Tribolium (beetle)

Tribolium is a genus offlour beetles in the familyTenebrionidae.^[1] They are known by various common names includingflour beetles,flour weevils,red weevils andbran bugs.^[2]

Tribolium

Tribolium castaneum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Coleoptera
Suborder:	Polyphaga
Infraorder:	Cucujiformia
Family:	Tenebrionidae
Subfamily:	Tenebrioninae
Tribe:	Triboliini
Genus:	Tribolium Macleay 1825

Description

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AdultTribolium are beetles 3–6 mm in length and with colours ranging from reddish-brown to black. The clypeus is enlarged and has genae forming shelf-like projections extending around the sides, partly dividing the eyes. Theantennae are 11-segmented and either expand towards the ends or have terminal clubs. The prothorax is nearly square in shape and rounded on the sides. Theelytra are striated. They possess well-developed wings, but at least one species (T. confusum) is unable to use them to fly. The two sexes are similar in external appearance, i.e. there is nosexual dimorphism.^[2]

The eggs are 0.6 mm long, oblong in shape and whitish to nearly transparent in colour.^[2]

Thelarvae are elongate and approximately 6–7 mm long when mature. They can be distinguished from some other tenebrionid larvae by the last abdominal segment ending in two points (urogomphi). They are mostly yellowish-white except for: the dorsal part of the head, the tips of the claws and thetergites, which are slightly darkened; and the urogomphi and the tips of the mandibles, which are reddish-brown.^[2]

The pupae are initially white, but turn yellow after a day or two, then gradually turn darker.^[2]

Habitat

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In the wild,Tribolium occur under bark and in old logs. A number of species have adapted to living in stored plant products.^[2]

Life cycle

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Tribolium lay their eggs in a food source such as flour. These hatch into larvae which proceed to feed and grow within the food, avoiding light. There are usually 7 or 8 larvalinstars, but there can be as few as 5 or as many as 11 depending on food, temperature and humidity. The larval stage can take 22 days to over 100 days, depending on food, temperature and species (T. confusum takes longer to develop thanT. castaneum). Once development is complete, larvae come to the surface of their food or to some sheltered space or crack, where they become pupae. They use empty pupal cells left behind byMediterranean flour moths when available. The pupal stage lasts for 5–12 days depending on temperature and light. Adults emerge from pupae to continue the next generation. Adults are among the longest-lived of stored product insects, able to survive for over three years. Males (but not females) can continue reproducing even after their third year.^[2]

Distribution

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The original distribution ofTribolium is suspected to be in the region ofIndia, southwesternAsia and the easternMediterranean. The genus has since spread worldwide via human trade.^[2]

Pest status

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Two species ofTribolium,T. castaneum andT. confusum, are probably the most common secondary pests of stored plant products. A few other species are occasional minor pests.^[3]

They are known to feed onwheat,maize,flour,starch,peas,beans,nuts,dried fruit,spices andherbarium specimens, and food products made from these such asbread andcakes.^[2]^[3] They cannot attack intact grains, seeds or nuts, but can do so if these products are broken/cracked.^[2] They can also attack animal products such as preserved insect specimens, hides, bird skins andmilk powder.^[2]

Both adults and larvae cause damage. They produce toxicquinones which contaminate plant products.^[3]

Natural enemies

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Tribolium can be infected by protozoans of the genusAdelina.^[2]

They are parasitised by various mites, an example being theacarophenacid miteAcarophenax lacunatus which feeds on the eggs.^[4]

Theparasitoid waspHolepyris sylvanidis feeds onTribolium larvae and pupae, and lays its eggs on the larvae.^[5]

Thelyctocorid bugXylocoris cursitans is presumed to feed on the larvae. Mice are another (occasional predator) of the beetles.^[2]

Phylogeny

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Phylogenetic analysis of the genusTribolium indicates that it has two species groups,castaneum andconfusum, which are bothmonophyletic and are most closely related to each other. Several species which were once in the genus, such asT. brevicornis and its relatives, have since been removed to genusAphanotus.^[1]