CYCLOMINAE Schönherr, 1826
Two species are included on the British list. Gronops lunatus is a local coastal species, while G. inaequalis is a very rarely recorded introduction.
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POLYPHAGA Emery, 1886
CURCULIONOIDEA Latreille, 1802
Gronops Schönherr, 1826
G. inaequalis Boheman, 1842
G. lunatus (Fabricius, 1775)
3.1-4.8mm
Introduction
Formerly known as Rhytirhininae, this large subfamily is most diverse in Old-World tropical regions, more particularly Australasia, and especially Australia, and South America with only a few genera occurring in New Zealand, tropical Africa and elsewhere, although western Mediterranean Europe and North Africa are also very diverse. The limits of the family are not well defined and it currently includes 8 tribes, some of which are speciose and confined to Australia, but several tribes have recently been removed and some former subtribes are now considered to be full tribes, and because of the mostly tropical diversity this shuffling is likely to continue. The group is relatively poorly represented in northern temperate areas and then by only a few widespread or localized genera e.g. the Nearctic fauna includes about 85 species of 2 genera which, through introductions, have become almost Holarctic in distribution, Listroderes Schönherr, 1826 (3 spp.), a Neotropical genus which has spread with human trade, and Listronotus Jekel, 1865 (>80 spp.), which occurs throughout the New World south to Chile and Argentina, both included in the tribe Listroderini LeConte, 1876. The Palaearctic fauna is more diverse although the majority of species occur in warmer southern regions; about 200 species of 6 genera and 4 tribes are represented although the Listroderini is known only from introductions; Listroderes difficilis Germain, 1895, recorded from Spain as well as various parts of Asia is a Neotropical species that has become established in tropical and subtropical regions worldwide, and Listronotus cyrticus Desbrochers, 1898, recorded from France, is native to the Neotropical region. Notiomemetini Wollaston, 1873 is represented by the single species Isonycholips goroi Chûjô& Voss, 1960 from eastern Siberia, Japan and Korea. Dichotrachelini Hoffmann, A., 1957 is represented by about 65 species of Dichotrachelus Stierlin, 1853, the vast majority occur around the western Mediterranean, including north west Africa, and Spain, France and Italy are particularly diverse, to the north the genus extends into Switzerland and Austria and none have been recorded from the UK. The Hipporhinini Lacordaire, 1863 is represented by about 135 species of 3 genera. Borborocoetes Schönherr, 1842 is known from 4 Asian species; it occurs as far west as the Middle East but has not been recorded from Europe. Gronops Schönherr, 1823 is a widespread Eurasian and African genus represented by 20 Palaearctic species (30 in total), 7 occur in Europe, most are of very limited distribution but 2 of the most widespread extend north to the UK and are the only members of the subfamily to do so. Entomoderus Raffray, 1873 includes about 110 western Palaearctic species, the majority of which are endemic to Portugal, France, Spain, Italy and Mediterranean North Africa.
Description
Members of this subfamily are very typical weevils; most are convex and elongate and resemble many Curculioninae or Entiminae etc., they are characterized by a short and robust rostrum which is often square or rectangular in cross-section, distinct post-ocular lobes (absent in some Neotropical genera) and lack of a prosternal channel or ascending mesepimera, These characters together with the elongate and rather parallel-sided form are a good guide to the group but fortunately our 2 UK representatives of the genus Gronops are readily distinguished from our other weevils by a combination of easily appreciated features that give them a characteristic appearance. Most members of the subfamily are small, 3-6mm, but many exotic species exceed 20mm, and in temperate regions they tend to be drab brown or grey with patterns made up of these colours but there are many very splendidly-coloured The head is elongate with weakly convex or flat eyes that are usually strongly transverse or reniform and well-separated, the interocular distance being about the same as the width of the base of the rostrum, the vertex is smooth and the clypeus variously impressed, usually with longitudinal furrows that extend onto the rostrum, the scrobes are lateral and extend almost to the anterior margin of the eyes where they usually turn down and may continue onto the ventral surface and the rostrum is often expanded towards the apex, the mandibles lack a deciduous process and so do not display a lateral scar. Antennae 11-segmented with a long scape that is strongly broadened towards the apex and a distinct club that appears 4-segmented as the eighth segment is transversely expanded and closely approximated to the ninth, the first funicular segment is long and generally thickened apically and the rest quadrate to transverse. The pronotum is quadrate and varies from smooth to heavily sculptured (as seen in most Gronops), there is no rostral channel and the front coxae are rounded, very convex and closely approximated. The elytra are also very variable, in many they are smooth and simply striate but various interstices may be raised or ridged and there may be strongly developed tubercles or raised areas before a steep apical declivity, the apical margin is continuously rounded and completely covers the abdomen. The legs are usually long and robust, the femora lack a ventral tooth and the tibiae lack an apical hook and the third tarsomere is bilobed but may be narrow and hardly wider than the second, and the claws are smooth and separated from the base.
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Ecology
The biology of most species is unknown, three European species of Gronops are reasonably well-documented and plant associations are known for many northern hemisphere species, adults are mostly ground-living and the subfamily includes many exotic aquatic or semi-aquatic species. Some Australasian ground-dwelling genera have very reduced eyes and are thought to spend their entire lives among roots. It is thought that most genera include phanerognathous (root-feeding) larvae and flightless or only weakly-flying adults, which may partly explain why some areas e.g. around the Mediterranean, have become species ‘hotspots’ although the Neotropical native Listroderes costorostris Klug, 1829, the so-called ‘vegetable weevil’, is very widely polyphagous, in common with other members of the genus, feeding on all parts of its various hosts, and has become established in warmer areas worldwide.
UK species
Gronops lunatus (Fabricius, 1775)
This very local species is restricted to western Europe, extending east to Poland, Austria and Italy, it is absent from most of the Balkan peninsula but occurs in Greece and is present on many Mediterranean islands, to the north it extends to the UK, Denmark and the southern provinces of Sweden, where it seems to have suffered a recent decline, but is otherwise absent from most Baltic countries. Here it is a very local and mostly coastal from the Humber estuary to Dorset and around the Welsh coast, there are also inland records, mostly from East Anglia and Kent but also from the midlands. Typical coastal habitats are salt marshes and sand dunes while inland they occur on exposed sand or light sandy soils, adults are present from March until late summer and may be locally abundant but are infrequently recorded as they are nocturnal and spend most of their time in the soil around host roots. Adults have been observed mating in June and host plants include various Caryophyllaceae; Spergularia spp. (sea spurreys), Spergula arvensis L. (corn spurrey) and Cerastium spp. (mouse-ears) but little is known of the beetle’s biology in the UK. In northern Europe they are mainly recorded from Spergularia rubra (L.) (red sand-spurrey) and S. salina J. & C.Presl (salt sand-spurrey) in dry sandy habitats but also occur on damp sand and around ponds in dunes, adults are recorded mainly in spring and autumn and larval development occurs during the summer among host roots. Adults may be sampled by pitfall trapping or sweeping as they roam the soil surface at night and occasionally rest on plant stems during the day.
Adults are small, 3.0-3.9mm, but very distinctive due to the raised elytral intervals which terminate in an abruptly raised prominence above the elytral declivity. Body, including the rostrum, clothed in pale and dark grey broadly rounded scales which usually form a pattern of two transverse pale bands on the elytra although extensively dark specimens occur. Head widest at the base and narrowed to a broad and only slightly elongate rostrum, frons concave with a small and shallow median impression and only weakly developed supra-orbital keels, distal part of scrobes narrowly visible from above, at the base they curve down in front of the weakly-convex eyes. Antennae inserted halfway along the scrobes, scape robust and distally thickened, and funiculus appearing 6-segmented as the last segment is expanded and pubescent and forms part of the club. Pronotum quadrate or nearly so and only slightly widened in front of a sub-parallel basal half, fronto-lateral post-ocular lobes well-developed and surface with irregular raised areas and three variable longitudinal impressions. Elytra broadest at the base and gradually narrowed to a continuously rounded apical margin, base curved and much wider than the pronotum, striae deeply impressed and punctured, each with a row of fine seta-like scales, interstices convex; 3 and 5 raised into distinct ridges, 1, 3 and 5 raised above a steep declivity. Elytral colour usually consists of two pale bands among a mixture of lighter and darker scales but beyond the declivity the scales are usually extensively dark. Femora without ventral teeth, tibia not produced apically, claws separate from the base. Smooth and lacking a basal tooth.
Gronops inaequalis Boheman, 1842
During the nineteenth and early twentieth Centuries this species was known only from Eastern Siberia, it was then widely recorded in western Russia during the second world war and in 1946 from southern Sweden, it first appeared in Denmark on 1960 and since that time has spread rapidly across central and northern Europe, the first UK record was from a landfill site in Kent in 1982 and since that time it has appeared in Suffolk, North Lincolnshire, Wales and possibly Surrey. Adults occur from June until August and are most often recorded from ruderal sites, in the UK it is often coastal but on the continent appears on roadsides, dunes, wooded margins by the coast and dry hillsides, they are nocturnal and spend the day in the ground or among host stems where they are cryptic and very difficult to find. In the UK larvae have been associated with various Chenopodiaceae; Chenopodium album L. (fat hen), Atriplex prostrata Boucher ex DC (spear-leaved orach), A. patula L. (common orach) and Bassia scoparia L. (summer-cypress), while on the continent they are also recorded from Cynoglossum officinale L. (houndstongue) (Boraginaceae), Carduus crispus L. (curly plumeless thistle) (Asteraceae) and have been found on peas. Mating occurs in the summer and larvae develop among roots but the biology is otherwise unknown.
Adults are small, 3.9-4.8mm but very distinctive due to the widely depressed head and tuberculate elytra. Upper surface usually with extensive dark rounded scales; the head and rostrum, pronotal disc and two lateral bands on the elytra variably lighter. Head widely depressed between weakly convex eyes, supra-orbital keels well-developed and sharp, rostrum broadest at the base and gradually narrowed to the apex, the lateral scrobes narrowly visible from above. Antennal scape strongly and rather abruptly thickened towards the apex, funiculus apparently 6-segmented due to the broadened and pubescent seventh segment which forms part of the club. Pronotum distinctly transverse and broadened apically, the surface with variably developed tubercles and raised and depressed longitudinal areas and the antero-lateral margin with well-developed post-ocular lobes. Elytra with very strongly punctured striae and intermittently raised or tuberculate interstices; the third and fifth (and often the sutural) usually tuberculate about the middle and above the declivity, the third sometimes very strongly so, and each with a row of elongate semi-erect scales above. Legs usually chequered with light and dark scales, femora smooth below, tibiae not produced apically, basal segments of front tarsi bilobed but narrow, claws separate from the base and smooth internally.