Sitona lineatus (Linnaeus, 1758)
POLYPHAGA Emery, 1886
CURCULIONOIDEA Latreille, 1802
ENTIMINAE Schönherr, 1823
SITONINI Gistel, 1848
Sitona Germar, 1817
Native to the Western Palaearctic this species is now Holarctic in distribution following introductions; it was first recorded in the Nearctic in the 1930’s in British Columbia and is now widespread throughout Canada and the United States. It is generally common or abundant throughout Europe including England and Wales north to the Scottish border. Host plants include legumes of the family Fabaceae; peas, beans, lucerne and clovers etc. and in some countries it is a severe pest of commercially grown plants. Adults occur year-round and are likely to be found in just about any habitat that is not too wet; they overwinter among grass, straw or clover stubble and become active early in the spring when they begin feeding upon fresh foliage, they feed from the leaf edges producing characteristic semi-circular notches which may join to give the leaves a scalloped effect, and in severe infestations young plants may be defoliated and the stems consumed to the ground. Mating occurs throughout the spring and early summer and oviposition is continuous from April to July, after which the adults gradually die off; fecundity is high and up to 1600 eggs have been counted from single females, eggs are scattered in the soil near the host plants and larvae emerge after about 3 weeks and burrow down into the soil seeking the root nodules which they puncture and consume; most feeding occurs down to 15cm but in severe infestations they burrow deeper and may drastically limit growth or even destroy the plants. The larvae are fully grown after 6 or 7 weeks and measure 6 or 7mm, at this stage they construct an earthen cell about 5cm below the surface in which they pupate, and adults eclose after 2 or 3 weeks, as this new generation occurs beside the existing one the abundance increases through the summer. Fresh adults fly in search of new hosts and feed through the summer and autumn prior to overwintering; the females of this new generation remain immature for up to 5 months and so ensure the species is univoltine. Adults generally walk short distances to the overwintering sites, generally among soil or on field margins etc. and early the following year the males produce an aggregation pheromone which is attractive to both sexes and large numbers so large numbers start to appear on young plants at this time.
The vast majority of Sitona specimens taken will belong to this very common species, early or late in the year when they are fully scaled the pattern of stripes will be familiar but during the summer they lose their scales, often almost completely, and will need to be examined carefully. 3.3-5.3mm Upper surface with dense and variously metallic round or oval recumbent scales, the overall colour varies; golden, green or blue, and with pale stripes to the pronotum and elytra. Head and pronotum with fine elongate setae among the scales, obvious at X20. Head coarsely and closely punctured; the rostrum quadrate with scrobes angled down in front of the eyes and narrowly visible from above. Eyes round and moderately convex. Vertex convex, the central furrow broad, deep and extending back to at least the level of the posterior margin of the eyes; the basal extremity without a pit. Antennal scape sinuate at the base and abruptly thickened near the apex, funiculus 7-segmented; first and second segments elongate, 3-7 progressively shorter, the club narrow and pointed. Scape red, remainder dark. Pronotum shiny black and strongly punctured, the lateral curvature characteristic, broadest behind the middle. Elytra elongate; 2:1 or a little less, parallel sided and evenly rounded apically. Striae strongly punctured and the interstices flat or only weakly convex, the second and third of equal width towards the apex; lacking larger setae but alternate interstices with rows of semi-recumbent setae-like scales from behind the middle to the apex. Femora dark, clavate and without a ventral tooth. Tibiae and tarsi red. Claws separate to the base; strongly curved and without teeth.