Cryptolestes ferrugineus (Stephens, 1831)
Originally native to warmer regions of the Palaearctic, although possibly not to Europe, this species is now cosmopolitan in distribution, having been transported worldwide with the trade in foodstuffs. It is most prolific in warmer parts of the world where it is a pest of a wide variety of commodities including all types of grain and stored seed as well as oilseeds and cocoa but it also extends to temperate regions where it survives in heated or sheltered situations and is often among the most common pest of stored grain. Under good conditions it may be continuously brooded and, because of its small size and high fecundity, may go unnoticed until large populations develop. In general it occurs where large volumes of grain are stored or handled and it is common in cargo ships, warehouses, flour mills, silos and elevators, it also occurs among processed flour and sometimes alongside other members of the genus, is one of the most cold-tolerant grain pests and has one of the highest rates of population growth of any grain pest. In temperate regions the main hosts are wheat, barley, rye and oats but other grains are occasionally attacked, it is classed as a primary pest because both females and larvae are able to bore into healthy grains and do not rely on damage inflicted by other means, larvae destroy grains by direct feeding but they may also spread fungal spores through stored grain and large infestations mat cause grain to heat up and spoil. Mated females move freely among the host material, feeding and laying eggs but they disperse readily by flight and so new infestations constantly appear, each may lay between 200 and 500 eggs among grain debris and kernels over a long period, they do not fly when the temperature is below 23 Celsius but remain active; they may disperse among grain and are capable of climbing glass and so restricting a population is very difficult, many populations have also developed a very high resistance to phosphine, a grain fumigant, which further adds to the difficulty of controlling the species. Upon emerging from the eggs larvae bore into the grain and feed on the seed germ, consuming stored sugars and destroying the seeds ability to germinate, they remain within the host although are also capable of moving between grains if necessary, pupation occurs within the damaged grains and adults eclose after
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a few days. Under optimum conditions of 35 Celsius and 70% relative humidity larvae hatch within 4 or 5 days and the complete life cycle takes about 3 weeks but as the temperature drops adult reproduction slows considerably and when the relative humidity drops below 40% or the moisture content of the grain falls below 12% larval development is restricted. This species occurs sporadically though regularly throughout Europe, including England and Wales; it is usually associated with grains but may also occur dried fruits, nuts and rice where it is not a pest as the adults feed upon surface fungi and do not oviposit, in Europe they also occur in the wild under damaged bark on both coniferous and broadleaf trees.
Although tiny at 1.5-2.5mm this species is distinctive due to the wide transverse head, cordiform pronotum and entirely pale colour. Several other members of the genus are superficially similar but ferrugineus may be distinguished by the combination of a single lateral carina and sharp posterior angles to the cordiform pronotum. Habitus elongate and rather parallel-sided, with the base of the pronotum distinctly narrower than the base of the elytra. Head transverse, smoothly convex; without a transverse basal sulcus, and finely punctured and pubescent, transversely impressed anteriorly, with prominent convex eyes and short temples. Labrum rounded anteriorly, mandibles tridentate apically, curved and prominent; in males laterally expanded towards the base, antennae 11-segmented and about as long as the head and pronotum combined in both sexes, segments 4-10 quadrate to slightly elongate. Pronotum transverse, widest near the rounded anterior angles and narrowed to obtuse posterior angles, laterally sinuate in the basal half, surface finely punctured and pubescent, laterally with a single fine carina parallel to the margin. Elytra elongate, with rounded shoulders, weakly curved lateral margins and a continuously rounded apex, striae impressed and complete to the apex, interstices convex and finely punctured and pubescent; the first and second with four rows of punctures. Legs relatively long, with thickened femora and slender tibiae, tarsi 5-segmented in the female, 5-5-4 in the male.