Hylastinus obscurus (Marsham, 1802)
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POLYPHAGA Emery, 1886
CURCULIONOIDEA Latreille, 1802
HYLESININI Erichson, 1836
Hylastinus Bedel, 1888
This native western Palaearctic species is generally common across southern Europe from Portugal to Greece although it is not recorded from several of the Balkan countries, it is more sporadic and local in the north, extending to the UK, Denmark and parts of southern Sweden, the eastern limit of the distribution includes Belarus, Latvia and Serbia, and it is widespread across North Africa including the Atlantic islands. Following introductions in the late 19th century is now established and widespread in North America and is also recorded from several Neotropical countries. Here it is widespread though very local and generally scarce across Central and Southern England and Wales, including Anglesey and Man, and there are a few scattered records from Scotland and Northern Ireland. Adults are present for most of the year, they usually vanish for a month or two during the summer between the current generation dying off and the new generation appearing, and they are most common during spring and autumn. Host plants include various legumes, especially clovers (Trifolium L.), and with woody shrubs such as gorse and broom they tend to attack weakened or dying plants. Winter is passed in the adult stage, usually deep in the ground within roots but sometimes in the soil or among matted vegetation on the surface, in early spring when the ground warms up they make their way to the surface, feeding as they go, and once emerged most will disperse by flight. Adults usually migrate in numbers, climbing grass stems etc to take flight en masse, and here commercial growers use flight-interception traps to destroy them. Mating is believed to occur beneath the ground when they become active in the spring and so upon reaching new host material females will begin digging down to oviposit in host roots and stems, short oviposition galleries containing eggs and a female, sometimes also a male, have been found from April onwards and the female usually remains in place until the larvae emerge, at which time she, or the pair, will ascend to find new host material and may oviposit again. Later oviposition galleries tend to be shorter but contain more eggs, the female
will again remain to guard her eggs but adults die off from June and occasionally dead females remain in the galleries. Larvae bore straight or branched galleries between 20 and 40 mm long into roots, development time varies widely according to conditions and may last between five and seven weeks, they pass through at least three instars and fully-grown larvae are present from June. Pupation occurs at the end of the larval gallery and adults eclose after about ten days. The latest eggs will produce fully-grown larvae in October and these will overwinter within their feeding galleries but they represent only a small proportion of the overall number. Although typical of the family in most respects, bark beetles that feed on herbaceous and woody roots do not appear to be associated with symbiotic bacteria, rather they consume the plant tissue, in the present case both adults and larvae feed on root tissue, and in warmer areas of southern Europe and America, and sporadically elsewhere e.g. they are the main pest of clover in Chile, they can occur in large numbers and become a serious pest of cultivated legumes, causing damage both by direct feeding and by causing damage that allows fungal pathogens to enter the plant system. Adults can be sampled early in the year by sweeping or pitfall trapping and during the winter they sometimes appear in extraction samples, but otherwise they are mostly subterranean and rarely recorded.
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2.0-2.8 mm. Long-oval and almost cylindrical, parallel-sided and rounded anteriorly and posteriorly, entirely dark reddish-brown, anterior margin of head, legs and antennae paler brown. Head not, or only narrowly visible from above, smoothly convex and roughly rugose throughout, with reniform and widely transverse eyes and robust blunt mandibles, anterior margin of frons with forwardly-directed pale setae. Antennae inserted laterally in front of the eyes, 11-segmented and geniculate, scape long and gradually broadened from the base, funiculus 7-segmented, club flattened, oval and pointed, the three segments obvious. Pronotum slightly transverse, broadest near the base and smoothly narrowed to a rounded anterior margin, surface evenly convex, densely and moderately strongly punctured throughout and with sparse narrow pale scales, in some specimens there is a smooth longitudinal median line but this is very variable, basal margin bisinuate. Elytra with angled or narrowly-rounded shoulders, near-parallel lateral margins and a continuously rounded apical margin, lateral margins in side view distinctly sinuate, basal margin bisinuate and appearing finely toothed, striae regularly and strongly punctured to the apices, interstices punctured, finely pubescent and each with up to three rows of semi-erect setae, lateral interstices with fine tubercles towards the apex. In lateral view the elytral base is raised above the pronotal base, the dorsal surface is straight and the declivity evenly curved to a vertical sub-apical margin. Femora long and moderately broad, without teeth or spines. Tibiae broadened from the base, especially the middle pair, all with several strong external teeth towards the truncate apices. Tarsi pseudotetramerous with a strongly bilobed third segment. Claws separate from the base, smooth and without a distinct basal tooth.