The first thing to mention is literature - often books, sometimes online resources such as interactive keys - because without these, identification of many species with any certainty is difficult or more likely impossible. Personally I tend to buy what I can find/afford and borrow the rest (e.g. from biological records centres, museums, inter-library loans and so on). There is, for me anyway, great joy to be had in building up a good references library. Anyhow, onto the specimen - looking yellower than in reality due to lighting...
|Dorsal view (approx 5mm long, excluding legs & antennae)|
|Underside of the head|
|Side view of the head showing curved scrobes (dark antennal sockets in front of the eyes)|
|Scales on the elytra - pale and flat|
Though Duff (2008) includes several genera within the Sitonini, Morris (1997) keys them to species but considers them all to be within the genus Sitona - mentioned just to add a little taxonomic confusion to the mix. Anyhow, keying onwards...
The scutellum has no upright tufts of scales and has a rostrum about as long as wide. So, not S. griseus. The elytra have flat scale and no upright bristles; the dorsal surface has a striped (rather than tesselated) pattern and is scaled rather than pubescent - the overall appearance is brownish rather than black. The head is also more-or-less flat between the eyes (rather than being deeply dented) and the upper edges of the eyes are not obviously higher than the top of the head. OK so far, but it now gets a little trickier.
I can't see the median furrow on top of the head very clearly, but it does broaden out and reach further back than the midpoint of the eyes, ending without a clear pit. So, not the scarce S. puncticollis. Also, the elytra are more than 1.5 times as long as wide, and more-or-less straight-sided, and the scales are oval but not 3-4 times as long as broad (more like 2 times), so it isn't the common S. lepidus. The eyes are clearly convex rather than almost flat which leaves just two species - S. cylindricollis and S. lineatus. The separation is straightforward enough here - in S. lineatus the pronotum is widest behind the middle, and slightly narrower at the base than at the front (as here) and also has complete elytral stripes rather than just short segments as would be seen in S. cylindricollis. So, this is an overwintering adult of S. lineatus just emerged in the spring - widespread and abundant in lowlands, fully winged, and known to feed on pea and bean leaves in gardens and allotments as well as on vetches. Eggs are laid by these overwintered adults with larvae feeding on root nodules and the next generation adult by about July in the south, August in the north. So, I suppose it could be seen as a pest, but a few leaf-holes won't matter and so it has been released back into the garden. I fully expect to find more...
Duff, A.G. (ed.) (2008). Checklist of Beetles of the British Isles. A.G. Duff, Wells.
Morris, M.G. (1991). Weevils. Richmond, Slough. [Naturalists' Handbook no. 16; an excellent introduction]
Morris, M.G. (1997). Broad-nosed weevils. Coleoptera: Curculionidae (Entiminae). Handbooks for the Identification of British Insects 5(17a): 1-106. [Royal Entomological Society; a bit more advanced]