Red flour beetle

Tribolium castaneum (Herbst), the red flour beetle, is a small coleopteran imago with a reddish-brown coloration, about 4 mm in body length. Adults show a uniform habitus and are readily confused with other Tribolium congeners. Larvae are white with brown bands; just before pupation, the final instar reaches 8 mm.

A polyphagous, secondary stored‑product pest that thrives in warm, overheated grain. Adults and larvae feed on cracked kernels, screenings and flour, abrading the endosperm and causing weight loss and reduced test weight. Infestations contaminate lots with frass, exuviae and cadavers, downgrading quality and seed germination. When disturbed, it releases benzoquinone‑rich, malodorous secretions that taint milled products; at high densities, these secretions can impart a pinkish hue, rendering flour and meal unfit for consumption. Active populations generate metabolic heat, creating hot spots that accelerate moisture migration, caking and mold (e.g., Aspergillus) development. Damage is often insidious and hard to attribute, as this species readily colonizes cereals, oilseeds, pulses and processed foods. Early detection and rigorous sanitation are essential to prevent economic losses.

Signs of infestation by the Red flour beetle, Tribolium castaneum (Herbst), in stored grain: - Grain heating: metabolic activity creates “hot spots,” elevated probe temperatures, moisture migration, and caking. - Pinkish discoloration at very high densities, as fines and grain become stained by beetle secretions and frass. - Visible insects: reddish-brown, flattened adults (≈3–4 mm) and cream larvae in the fines and damaged kernels; dead insects may accumulate. - Frass and exuviae: fecal pellets, powdery frass, and cast skins mixed with broken kernels; increased “flouring” and fines. - Off-odors: musty/rancid smell with a characteristic sweet–pungent benzoquinone taint. - Kernel damage: feeding on cracked kernels and germ, superficial tunneling, and elevated dockage/dust. - Secondary spoilage: localized mold around hot spots and clumped/caked zones. - Monitoring evidence: captures in pitfall/probe traps or detection by sieving near warm, dusty areas.

Tribolium castaneum undergoes complete (holometabolous) metamorphosis adapted to stored commodities. Females oviposit 400–500 eggs directly in flour, grain dust, or cracked kernels. Eggs hatch into cryptic, actively feeding larvae that remain within the commodity and later pupate in situ. Newly emerged adults (teneral) harden and soon mate, producing overlapping generations. Reproduction occurs from 22–40 °C, with development most rapid at 32–35 °C. Among stored‑product insects, this beetle shows one of the fastest population increases, enabling swift outbreaks in warm storage.

Thrives in warm, dry stored-grain habitats: grain bins, silos, ships, railcars, and flour mills; occasionally retail stores and warehouses. Infests packaged foods; flies in hot weather. More common in granaries than T. confusum.

Confused flour beetle (Tribolium confusum) European black flour beetle (Tribolium madens) American black flour beetle (Tribolium audax) Broad-horned flour beetle (Gnatocerus cornutus) Longheaded flour beetle (Latheticus oryzae)