Hydronomus alismatis (Marsham, 1802)

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POLYPHAGA Emery, 1886

CURCULIONOIDEA Latreille, 1802

CURCULIONIDAE Latreille, 1802

BAGOINAE Thomson, C.G., 1859

Hydronomus Schönherr, 1802

Formerly included in subgenus Hydronomus of Bagous Germar, 1817 and is still widely referred to as such, this species occurs throughout Europe from the Pyrenees to Ukraine in the south; it is widespread in Italy but absent from most of the Balkan Peninsula and Greece, to the north it extends to the UK, Denmark and above the Arctic Circle in Fennoscandia but is absent from much of the southern Baltic region. Beyond this the species is known from Asia Minor and, sporadically throughout Asia to the Pacific coast of Russia. In the UK it is widespread though local throughout England and Scotland as far north as Glasgow, mostly coastal in Wales and occurs on Anglesey but not Man. Typical habitats are well-vegetated margins of swamps, ditches, ponds and slow-moving stretches of rivers etc. where the host plants are abundant. Adults occur throughout the year; they overwinter in host stems or adjacent litter and are active from April until September or October, peaking in abundance during June and July. The usual host in the UK is Common Water plantain (Alisma plantago-aquatica L.) but on the continent other plants are used including Arrowhead (Sagittaria sagittifolia L.), Water Horsetail (Equisetum fluviatile L.) and possibly other species of the genus, and Damasonium alisma Mill. Adults are mostly diurnal, they spend much of their time low down on host stems or on the surrounding substrate but become active in warm weather when they climb stems and may be seem basking on foliage, often with several specimens on a single plant, they feed on both surfaces of leaves, producing numerous small holes which sometimes become merged. Mating occurs from late spring and females lay batches of 5 or 6 eggs on leaves, binding them together with secretions. Larvae emerge within a week and begin feeding on leaves; they produce wide translucent mines which tend to be short as the larvae often leave the mine, move through the water and start fresh mines on nearby leaves. Individual leaves may host several occupied mines and each mine may contain several larvae. Mines tend to be conspicuous, even from a distance as they contrast against the leaves and the occupied end is red or reddish-brown. Fully-grown larvae bore into petioles where they pupate, this stage lasts about a week and new-generation adults appear during July. These fresh adults will feed during the summer and autumn before entering stems or substrate to overwinter.

Hydronomus alismatis

Hydronomus alismatis

© Lech Borowiec

2.7-3.5 mm. Very similar to various Bagous but with only a slightly-impressed rostral furrow on the prosternum, densely pubescent tarsi and having scrobes which do not reach the anterior margin of the eyes and are obscured with scales.  Elongate and rather narrow, black with variable patterns of grey or greenish-grey scales; typically to the pronotal sides and sometimes along the centre, the elytral base and apex and transversely across interstices 2-4 behind the middle, the darker areas often having a slight metallic shine. Antennae dark but the base of the scape and the funiculus is often paler, femora and tarsi dark grey, tibiae reddish. Head smooth between weakly convex eyes, rostrum short and broad, widest across the base and narrowed to the apex, antennae inserted about the middle. Pronotum quadrate or nearly so; broadest about the middle and evenly narrowed to perpendicular posterior angles and a weak sub apical constriction, surface evenly convex and without impressions or structure. Elytra slender, at least 1.6X longer than wide and only slightly dilated from broadly-rounded shoulders to a continuous apical margin. Striae well-impressed and only obscurely punctured, interstices wide and flat or very weakly convex. All Femora of similar length and width. Tibiae curved inwards towards the apex and produced to a sharp internal tooth at the apex. Tarsi pseudotetramerous with elongate basal segments, weakly bilobed third segment and diminutive fourth segment. Claws smooth and free at the base. The sexes are very similar but females have a slightly longer rostrum.