Tenebrio Linnaeus, 1758
Eleven species of this very distinctive genus have been recorded from the Palaearctic region of which three occur in Europe and two in the U.K. Tenebrio opacus Duftschmid, 1812 is widespread in Europe and extends into Russia but is absent from the U.K., unlike the other European species it is wholly saproxylic, being associated particularly with oak woodland, and is not synanthropic. Of the two U.K. species, T. molitor Linnaeus, 1758 is widespread and locally common although better known as the yellow mealworm beetle, the ubiquitous source of pet food etc. while T. obscurus Fabricius, 1792, the dark mealworm beetle, is much less common though no less exploited. They are readily separated as follows:
12-18mm. Appearance shiny, the dorsal punctures everywhere discrete; those on the elytra separated by at least a puncture’s width. Pronotum more transverse, with almost perpendicular posterior angles which are only weakly, if at all, produced backwards. Terminal antennal segment elongate-oval and longer than the penultimate.
14-18mm. Appearance dull, the dorsal punctures dense and often confluent; those on the elytra so close that they appear to form transverse ridges. Pronotum less transverse, the posterior angles acute and produced backwards. Terminal antennal segment quadrate or transverse, and rounded, as long as the penultimate.
Tenebrio obscurus Fabricius, 1792
Dark Mealworm Beetle
This species is widely distributed in temperate regions worldwide but is much less common than the yellow mealworm beetle, T. molitor Linnaeus, 1758. It has much the same general lifestyle in the wild i.e. univoltine and developing in a range of situations, and is also synanthropic and an occasional pest of stored grains etc. Like the yellow mealworm its origins are probably Western Palaearctic and it has been spread globally with human activity e.g. it was first recorded in the United States in the 1860’s. It is similarly used in the pet food industry and known as either the dark- or mini-mealworm due to its generally smaller and darker larvae. The species is native to the U.K. and widespread although generally uncommon; it occurs mostly near buildings or in stables, grain storage facilities and flour-mills etc. In appearance it resembles T. molitor but is less shiny, overall rather narrower and differs in details of the pronotum, antennae and punctation.
Tenebrio molitor Linnaeus, 1758
Yellow Mealworm Beetle
This species was probably native to the Mediterranean region before being transported worldwide by human activities; it is now cosmopolitan but most prolific in northern temperate regions and rather infrequent in the tropics as it cannot breed in high temperatures. Conversely, the larvae have been recorded surviving in dry conditions at -5oCfor 80 days, and at -15oC for 3 weeks, an ideal adaptation to being transported with refrigerated foodstuffs etc. The species is generally common throughout Europe, including the U.K., but is thought to have declined over recent decades. Under good conditions it is a prolific breeder, the larvae develop quickly and are one of the largest among stored product insect pests, and these factors have made the mealworm one of the most abused species worldwide. They are farmed in absolutely astronomical numbers for a wide variety of uses; they are used as live food for pet fish, birds, reptiles and insects etc. as well as for fishing bait, and dried as food for wild bird feeders etc., and for these uses they are available by the kg in any high street or DIY store. Other uses include scientific studies into evolution, biochemistry and immunology etc. and because they are rich in fats and protein they are used both fresh and dried for human consumption where they are baked, toasted or fried in quantities to add to various dishes or to be consumed by the bowlful, they are a common item on restaurant menus throughout the tropics and are gaining popularity in other areas. Many commercial farmers optimize their product by incorporating juvenile hormones into the diet so that they remain as larvae for longer and become much larger as a consequence. They are typically reared on grains, oats or wheat bran but they will consume a wide range of foods of both vegetable and animal origin e.g. they have been recorded consuming dead insects, rodents, birds and meat scraps, and developing on droppings in avian nests, carpets and underlay, damp straw-filled furniture, sacking material and accumulated material in refuse bins etc., and in 2015 it was discovered they can develop feeding upon polystyrene, consuming 30-40 mg per day. In parallel with this huge commercial increase in the use of mealworms they may be decreasing generally in the wild; farmed stock is highly inbred and this has been shown to reduce the male pheromone signalling capability and therefore their attractiveness compared to outbred wild males and this may drastically reduce their fitness under natural conditions. Under less insane conditions the adults are active between May and September in temperate latitudes and occur in a range of situations e.g. avian and mammal nests or accumulations of organic detritus but they are also synanthropic, they fly strongly and are attracted to light and so regularly enter houses etc. at night when they will rest on walls before seeking dark or neglected corners or larders where food is stored undisturbed, they will also infest materials such as nests under floorboards or in roof spaces. They generally live for 2 or 3 months but up to 2 years have been recorded; they mate and oviposit early in the year and may visit several sites looking for suitable substrate; eggs are laid singly or in small batches over a period of 3 to 20 weeks. Almost any organic material will support the developing larvae; bulk stored products are especially attractive to ovipositing females but spilled or loose grain or cereal, especially when this is damp and infected with fungi, are often chosen, and occasionally population explosions occur in badly stored or disposed of products e.g. litter in poultry houses or accumulated debris in storage bins. Females are very fecund and each will produce between 200 and 600 eggs over the course of her life. Larvae emerge after about a week and begin feeding upon whatever is available, at first they form local infestations but as they grow may wander far from the original food to find further sustenance or to pupate. They are very voracious feeders and tolerant of a wide range of conditions. They will generally be fully grown by the autumn and will overwinter, usually among the food or nearby detritus, and pupate in the spring. Larvae can survive extended periods of adverse temperature, humidity or nutrition and under such conditions have been recorded taking 2 years to develop. The pupal stage lasts between 7 and 25 days and often occurs remote from the development site in order to avoid cannibalism from other larvae. Because of their economic significance mealworms are not always considered as pests, but infestations in domestic premises can cause alarm as the adults are conspicuous and their origin is not always obvious, especially as the larvae may wander between food sources and may be very difficult to eradicate, but in general the adults are easily removed and the larvae, with a relatively long development time, are rarely a problem in where standards of hygiene are high.
12-18mm. mature adults vary from pale brown to black. The dorsal surface, except the head, is glabrous and densely punctured with the cuticle flat and shiny. The head is transverse with a distinctly impressed frontoclypeal suture; the clypeus is expanded laterally in front of the eyes so covering the antennal insertions, and narrowed to a curved anterior margin. Eyes reniform and widely transverse. Antennae short; not reaching the base of the pronotum, segments 1-4 elongate; 2 barely so, 5-7 globular, 8-10 transverse, and the terminal segment is oval. The pronotum is transverse and weakly convex, the lateral and basal margins bordered, broadest about the middle and sinuate before almost perpendicular posterior angles, the basal margin bisinuate. Usually with a distinct basal fovea either side of the middle. Scutellum transverse; almost parallel in the basal half and broadly triangular apically. The prosternum is finely punctured and transversely wrinkled throughout and the notopleural sutures are distinctly impressed. The pro-coxae are round and separate, the prosternal process widening to a broad and rounded apex, the meso-coxae round and separated by about a coxal width, the mesosternum finely punctured throughout and transversely wrinkled in the basal half. The meta-coxae are transverse, reaching almost to the elytral epipleura. Abdomen with 5 free ventrites, the first strongly produced between the meta-coxae. Elytra completely covering the abdomen, sub- parallel and smoothly rounded apically; each with 8 or 9 well-impressed but sometimes vaguely defined striae and an abbreviated scutellary stria. The legs are generally a little paler than the body; robust and relatively long, trocantins small and slender, the junction with the femora strongly oblique, the femora broaden medially, and the tibiae are slender and gradually broadened to a truncate apex. The meso- and meta-tibiae have a pair of stout spines at the inner apical angle; the pro-tibiae are sexually dimorphic; in the male longer, more curved, flattened ventrally and produced apically. Tarsi 5-5-4, the claws smooth and without a basal tooth.