Rice weevil (Sitophilus oryzae)

Adult small curculionid, 2.3-3.5 mm, cylindrical, dark brown to blackish, with a slender, downward-curved rostrum and geniculate, clubbed antennae. Pronotum punctate; elytra striate, each bearing four characteristic yellowish to reddish-brown maculae. Macropterous and capable of flight. At the adult stage it is externally identical to the maize weevil (Sitophilus zeamais); separation requires dissection. Larva white and apodous, endophytic within the grain; concealed development means larvae are not detected by sieving or Berlese funnel extraction.

A destructive internal feeder of stored cereals, this pest causes major quantitative and qualitative losses. Adults rasp and perforate whole kernels or feed on flour; larvae are endophagous, developing inside intact grains, large fragments, or compacted cereal products. Feeding hollows the endosperm, reduces test weight and milling yield, and leaves round emergence holes with powdery frass and insect fragments. Metabolic heat and respiration create hot spots, elevate interstitial humidity, and lead to sweating, caking, and condensation. The warmed, moistened mass favors mold growth, off-odors, and loss of germination. Infested bulks show increased fines, dockage, and grain breakage, inviting secondary pests (mites, psocids, other beetles). Damage also extends to biscuits, bread, pasta, and buckwheat, compromising product integrity and shelf life. In tropical and temperate stores, populations build rapidly, accelerating spoilage and downgrading lots.

Signs of infestation by the Rice weevil, Sitophilus oryzae (L.), in stored grain: - Elevated intergranular moisture and relative humidity; localized condensation and mold. - Metabolic heating in surface layers (hot spots) detectable with probe thermometers. - Clean, circular exit holes in kernels from emerging adults; perforated, hollowed kernels. - Fine, powdery frass and enriched grain dust; caking or clumping of grain. - Lightweight “floaters” during sieving/float tests from larval endophagy in the endosperm. - Musty/yeasty odor due to insect activity and secondary fungal growth. - Live adults visible on the grain surface and in samplers; occasional cast cuticles. - Increased IDK (insect-damaged kernels) and downgraded grain quality in inspections.

Rice weevil, Sitophilus oryzae (L.), is a holometabolous, endophagous pest of stored cereals. Adults can fly and live 4–5 months. Females oviposit directly into kernels; several eggs may be laid in one grain, and more than one larva can develop per kernel. All immature development (egg–larva–pupa) occurs concealed within the grain at temperatures above 13°C. In tropical climates the full development is rapid, taking about one month; in warm regions, oviposition often begins in the field on ripening cereals near harvest, seeding infestations into storage. Newly emerged adults exit by boring a small, round emergence hole, whereas other grain borers produce larger, oblong perforations.

Thermophilic and hygrophilous, Sitophilus oryzae thrives in warm, humid microclimates of grain stores—silos and warehouses—where elevated temperature and moisture in bulk cereals accelerate development and infestations.

Granary weevil (Sitophilus granarius), maize weevil (Sitophilus zeamais) — practically identical morphologically.