Stegobium paniceum (Linnaeus, 1758)
Drugstore Beetle
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POLYPHAGA Emery, 1886
BOSTRICHOIDEA Latreille, 1802
ANOBIINAE Fleming, 1821
STEGOBIINI White, 1982
STEGOBIUM Motschulsky, 1860
This is the only member of the genus Stegobium Motschulsky, 1860. It has a worldwide distribution and is one of the most frequent pest species, infesting a very wide range of stored products, typically bread, cake and biscuits but also e.g. flours, spices, wool, hair, leather, museum specimens and even strychnine, a poisonous plant extract. It has been recorded sporadically and commonly throughout the U.K. The adults do not feed but have been recorded boring through wood, tin, aluminium and lead as well as into books. Larval feeding accounts for most of the damage and spoilage; they live in symbiosis with yeasts that are passed from the adult to the egg, and this allows them to develop on foods of poor nutritional quality. In temperate climates the species occurs in warehouses, stores and domestic premises etc. and avian and mammal nests in roofs etc. have been found to host them in large numbers. In general they inhabit warm and dark situations that remain undisturbed for long periods, they fly strongly and usually the first signs of their presence are adults on walls and other surfaces, especially in warm weather. They are attracted to light and commonly enter houses on warm summer evenings. Females lay around 75 eggs at a time on or near to suitable host material, and the newly hatched larvae, less than a millimetre in length, begins to search for food, boring into packaging etc. if necessary in order to find it. Development is strongly dependent upon temperature, moisture content and the quality of the food; in temperate climates there is a single generation each year while towards the tropics there may be 4 or 5. Optimum conditions are around 30°C and 60-90% relative humidity with a suitable food source. Pupation occurs in a silken cocoon spun inside a chamber bored into the host material and lasts between 12-18 days. The entire life cycle takes between 2 and 7 months and adults live for 2 to 10 weeks; the eclosion period may be very long in temperate climates and so adults may be observed throughout the season. We maintained a breeding population for several years in a small, 50 gram sample of unsalted peanuts and the adults were observed from April to September with occasional specimens during the winter.
This species will soon become familiar, some Anobium species are superficially similar with elongate terminal antennal segments and striate elytra but they are larger, darker and have the pronotum distinctly raised and angled above in side view; in the present species the dorsal surface of the pronotum is evenly curved in side view.
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2-4mm. Entirely shiny pale to dark brown with the appendages pale, the entire dorsal surface has dense short and recumbent pubescence as well as sparse longer setae. The head is prognathous and generally not visible from above, being mostly hidden under the extended pronotal anterior margin. The frons is flat between the characteristic antennae; filiform with the last 3 segments elongate and flattened, a little longer in the male. The insertions are further apart than the length of the first segment. The eyes are large and round with the anterior margin weakly sinuate behind the antennal insertions. Pronotum smoothly convex and finely bordered laterally, broadest in front of rounded hind angles and, from above, narrowed to an evenly rounded anterior margin. The surface is uneven; rugose and punctured with patches of dense pubescence at the hind angles and about half way along the lateral margins. The scutellum is small and triangular. Elytra parallel, entirely covering the abdomen and evenly rounded at the apex. Each with well impressed striae, including a scutellary stria, which are only finely punctured, the interstices wide and flat, roughened and finely punctured. The legs are slender and relatively long. Tarsi 5-5-5 with only the basal segment elongate, 2-4 are variously lobed below and the terminal segment is quadrate.