Nacerdes melanura (Linnaeus, 1758)
Wharf Borer
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POLYPHAGA Emery, 1886
TENEBRIONOIDEA Latreille, 1802
OEDEMERINAE Latreille, 1810
NACERDINI Mulsant, 1858
Nacerdes Dejean, 1834
Originally native to the western Palaearctic this species is now more or less cosmopolitan and established in most parts of the world although in tropical regions it tends to be sporadic as the upper limit for its eggs to develop is between 30 and 35oC and a relative humidity of at least 20%. It develops in old damp timber which has been infected with fungi; softwoods seem to be preferred but a wide range of hardwoods including fig and eucalyptus have also been recorded hosting the beetle, it will attack damp structural timber as well as decaying parts of living trees, and it can withstand a very wide range of environmental conditions. The larvae are classed as secondary pests as they only consume infested timber and may greatly accelerate the spread of the fungus. The host material must be permanently damp or wet and fungal infection must have begun before the beetle will choose it, buried wood and structural timbers regularly submerged by the tide are particularly prone to attack as the larvae can withstand prolonged immersion in both salt and fresh water, they have historically been notorious pests of wooden ship hulls and structural maritime timber-hence the common name-and infested timber is often partly buried e.g. pilings or joists in basements, or concealed and inaccessible as in the submerged parts of ships or loch gates etc. The larvae can be long-lived so that damage can be extensive before an infestation is discovered and often the first sign of the beetle is the mass emergence of adults, and so the global spread among timbers of sailing ships was inevitable. In the U.K. it is widespread although local; there are many coastal and estuarine records and formerly it has been common around the canal system in the midlands, there was an increase in abundance following the war when large quantities of timber became buried as a result of bombing and for a while the base of telephone poles were commonly infested, during the 1970’s it became common in damp basements in London and other large cities and it also infests trees and fallen timber near rivers and lakesides etc. In temperate regions the adults have a long season, from May to September, but they are
short-lived, generally a week or two although in general this is reduced at higher temperatures, and they usually emerge together and swarm, they are mostly diurnal or crepuscular but they often come to light, sometimes in numbers, they do not feed upon the host timber but have been observed consuming pollen from a range of flowers. There is a single generation each year but in warmer climates they have been recorded breeding continuously. They detect suitable host material by smell and may suddenly appear swarming around suitable oviposition sites, here they mate and the females ‘stick’ the eggs to the timber surface where they are subjected to a wide range of conditions; under ideal conditions, 20-30°C and 20-40% r.h., they will hatch within 5-10 days but they are otherwise resilient and can take much longer if subjected short-term immersion or desiccation. Freshly emerged larvae immediately burrow into soft and damp wood, feeding initially within a centimetre of the surface but as they grow they burrow deeper, producing randomly-shaped galleries up to 30cm long and reaching down to solid wood; they produce cellulose which allows them to feed directly upon cellulose and hemicelluloses. The length of the larval stages depends upon the temperature, moisture content and nutritional value of the wood, and the total developmental time may take anywhere between two months and two years although under wild conditions it is usually completed over a single season. Upon reaching a certain size, generally in spring or early summer, they pupate within the host and adults eclose between one and three weeks later. The pupae are very resistant to desiccation but will only develop within the temperature range 10-30°C. Although saproxylic development is common in oedemerids this is the only problematic wood-boring species.
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Among the U.K. fauna this species should be unmistakable although the cantharid Rhagonycha fulva and the cerambycid Phymatodes testaceus are superficially similar, in both cases the heteromerous tarsi of Nacerdes are distinctive.
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9-13mm. Elongate and parallel-sided; cantharid-like in appearance, testaceous or dark testaceous with the elytral apices darker. Head prognathous or somewhat hypognathous, widest across the eyes which are emarginate anteriorly, and with temples that narrow to the pronotal margin. Surface shiny, finely punctured and pubescent, the frontoclypeal suture absent or only weakly impressed, the mandibles are long, prominent and bifid apically. The last segment of the maxillary palpi is elongate and gradually widened towards an obliquely truncate apex. Antennae filiform; 11-segmented in the female and 12 in the male. Pronotum slightly transverse, cordiform and broadest in front of the middle, without lateral borders but with the anterior margin raised and the posterior margin distinctly bordered. The surface is finely punctured and has fine recumbent yellow pubescence. The anterior margin of the prosternum is wide and the pro-coxal cavities are closely approximated and open at the base. Abdomen with five visible sternites; all sutures distinct and visible. Elytra proportionally very long, with fine recumbent pubescence; oblique near the suture and straighter towards the lateral margins, each elytron with four longitudinal costae which extend to the apical quarter, the third often obliterated apically. Legs long and slender, the colour varies from testaceous to almost black. Protibiae with a single spur on the inner apical angle, meso- and meta-tibiae with two well-developed spurs on the inner apical angle. Tarsi 5-5-4, the penultimate segment bilobed. Claws smooth.