Anoplus Germar, 1820
This small genus includes four Palaearctic species; A. japonicus Morimoto. 1983 is endemic to Japan while the others are widespread, A. plantaris (Naezen, 1794) occurs throughout the region, including Japan, while the others are restricted to the west; A. roboris Suffrian, 1840 is widespread in Europe and also occurs in Turkey, and A. setulosus Kirsch, 1870 is widespread, extending into Asia Minor, but does not occur in the UK, and the subspecies A. s. pericarri Tempére, 1972 is endemic to Corsica. Our two UK species may be distinguished by the form of the tarsi; three segmented with the terminal segment widely bilobed. They are small, <3 mm, elongate weevils with broadly oval elytra and a short and broad rostrum, the forebody is strongly punctured and the antennae are geniculate with a 7-segmented funiculus, the elytra have punctured and strongly impressed striae and the interstices are convex. The legs are short and robust with unarmed femora and the front tibiae have an inwardly-curved apical hook. All are drab coloured, black or dark grey with the antennae variously pale towards the base, and have fine pubescence to the body and legs. Our two UK species are easily distinguished as follows:
-Larger, 2.3-2.8 mm. Pronotum microsculptured between the punctures, elytral pubescence includes more conspicuous, apically truncate hairs.
-Smaller, 1.7-2.2 mm. Pronotum lacking microsculpture, smooth and shiny between the punctures, elytral pubescence fine and pointed.
Anoplus plantaris 1
Anoplus roboris 1
© Mark Gurney
Anoplus plantaris 2
©Lech Borowiec http://www.cassidae.uni.wroc.pl/Colpolon/index.htm
Anoplus plantaris 3
Anoplus plantaris (Naezen, 1794)
This is the most common member of the genus, it widespread and locally common from lowland to mountain altitudes across central and northern Europe, extending to the far north of Fennoscandia and east through Russia and Siberia, in the UK it is common across England and Wales though scarce in the West Country, more local in the north and there are a few records from the Scottish Highlands and Northern Ireland. Adults are present from April until late in the year and occur on birches; Silver birch, B. pendula, and downy birch, B. pubescens, as well as various hybrids, and on the continent they also occur on alder, Alnus glutinosa. Typical habitats are wherever the hosts occur, woodland and wooded parkland, heaths and moorland etc, and adults will usually be found in numbers. Mating occurs early in the year and oviposition begins in early May, eggs a single egg is laid on the underside of a developing leaf, usually near to the central vein in the distal half where the larva will begin to mine, affected leaves are usually much broader than others and it is thought that secretions from the female during oviposition retard the longitudinal expansion of the leaf. The larva enters the leaf at the oviposition scar which may become swollen and resemble a gall, it mines towards the leaf margin and then the mine usually loops around the edges, often isolating areas of leaf which then die-off, mines are translucent, they penetrate to both surfaces and have a thin dark central line of frass throughout their lengths. Larvae are fully-grown by June when they cut out an area of leaf around the end of the mine, this will fall to the ground and the larva will pupate within. New generation adults occur from June or July, these will go on to overwinter and there is only a single generation each year. Adults may be beaten or swept from host foliage during the season and may be found among litter around the base of host trees during the winter.
1.7-2.2mm. Entirely black with the legs dark brown and the antennal scapes yellow. Readily identified by the 3-segmented tarsi. Head transverse with weakly convex eyes and short diverging temples, vertex strongly punctured, clypeus and rostrum more sparsely so, rostrum short and broad with lateral scapes visible from above towards the apex. Antennae geniculate, scape rather abruptly thickened towards the apex, funiculus 7-segmented, club broadly-oval. Pronotum transverse, broadest in the basal third and narrowed to a straight apical margin, basal margin sinuate medially, surface strongly punctured and shiny, lacking microsculpture. Elytra shiny black, broadest in the apical half and weakly constricted before a continuously rounded apex, striae deeply impressed and indistinctly punctured, interstices convex, finely rugose and each with a more-or-less regular row of pale semi-erect setae. Femora unarmed, middle and hind tibiae unarmed, front tibiae with a small incurved spur at the apex.
Anoplus roboris Suffrian, 1840
This species is locally common throughout Europe, extending north to the UK and all but the northernmost provinces of Fennoscandia, and occurring from lowlands to middle mountain altitudes. In the UK it local across the south of England and Wales and very local and generally scarce further north into central Scotland and across Ireland. Typical habitats are wetland margins and damp deciduous woodland but they may also occur on trees in parkland and wasteland; the species is oligophagous on Alder, in the UK probably exclusively on European Alder, Alnus glutinosa (L.) but also commonly on Grey Alder, A. incana (L.) on the continent. Adults occur from April until September, peaking in abundance during late May and June, and mating pairs can be seen on foliage during spring and early summer. Mated females chew small cavities into the base of the midrib or a thick lateral vein and insert a single egg or a small group of eggs, this oviposition site usually develops into a large swollen scar which is obvious on the underside of the leaf. Following emergence the larvae enters the leaf and mines towards the margin, often producing widely meandering mines that may join to isolate small areas of leaf which die off, development takes only a few weeks and the mine terminates near the edge of the leaf, the corridor being lined with a narrow central line of frass, the fully developed larva cuts around the leaf margin causing it to fall to the ground and then pupates in a chamber between the upper and lower leaf epidermis. Leaf mines are plainly visible from above and below and after drying out for a week or so the upper surface often splits along the mine. New generation adults occur from June or July and these may feed but will not reproduce until the following spring. Adults may be sampled through the season by beating or sweeping foliage and they sometimes occur among leaf litter extractions through the winter.
Very similar to the previous species but larger and differing as above, they become obvious in series but specimens will need to be taken for critical examination for certain identification.