Sitona Germar, 1817
Pea and Bean Weevils
As currently accepted, Sitona includes about 100 species; it was formerly much larger but several subgenera have been given generic status. Among the British fauna these include the monotypic Andrion Velázques de Castro, 2007, Charagmus Schönherr, 1826, which includes 9 Palaearctic species of which 2 occur in Britain, and Coelositona González, 1971, which includes 9 species of which 3 occur in Britain. Sitona is native to the Holarctic region although only 11 species occur in North America and of these only about half are native, the rest introduced from Europe. Several European species have been transported with trade and have become established in Australia, New Zealand, South Africa, and South America, and species are often reported worldwide with imported food material. The European fauna includes 48 species (and about 11 subspecies) and 15 of these extend to the UK. Most of our UK species occur throughout the Palaearctic region, and of these all extend north into Scandinavia and many also occur on the Atlantic islands. Only three are restricted to the Western Palaearctic region: S. gemellatus Gyllenhal, 1834 is a rare species of Southern and Central Europe, S. striatellus Gyllenhal, 1834 occurs throughout Central Europe and extends east to the Caspian Sea; it is generally absent from Fennoscandia but is widespread in the UK, and S. waterhousei Walton, 1846, which is widespread though local in Europe including the UK but not Fennoscandia. Most of the remaining European species are either much more restricted in distribution or are endemic to certain countries e.g. S. treneri Solari, 1948 occurs in Italy, while S. anchora Gyllenhal, 1834 and S. canus Gyllenhal, 1834 occur in Ukraine, a very few are fairly widespread in Mediterranean regions, and two species, S. longulus Gyllenhal, 1834 and S, inops Gyllenhal, 1832, occur sporadically from the Mediterranean to the Baltic and so might be expected in the UK.
So far as is known all species of Sitona feed on various Fabaceae (Leguminosae), both as larvae and adults. The basic biology probably applies to all species and is as follows. Adults overwinter among litter or in the soil, and usually become active when the temperature reaches about 15°C. They are good fliers and soon move to feeding sites, here they feed on foliage and may aestivate while they become sexually mature, and then begin mating. Eggs are laid in the soil around suitable host plants, and larvae enter root nodules to begin feeding. Young larvae will consume most of the nodule and may move to others, but larger larvae generally complete their development by feeding externally on roots. Pupation occurs in the soil, usually about 5 cm below the surface, and the first new-generation adults emerge in early summer. Several species have become serious agricultural pests and they may occur in vast numbers on crops during the summer; in Europe, including the UK, this is the case with S. lineatus (Linnaeus, 1758). Here the adults cause notches in leaf margins as they feed, and this can be extensive and unsightly, especially around field margins where damage is usually most severe, but this hardly seems to affect the yield. Larval feeding, however, can cause extensive damage to nitrogen-fixing root nodules, and this can seriously reduce yields of pea and bean crops. Such pest species are monitored by using pheromone traps in fields where crops are emerging or have been previously grown. When adult beetles reach a certain daily number in the traps other measures, either chemical or biological, may be taken although pyrethroids resistance has recently been confirmed in some species in the UK. Crop rotation has been found effective, at least in the short term, but increased agricultural pressure is making the situation very complex and difficult. As with any native species, a certain equilibrium exists with pests and diseases etc., but in novel environments these weevils may become very troublesome indeed. The widespread Southern European S. discoideus Gyllenhal, 1834 was first recorded in Australia in 1954, in New Zealand in 1974, in Chile in 1983, in Argentina in 2018 and in the United States in 1983, and since then has become a serious pest of lucerne. Losses of up to 40% have been recorded in New Zealand, and here it is now being controlled by the parasitoid, Microctonus aethiopoides Loan (Hymenoptera: Brachonidae), which was introduced from Morocco in 1982. This tiny wasp attacks a range of weevil species; it is has been widely used to control the Alfalfa Weevil, Hypera postica (Gyllenhal, 1813), but has proved equally valuable against S. discoideus. Several of our British species are widespread and common and will soon be found by sweeping suitable crops or low vegetation in almost any fairly open situation, including disturbed sites such as roadsides and domestic gardens.
Sitona is not defined by any definite synapomorphy, and so it is a potentially artificial group, however the species are generally distinctive and with a little familiarity will soon be recognized on sight. There are a few superficially similar species e.g. Polydrusus confluens Stephens, 1831 (which has claws fused at the base), but these will soon be rejected by the following list of features. The species are small, generally between 3 and 6 mm, elongate and rather parallel-sided, with prominent shoulders and elytra broader than the pronotum. Most have a characteristic appearance, with longitudinal strips of lighter scales on a background of grey or brownish metallic scales, in most there are also semi-erect fine setae, and some have lines of longer erect setae on the elytra. Important features of the head include the development and length of a median longitudinal impression, and the form of the eyes which vary from almost flat to very convex and prominent. In most species the temples are well developed. The rostrum is quadrate or nearly so, with lateral scrobes that are not fully visible from above and that curve down, reaching back at most to the ventral margin of the eyes, in some species there are oblique or longitudinal ridges to the dorsal surface which may aid identification. The antennae are of typical geniculate form with a short scape and elongate and pointed club. The pronotum is transverse to quadrate and lacks borders, it is usually smoothly convex, strongly and quite densely punctured and often has three longitudinal strips of pale round or slightly elongate scales against a darker background, the anterior margin lacks a uniform series of long setae but usually has short overlapping setae, similar to those on the pronotal disc. The front coxae are large, closed and close together; in some species they are situated near the middle of the prosternum while in others they situated more anteriorly, adjacent to the subapical constriction. The scutellum is small and clothed with decumbent and longitudinally oriented scales, a feature which will distinguish Sitona from Charagmus in which the scales radiate from the midline or form two distinct lateral ‘tufts’. In lateral view the pronotum and elytra form a more-or-less continuous line (in Andrion both have a strong declivity and so the outline is discontinuous). The elytra have broadly-rounded shoulders and a continuous apical margin, the striae are strongly punctured and for the most part parallel although the inner striae may diverge towards the apex and this is sometimes an important identification feature. The legs are long and robust, with unarmed femora and narrow tibiae that may be expanded or toothed apically. In males of most species the inner keel of the front tibiae is expanded into a small apical tooth. The claws are smooth and not fused at the base.
The genus is generally considered to be difficult to deal with as some species are closely similar and the differences are difficult to describe, but identification becomes easier with experience, especially once the abundant S. lineatus can be identified by sight. This experience is vital because the majority of specimens will be lineatus, and these occur in numbers absolutely everywhere. Once lineatus becomes obvious in the field other specimens can be taken for close examination. This is where things soon become difficult, but there are some very good keys, the Royal Society Handbook by Mike Morris covering Broad-Nosed Weevils (Vol.5 Part 17a 1997), and Vol 4 of Beetles of Britain and Ireland by Andrew Duff (2016) are excellent, but the online key by Arved Lompe is also very good although it deals with a wider European fauna (no bad thing), and there is also a detailed key presented by Mike Hackston.