Adistemia watsoni (Wollaston, 1871)
Native to Europe this species has become established through trade throughout the Palaearctic region and occurs sporadically across the globe among transported foodstuffs, it rarely occurs in the wild but under artificial conditions may occur anywhere; it has been recorded across Europe and North Africa including the Atlantic islands etc and the UK but surprisingly it was first recorded from Denmark in 1991, Norway in 2010 and Greece in 2011. There are only a few records from the UK, these are scattered across England and Wales and many are pests from museum collections, but the species might occur among stored products anywhere. In the wild the most likely habitats are among decaying organic matter such as hay, straw, decaying potatoes and other tubers or old bird nests and the associated accumulated droppings infested with fungal hyphae and spores upon which both the adults and larvae feed, but under artificial conditions they may occur among any products hosting fungi e.g. it was found in Greece feeding on Penicillium conidia on stored corn and plums. The most frequent habitats are warehouses, granaries and food processing plants as the only requirement for the beetles is sufficient moisture to allow fungal growth, for this reason they are less frequent in shops and houses but they are still regularly recorded in dwellings throughout the world. Under artificial conditions they breed at any time of year and are continuously brooded, they are attracted to fungus and so foods stored under dry conditions are rarely affected but adults are tiny and easily overlooked and so readily transported with fruit and vegetables etc, Oviposition occurs over a long period, females seek relatively dry conditions and moderate temperatures to oviposit and usually lay a single egg directly onto the fungal substrate and then move on to new host material, they have not been observed to fly and both adults and larvae are photophobic and so it is likely that all infestations are directly from infected introduced material. Larvae emerge within 6 days and remain among the host material, moving only when this becomes scarce, they are fully developed within 2 weeks and attach themselves to the fungus to pupate. The entire cycle from egg to adult varies from 21 to 35 days and all stages are likely to be present in an infestation. Both adult and larval mortality increases with temperature and humidity and so fairly stable conditions are needed to maintain a population e.g. 4% of adults die over 7 days at 32°C while 98% die over 7 days at 38°C and at 43°C all adults die within 24 hours. Adults are usually sampled in commercial traps designed to monitor infestations but they have often been recorded on walls in domestic premises, they disperse at night and sometimes occur in numbers on damp wallpaper or among crumbling plaster.
Adults are tiny, 1.2-1.5 mm and easily recognized by the narrow elongate form, more especially the long head and smoothly-oval elytra, entire body and appendages pale brown, usually with the forebody a little darker, and extremely finely pubescent. Head elongate with small convex eyes in which the few individual facets are easily seen, the temples are long and rounded and the cheeks are long and converging towards an apical expansion, surface evenly convex and densely punctured, without depressions or other structure. Antennae inserted on the dorsal surface of the anterior margin, 11.segmented with a well-defined 3-segmented club. Pronotum elongate, broadest in front of the middle and narrowed to distinct posterior angles and a rounded apical margin, surface without sculpture, strongly and densely punctured throughout. Prosternum long in front of nearly contiguous coxae, middle coxae very close, hind coxae widely separated and the first abdominal ventrite fused to the metasternum. Elytra long (9:4) and smoothly oval, surface strongly sculptured; with rows of large round punctures, most of which are separated by transverse ridges, the sutural, third and seventh interstices strongly raised into almost complete ridges, the lateral margin under the seventh interstice steeply angled to a narrow explanate margin. Legs long and slender, femora and tibiae smooth and the tibiae with only very fine apical spines, tarsi 3-segmented, the apical segment at least as long as the others combined.