Isochnus Thomson, C.G., 1859
Isochnus foliorum (Müller, O.F., 1764)
Widespread but discontinuous and generally very local in central and southern Europe; known from France, Italy, Yugoslavia, Slovakia, Bulgaria, Germany, Poland and Ukraine, but widespread and locally common throughout Fennoscandia to the far north of Norway and Finland, also known from Svalbard and Iceland where it was discovered in the 1970s and is now known to be widespread in south western parts of the country. In the UK it is widespread but very local and generally scarce across England, Wales and Scotland north to the Moray Firth and in Ireland. Typical habitats are humid woodlands, wetland margins, upland valleys, peat bogs and ditches etc where the host plants are common; in Europe these include various willows, and adults sometimes occur in numbers on Alaska Poplar (Populus trichocarpa Torr. & Gray.) in urban gardens. Adults have been recorded throughout the year; they overwinter in the ground below host trees and are active over a long period from early spring until late in the autumn, peaking in abundance during May and June. After emerging in the spring they feed on host foliage for several weeks before mating and females oviposit on host foliage. Larvae enter the leaves and produce round blotch mines about a cm in diameter; these are lined with a dark secretion which makes them obvious on infected plants. Larvae develop through the summer and autumn and pupate in situ late in the year, falling to the ground along with the leaves. New-generation adults appear from late summer but they generally remain in the leaves until the following spring. This is probably typical of the development in Europe, but a rather different account is given of the development in Iceland where larval mining induces gall-like swellings in which they live until fully-grown, at this point they bore out of the mine and fall to the ground on a silk thread, here they enter the soil to pupate and overwinter, and adults emerge towards the end of May. Adults may be swept from foliage and they will usually appear in numbers (strange how the beetles are so scarce when the hosts are common everywhere), but mines are always worth collecting and keeping outside until the spring.
Isochnus foliorum 1
Isochnus foliorum 2
1.5-1.9 mm. Dorsal surface shiny black with sparse pale pubescence, scutellum with dense white pubescence, antennae pale with darker clubs, femora dark, or at least darkened, tibiae and tarsi pale, tarsi sometimes darkened. Head narrow and sparsely punctured between large and convex eyes, temples short and diverging, rostrum broadened from the base and rounded apically, scrobes very narrowly visible from above. Antennae geniculate; basal segment as long as 2-4 combined, funiculus 7-segmented and club long and narrow. Pronotum transverse, broadest slightly behind the middle and narrowed to obtuse angles, apical margin curved, basal margin straight, surface regularly and moderately strongly punctured throughout. Mesepisternum, metepisternum and metasternum clothed with dense white scales. Elytra elongate, broadest behind the middle and evenly curved from rounded shoulders to a continuous apical margin, striae well-impressed and punctured, interstices convex, finely punctured and sometimes distinctly cross-rugose. Legs long and slender with all femora of similar width. Middle and hind tibiae simple, front tibiae with a hook-like process on the outer apical angle. Tarsi pseudotetramerous, claws appendiculate.
Isochnus sequensi (Stierlin, 1894)
This widespread Palaearctic species is locally common across central Europe from the Pyrenees to the UK and southern Fennoscandia, to the south it occurs throughout Italy and is present on Corsica, it extends east into Siberia and, following introductions from Europe, has become established in north eastern United States and Canada. In the UK (where it was formerly referred to as I. populicola Silferberg, 1977) it was first reliably recorded in the mid-twentieth century from Canterbury and since that time has spread across the south east; the range appears to be increasing and it has become locally common in many areas. Adults are active over a long season, from March until October or November and will usually be found in numbers, the usual habitat is damp woodland and wetland margins but they may occur in any permanently damp situation where the hosts are common. The most frequent host in the UK seems to be crack willow (Salix fragilis L.) but a wide range of willows (Salix L.) and a few species of poplar including Populus balsamifera L., P. canadensis Moench, and P. nigra L. have been recorded hosting the species. Breeding occurs during the summer and females oviposit into small notches they chew into the under surface of a leaf, each notch will accommodate a single egg but several may be laid into a single leaf and up to 20 have been recorded. Larvae feed within the leaf, producing round blotch mines up to 10mm across which dry and become brown from above but may be dark below as they become coated with tar-like frass, they pass through 3 instars and measure about 6mm when mature, they are legless and white with the prothorax and head darkened, each abdominal segment is rounded laterally and the thorax is much wider than the head and abdomen. Pupation occurs within the mine and the black pupa, which lies on its back, is usually obvious within. Adults emerge during late summer and autumn and will overwinter near the host. The species cannot be identified from the form of the mine as it is very similar to that produced by Isochnus foliorum (Müller, O.F., 1764) which is much rarer but they overlap in distribution, but adults are easily reared by keeping leaves containing mines in a cool and damp box in a dark place and checking regularly for beetles; if mines are found containing pupae they adults will usually emerge within a few days. By keeping mines this way it might also be possible to record parasites; at least 4 eulophid wasps are known to parasitize Isochnus sequensi; Cirrospilus pictus (Nees, 1834), C. diallus Walker, 1838, Minottetrastichus frontalis (Nees, 1834) and Chrysocharis nepereus (Walker, 1839) (Hymenoptera, Eulophidae), these were recorded from the St. Petersburg region of Russia but all 4 parasites occur in the UK. Adult beetles are readily swept or beaten from host foliage but may occur more abundantly on surrounding vegetation; we found them in large numbers on grassy slopes around a reservoir in South Buckinghamshire during May and June 2017 but they were absent from nearby willows and present only in small numbers on young poplars growing beside the water.
Adults are small, 2.0-2.5mm, and entirely shiny black with pale red or yellowish appendages although the hind femora are usually darkened, the upper surface is sparsely clothed with very fine recumbent pale pubescence and the scutellum, and sometimes the adjacent pronotal border, has contrasting dense white pubescence. Isochnus is the only genus of the tribe that does not possess greatly enlarged hind femora, it is otherwise distinct in having geniculate antennae and the mesepisternum and metasternum clothed with dense pale scales. The eyes are closely approximated at the vertex and bordered with elongate pale scales and the rostrum is broad and strongly curved downwards, flattened above and expanded beyond the antennal insertions. The antennal scape is about as long as the following 2 segments, the funiculus is 6-segmented and the club is only slightly darkened towards the apex. The pronotum is transverse, broadest about the middle and the anterior and posterior margins are straight, the surface is strongly though shallowly punctured, on the disc the punctures separated by about their own diameter. Elytra broadest behind the middle and continuously rounded apically, striae strongly punctured and complete to the apex, interstices cross-rugose and weakly convex. Femora without a ventral tooth, front tibiae with an inwardly hooked process at the outer apical angle. Claws appendiculate.