Dark mealworm beetle

Adults of the dark mealworm beetle, Tenebrio obscurus F., resemble the mealworm beetle but are typically a paler brown. Body length 12–18 mm. At the adult stage, it is distinguished by denser punctation on the head and pronotum. Larvae are cylindrical and actively move within infested commodities. Initially white, they progressively darken and reach 25–30 mm at maturity.

Damage in stored grain is diffuse and non-diagnostic. Both larvae and adults feed opportunistically on broken kernels, fines, and germ, abrading the pericarp and shallowly tunneling into softer endosperm. Expect generalized surface grazing, irregular notches, and partial hollowing rather than the clean perforations typical of primary weevils. Infested lots show comminution to meal, abundant friable frass, accumulations of cast exuviae and dead insects, and silk-free clumping. Metabolic heat and moisture from respiration promote localized hotspots and caking, encouraging mold growth and musty, rancid off-odors. Consequences include dry matter loss, reduced test weight, injury to embryo (lower seed viability), aesthetic and sanitary contamination with insect fragments and allergens, and overall quality downgrading. Packaging and sacking may also be gnawed, spreading fines and contamination. The damage pattern is generalized and mirrors that of other secondary stored-product beetles.

Signs of infestation indicating the presence of the Dark mealworm beetle, Tenebrio obscurus F., in stored grain: - Readily visible life stages: large, cylindrical, sclerotized larvae (“mealworms”) and dark, elongate adults, often active in surface layers, seams, and residues. - Odor: a pronounced rancid/musty, sometimes phenolic taint from insect-degraded grain; often detected before insects are seen. The insects themselves can emit an unpleasant smell. - Feeding damage: kernels with irregular gnawing, hollowing, and powdery interiors; elevated fines and dust. - Insect traces: frass and pelletized droppings, cast larval exuviae, dead adults, and pupal cells formed in caked grain or soft packaging. - Secondary symptoms: localized clumping, warming “hot spots,” and moisture pockets driven by insect metabolic activity.

Tenebrio obscurus (F.), the dark mealworm beetle, is holometabolous. Females oviposit 400–500 eggs, scattering them loosely on stored grain and other commodities. After eclosion, larvae pass through multiple instars, feeding on the commodity and frequently preying on other insects (facultative predation). Mature larvae pupate at the surface of the colonized substrate as exarate pupae, without constructing a cocoon. Newly emerged adults (teneral at first) then sclerotize, mate, and resume oviposition, driving reinfestation. Complete development from egg to adult typically takes 8–12 weeks.

Hygrophilous and scotophilous, it thrives in moist, moldy stored grain within dark, humid facilities—grain bins, feed bags, poultry litter, barns, mills, silos, warehouses, and feed plants—and commonly co-occurs with Tenebrio molitor.

Mealworm beetle (Tenebrio molitor).