Hydronomus alismatis (Marsham, 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.