Granary weevil (wheat weevil; Sitophilus granarius)

Adult granary weevil (Sitophilus granarius) is a small, uniformly dark brown to brown‑black coleopteran, 2.5–5 mm long; freshly emerged adults are reddish‑brown. The head is extended into a distinct rostrum. Adults are flightless. Larvae are white, apodous, and develop concealed within individual grains, remaining undetected in routine samples processed by sieving or Berlese funnels.

This primary internal feeder of stored cereals causes both quantitative and qualitative losses. Adults perforate intact kernels to feed and oviposit; the concealed larvae tunnel within the grain, consuming endosperm and embryo and leaving shells packed with frass. Development also occurs in broken kernels, compacted flour, and even pasta products. Heavy infestations can virtually destroy entire lots. Continuous feeding and respiration create “hot spots,” raising grain temperature and adding metabolic moisture. The result is caking, localized condensation, and a sharp rise in relative humidity that favors mold growth and off-odors. Damage manifests as reduced test weight, increased fines, broken kernels, characteristic round exit holes, and contamination with frass, exuviae, and dead insects. Seed viability declines, processing yields drop, and the commodity is downgraded or rejected. Left unmanaged, these cascading effects extend to secondary invaders—storage fungi and mites—accelerating spoilage throughout the bin.

Key signs of infestation by the granary weevil, Sitophilus granarius (L.), in stored grain: - Surface “hot spots” and localized moisture increase from insect respiration, leading to warming, condensation, and caking at the top layer. - Oblong adult emergence holes in kernels (caryopses), often with clean, beveled margins. - Oviposition punctures on individual kernels sealed with a translucent waxy plug (operculum). - Live adults (reddish‑brown imagines with a pronounced rostrum) and concealed, creamy‑white, apodous larvae feeding internally on endosperm. - Powdery frass and fine boring dust in bins and handling points; hollowed, lightweight kernels that crush easily or float during cleaning tests. - Musty/off odors, elevated CO2 in the headspace, and grain clumping due to exothermic insect activity. - Increased insect fragments and exuviae in sievings and on inspection screens. Together, these indicators strongly suggest an internal-feeding Sitophilus infestation requiring prompt sanitation and targeted control.

Granary weevil (Sitophilus granarius (L.)) undergoes complete metamorphosis entirely within a grain kernel. Females live 7–8 months and lay about 150 eggs, each oviposited singly into a grain. The endophytic, apodous larva feeds on the endosperm through successive instars, then pupates inside the kernel. The newly formed adult completes sclerotization and exits by chewing an oblong emergence hole; by contrast, the rice weevil’s hole is smaller and round. Development from egg to adult requires roughly 40–120 days, depending on temperature: infestations can persist at ~15°C but progress slowly, while 26–30°C and ~70% RH favor rapid development.

Strictly synanthropic, Sitophilus granarius prefers stored-product habitats: granaries, silos, bins, warehouses, within bulk cereals. It cannot reproduce in nature; populations persist only where grain is stored.

Rice weevil (Sitophilus oryzae), maize weevil (Sitophilus zeamais).