare in South East England. B. frit (Herbst, 1795) is known from Wales and the south of England, and B. agrillaceus Gyllenhal, 1836 is a rare species of brackish coastal ponds in South East England. Both B. subcarinatus Gyllenhal, 1836 and B. nodulosus Gyllenhal, 1836 are rare and restricted to Southern England. Our remaining species are widespread though generally very local and scarce across England and Wales, a few extend into Scotland and a few occur in Ireland but more careful searching is needed and is likely to reveal many more records. Hydronomus alismatis (Marsham, 1802), generally referred to in the literature under Bagous, is widespread and sometimes locally common throughout England and Wales, except for The West Country, and extends north to Edinburgh in Southern Scotland. Identification is generally difficult and may require dissection but all species are dealt with in the 2002 RES Handbook by Mike Morris and in the fourth volume of Beetles of Britain and Ireland (2016) by Andy Duff.