Sitophilus oryzae (Linnaeus, 1763)
This is primarily a species of warmer countries and occurs naturally in tropical regions throughout the world but following its transportation with the shipment of grain and other food products it is now cosmopolitan and established generally in temperate regions. Historically there has been confusion between this and the closely similar S. zeamais Motschulsky, 1855 and so the increase in its distribution has become uncertain but it is now understood to be widespread; both species are distinct from S. granarius (Linnaeus, 1758) in having fully-developed wings and being able to fly. This is one of the most serious pests of stored grain worldwide but it also develops on a wide range of other host material including dried pulses, nuts, cereal products, grapes, pasta products and dry flour, and adults will feed on a range of non-citrus fruits, hollowing out cavities where they may remain concealed and transported along with the produce. Females are very fecund, under favourable conditions producing between 300 and 400 eggs although this is highly dependent on temperature; below 7 Celsius they may remain dormant for long periods and perish after a week or two at freezing temperatures or within an hour at 49 Celsius, and optimum development occurs between 27 and 30 Celsius. The female will search through a sample to find a suitable grain and then bore into it with her rostrum and deposit a single egg, then afterwards seal it with a gelatinous secretion which will protect the developing larvae and indicate to other females that the grain is unsuitable host material. The small, white and legless larva emerges within a week or so and begins to consume the starchy contents; it passes through 4 instars and may be fully grown within 2 weeks. Pupation occurs within the grain and adults eclose after a week or so, they remain within for 3 or 4 days to harden and mature and then bore their way out. After emerging the females will crawl up to a high surface and release sex pheromones which will attract more males to an infestation. Under favourable conditions the entire cycle from egg to adult may occur within 26 days and the
Sitophilus oryzae 1
Sitophilus oryzae 2
Sitophilus oryzae 3
beetle may be continuously brooded with all stages being present in a single infestation. Unlike S. granarius, the adults may disperse by flight from stored products and infest crops growing in the open, these infestations generally continue after harvesting and so the species may be very difficult to control. Early detection of an infestation is difficult; adults leaving the grain will leave dust and frass which, along with the emergence holes, is obvious, infected grains are more buoyant when immersed in water, and a large infestation will produce an increase of temperature within the sample, the most obvious indication is the presence of adults which will very quickly become abundant; in one study 100 adults per kilo of grain were found within 5 weeks of infestation. In temperate regions this species tends to be most prevalent in southern areas and displaced by S. granarius further north; in the UK it is frequent in stored grains etc. and generally widespread across England and Wales and there are scattered records further north to the Scottish Highlands.
A small and characteristically-shaped weevil readily separated from S. granarius by the pronotal punctures which are dense, large and slightly elliptical, but it is very similar to S. zeamais from which it may often be distinguished solely by the form of the male genitalia; in the present species the median lobe is smoothly convex externally while in zeamais it is has 2 longitudinal furrows. In general oryzae is darker brown with 2 diffuse pale spots to each elytron and the pronotal punctures are elliptical rather than circular as in zeamais. 2.3-4.0mm.