Cryptolestes pusillus (Schönherr, 1817)
This is one of the most common pests of stored grains, thought to be originally native to warmer areas of Europe it is now most prevalent in tropical and subtropical regions but regularly occurs in all temperate areas through the worldwide trade in foodstuffs and may occasionally become established in artificially heated food production premises. It is a secondary pest of grains, attacking only seeds that have been damaged by physical handling or by infestations of other pest species e.g. it often occurs among large populations of the rice weevil, although on softer stored fruits it can feed on the surface or burrow into undamaged material. Among the most commonly affected hosts are coffee, barley, rice, groundnut, sorghum, cocoa, wheat and maize but they have also been recorded from a wide range of dried stored products including processed flour. Adults fly well and may quickly find suitable host material but they are attracted to volatiles produced by damaged grains and so under suitable conditions of temperature and humidity such material is quickly infested. Adults of both sexes feed on or within grains etc. they mate among the host material and females live for up to a year, laying eggs over most of this time. Fecundity is greatest at higher temperature and humidity (> 33 C and 90% r.h.)and under ideal conditions a female may lay more than seven eggs each day among damaged host material. Larvae cannot penetrate intact grains but quickly find damaged areas and bore into the endosperm or the germ to begin feeding. The duration of larval development depends critically upon temperature and humidity but under ideal conditions they are fully grown within ten days, pupation occurs within the grain and the whole life-c cycle may be completed within five weeks although more generally this takes about two months given variations in nutrition, temperature and humidity. In temperate regions the adults are occasionally recorded among grain in the wild but are generally unlikely to survive winter conditions, they are usually the result dispersal from large populations although they are known to tolerate high densities among stored grains and with sufficient host material long-range dispersal is rare, rather they are translocated along with the host. Infestations are rare in the UK although may be under-reported, we once found huge numbers in a sample of flour infested with Tribolium castaneum (Herbst, 1797)
and no doubt the source of this sample was extensively infested, but imported material is occasionally infested and may lead to large populations in long-term storage facilities. Large populations can spread moulds etc. and may cause host material to heat up and become damp and so economically worthless and so regular monitoring is standard, control measures include lowering the host temperature by aeration or chilling, fumigation and irradiation but complete removal usually includes sanitizing the entire facility as the tiny beetles may endure in crevices etc wherever host material accumulates.
Adults are tiny, 1.4-1.8mm and can be very difficult to identify from other members of the genus but a good key can be found HERE. A flat, elongate and rather parallel-sided species which varies in colour from pale brown or grey to very dark grey, the head and pronotum are finely and sparsely punctured and have rather long, randomly arranged, pubescence, that on the elytra being arranged into longitudinal lines between the striae. Head transverse with large coarsely-faceted eyes and raised lateral ridges which are linked across the base of the head by a fine impression, this is usually only visible if the head is extended, frontoclypeal suture very fine but distinct and labrum truncate or weakly rounded. Antennae 11-segmented and inserted laterally in front of the eyes, all segments distinctly, if sometimes only weakly, elongate with the third and forth segments subequal, dimorphic; in the male as long as the pronotum and elytra combined, in the female much shorter. Pronotum transverse and almost as wide across the base as the elytra, widest in the apical half and narrowed to perpendicular posterior angles, anterior and basal margins only slightly curved, surface with a single longitudinal carina parallel to each lateral margin. Elytra elongate, 1.3-1.5X longer than wide, with distinct shoulders and a continuously rounded apical margin, each with four very fine raised striae and wide, finely punctured interstices; on the disc with four distinct rows in the inner interstices. Legs relatively long with broad femora widely visible from above, tibiae only weakly broadened towards the apex, tarsi short and narrow with the terminal segment elongate, 5-segmented in the female, 5-5-4 in the male. Very similar to C. ferrugineus but that species lacks the transverse impression linking the lateral ridges on the head, male ferrugineus have a large internal tooth on the mandibles which is lacking in the present species, and the antennae are only slightly longer than in the female.