Hypera nigrirostris (Fabricius, 1775)
Originally native to Europe and northwest Africa, this widespread and generally common weevil occurs throughout Europe to the north of Fennoscandia and the UK and extends east through Asia Minor, Kazakhstan and Russia to eastern parts of Siberia, and following accidental introductions it has become established in Japan and across North America. Here it is locally common across England and Wales though generally more coastal in the west, and very local and rare in Scotland including the Hebrides. Host plants include a range of Fabaceae such as various clovers; T. incernatum L., T. alpestre L., T. hybridum L. and, more especially, T. repens L. and T. pratense L. but also occasionally on Medium medium L., Ononis spinose L., Lotus corniculatus L. and Medicago sativa L., lucerne or alfalfa, the species is an occasional pest of commercially-grown clover on the continent but its effects are insignificant in the UK where it rarely occurs in large numbers. Adults are present year-round, they overwinter in the soil or beneath debris among crops or on headlands and become active in the spring when the temperature reaches 7-9 Celsius, they may remain close to host plants from the previous year or may disperse and so are likely to occur wherever suitable hosts are abundant such as grassland, gardens, wooded margins, roadsides and wasteland and, because of the speed at which the hosts can spread and establish, they often occur on ruderal sites. Adults feed on host foliage for a while before mating and oviposition begins in the spring, females chew holes in the epidermis and lay either a single egg or a small group, usually high up on the plant, and each will lay up to 800 eggs over a few weeks during May and June. Larvae emerge after two or three weeks and feed in leaf sheaths, inflorescences or within unopened leaf-rolls, they develop rapidly, pass through four instars and are fully-grown within three weeks. The mature larva constructs a delicate net-like cocoon among the foliage or sometimes on the soil around the stem and pupates a few days later, the first pupae appearing from late June and adults eclose within ten days, the cycle from egg to adult taking five to seven weeks in mild northern latitudes and a peak in adult abundance occurs during July.
Hypera nigrirostris 1
Hypera nigrirostris 2
Hypera nigrirostris 3
Adults continue to feed through the spring and early summer but generally enter the soil to diapause during August and become active again in September. Where large populations build up it is the larvae that are most destructive; adults chew small holes in leaves and stems but larvae usually move between the largest leaf buds and destroy growing tips, each consuming three or four buds during its development. Damage is often more severe as the brentid, Protapion apricans (Herbst, 1797) often occurs among the same crop and on the same plants; nigrirostris appears a week or two earlier in the year but the cumulative damage can be severe. Adults may be sampled by sweeping host plants but they tend to occur only singly or in small numbers.
3.3-4.1mm. Generally distinctive due to the green or golden-green dorsal scales and striped pronotum but teneral adults are pale brown and mature brown forms appear on the continent, the appendages vary from pale to dark brown, usually with the femora and antennal club darker. Head transverse, laterally almost entirely comprising the elliptical eyes, the temples being only narrowly visible, vertex narrower than the width of an eye when viewed from above, rostrum dilated towards the apex, about three times longer than wide and smooth above; without furrows above the scrobes. Antennae long and slender, the scape rather abruptly thickened in the apical third, funiculus with seven segments but the tiny third segment needs to be looked for carefully, club long, slender and pointed. Pronotum transverse, evenly curved laterally and widest slightly behind the middle, scales narrow and deeply bifid; divided almost to the base, metallic green with a distinct pale median strip and a paler stripe towards each lateral margin, setae short, semi-recumbent and visible along the margins. Elytra elongate with broad shoulders that are much wider than the pronotum, parallel in the basal half and continuously curved apically, scales deeply bifid, as on the pronotum, metallic green but generally paler stripes and sometimes the sutural interval is darker, especially in the apical half. Interstices each with a row of long, backwardly-curved pale setae, those on the inner interstices generally confined to the apical half. Fore tibiae without an internal tooth, male hind tibia with a small internal tooth at the apex.