Rhyzopertha dominica (Fabricius, 1782)
Thought to have been native to South-eastern Asia and spread to tropical regions worldwide, this naturally saproxylic species has become a serious pest of various stored products in temperate regions worldwide. It occurs frequently though sporadically across Europe with imported products and sometimes becomes temporarily established in the wild but permanent or long-term populations will probably survive the temperate climate only in the extreme south, in northern areas such as the UK it is infrequently reported and then mostly near areas processing imported goods, and sustained infestations can only occur under artificially heated conditions. In tropical climates its hosts include a range of hardwood trees and shrubs where the larvae develop under bark and within xylem, they develop quickly and may produce large populations, the adults fly-well and may disperse over long distances, they can bore into both natural and processed hardwoods and other materials and have a lifespan of between four and nine months, all of which equip them to become serious pest species under artificial conditions outside their natural range. Under artificial conditions their hosts include a very wide range of stored foods including wheat, barley, rye, rice, maize, millet, beans, flour, biscuits and a range of herbs and spices. The species is a primary pest, it can detect and bore into undamaged grains and therefore render products susceptible to secondary pests such as grain weevils etc., it can also introduce and spread fungal infections and the females are very fecund and so outbreaks in storage facilities can be very serious. Each female can produce between 200 and 500 eggs over a long period, they are small groups between 2 and 30 on the surface of host material, depending on the nature of the host most emerging larvae will bore into grains etc. and develop within, but they can also develop as free-living insects, feeding on debris and dust and developing at about the same rate. Under ideal conditions of about 12% r.h. and 30°C the life-cycle takes about 25 days and although adults fly readily they tend to remain and oviposit where the host is abundant so that very large populations can quickly build up. Larval development varies according to the host material, there may be between 2 and 7 instars and pupation occurs within the host unless the larvae are free-living. Both adults and larvae feed among the host, they leave empty grains with characteristic emergence holes, large amounts of frass and give the material a distinctive aroma and so, at least to the experienced, infestations can easily be detected.
Rhyzopertha dominica 1
© Lech Borowiec http://www.cassidae.uni.wroc.pl/Colpolon/index.htm
Rhyzopertha dominica 2
© U.Schmidt https://www.kaefer-der-welt.de/index.htm
Adults are small, 2-3mm, elongate and cylindrical in form and entirely dark brown to black in colour, usually with the appendages a little lighter, dorsal surface with sparse pale pubescence. Head from above covered by the arched prothorax, with large convex and protruding eyes and curved, well-sclerotized mandibles, vertex and frons very finely punctured and with scattered larger setiferous punctures, frontoclypeal suture well-impressed and labrum evenly curved anteriorly. Most of the head consists of a hugely expanded globular capsule, usually just visible behind the eyes but generally mostly concealed within the prothorax. Antennae 10-segmented, 2 basal segments enlarged, 3-7 small and transverse and 8-10 forming a loose asymmetrical club, the lack of a distinct scape and the form of the labrum will separate this species from superficially similar scolytids. Pronotum flat across the disc then strongly declined anteriorly, broadest before rounded posterior angles and narrowed anteriorly, surface entirely tuberculate, randomly and moderately strongly across the disc and becoming stronger and arranged into circular rows towards the apex, giving the apical margin a toothed appearance. Elytra slightly broadened towards the apex, with angled shoulders which are slightly narrower than the pronotum and a continuously curved apical margin, each with 10 regular and strongly punctured striae. Legs long and slender, with closely approximated coxae and only slightly widened femora. Tibiae long and slender, each with a series of stout teeth along the external margin, most developed on the fore and mid tibiae, apex developed externally into a sharp tooth and fore tibiae produced internally into a sharp hook-like process. Tarsi long and slender, basal segments short and lobed below, terminal segment long and curved, claws curved and smooth, without a basal tooth or process.