Sitophilus granarius (Linnaeus, 1758)
Originally native to eastern Mediterranean regions this species has become established throughout cooler regions of the world due to the transportation of cereal grains, it is among the most destructive pests of stored grains and may cause the complete destruction of grain in silos, bins or even in ships while being transported. In general it has moved from natural and cultivated grains in the wild to developing solely among grain in storage and has become dependent on humans for its distribution. There is some evidence that acorns may also have hosted the weevil as they have infested cracked acorns and have preferred them when offered alongside grains, and it has been suggested that acorns may have hosted the weevil before the development of modern agriculture and that smaller strains of the weevil developed as adaptations to infesting stored grains. While it is a major peat of commercially stored grains it tends to be sporadic and unimportant in domestic situations as it will not infest broken grains or grain products but it occasionally occurs among bird food and stored cereals and it may be obvious as adults have a tendency to disperse and may be found on walls etc. remote from their host material. Adults are long-lived, up to 8 months, and the species is continuously brooded under most artificial situations, usually producing 4 generations each year but there may be more as the life-cycle varies from about a month to up to 5 months and among large infestations all stages may be present simultaneously. Females are very fecund, producing up to 250 eggs under favourable conditions, they search through host material and when a suitable grain is found a hole is bored into it with the rostrum and a single egg is deposited, and then it is sealed with a gelatinous substance which will indicate to other females that it is unsuitable as host material. The larva emerges within a few days and feeds on the starchy material within, eventually hollowing the grain, there are 4 instars and development usually takes between 3 to 5 weeks, pupation then occurs within the hollow grain and the adult will eclose after a week or two. While the females lay only very few eggs at temperatures
below 16 Celsius they are very resilient and may survive of several months at 2 Celsius, but they are not so tolerant of high temperatures and an hour at 49 Celsius is fatal. This species is primarily a pest of wheat, rye, barley and oats and its effects can be severe but its impact worldwide is poorly known as it is believed to be particularly bad in regions where grain harvests are not accurately measured or monitored for pests; the weevils may be hard to detect as when disturbed they pull in their appendages and play dead, and once found the entire grain sample is usually destroyed. Pesticides e.g. Deltamethrin powder, and odour-masking chemicals have been used to protect stored grain and the introduction of predators has been trialled but early detection and safe and hygienic storage are the most effective methods of control; a visual examination of infested grains will soon reveal exit holes, and immersing a sample may be a good way of finding an infestation as infected grains are much more buoyant. In the UK it occurs sporadically through Wales and the south of England, usually in stored granary products or granary waste, also among grain in bird feeders, and samples of live adults, supplied breeding among grain samples, are widely available for exotic pet food.
This small and flightless weevil is very distinctive among our fauna due to the elongate compact form and the large prothorax. 3.0-4.3mm. Entirely shiny dark brown, sometimes almost black. Head triangular and evenly convex, with flat eyes and a long weakly curved rostrum that is characteristically dilated in front of the base. Antennae inserted on the rostral dilation, scape short and widened towards the apex, funiculus 6-segmented and the club elongate. Pronotum elongate, constricted sub-apically and widest behind the middle, surface punctures elongate and moderately dense. Elytra broadest behind the shoulders and regularly narrowed to a continuously curved apex which entirely covers the abdomen, striae with narrow and elongate punctures and equal to or narrower than variously convex interstices. Fore tibiae with a characteristic long and inwardly curved apical ‘hook’.