Foreign grain beetle

Flat grain beetle, Ahasverus advena (Waltl). Adults are minute, brown imagines about 2 mm long. In habitus they superficially resemble the “red cucujid.” A practical diagnostic trait: unlike Cryptolestes spp., adults readily scale the smooth walls of glass containers. Larvae are very small, slender, and pale, ranging from whitish to yellowish.

This species is a secondary, mycetophagous pest of stored grain. Adults and larvae develop on moldy fines, broken kernels, and caked residues where moisture is elevated. They seldom attack sound kernels; direct feeding injury, weight loss, and kernel perforation are minimal and rarely reach economic thresholds. The principal impact is qualitative: contamination of lots with live/dead insects, exuviae, and frass, leading to downgrading or rejection. Aggregations typically mark spoilage “hot spots” and signal high moisture, heating, and fungal growth—clear indicators of inadequate drying, aeration, or sanitation. In short, it serves more as a bioindicator of grain deterioration than a primary destroyer of grain.

Signs indicating Ahasverus advena (Waltl) in stored grain: - Grain heating with localized hot spots; rising interstitial humidity, caking/clumping; sweating and condensation on bin walls or roof. - Visible fungal growth (mycelium on kernels) and a musty, moldy odor—this mycophagous, secondary pest proliferates in out‑of‑condition grain. - Adults: tiny (≈2–3 mm), flattened, reddish‑brown beetles running rapidly in damp pockets, under crusts, and within fines/screenings. - Larvae: slender, cream to pale yellow with a brown head capsule; presence of fine frass and shed exuviae in moist fines. - Elevated counts in pitfall/probe traps in damp layers; adults readily recovered by sieving the top 10–30 cm. - Secondary deterioration rather than primary kernel damage: reduced grain flowability, off‑odors, increased respiration. - Frequently co‑occurs with moisture‑loving fungi and other secondary pests (e.g., psocids), confirming out‑of‑condition storage.

Ahasverus advena (foreign grain beetle) has a rapid, polyvoltine cycle in warm, humid stores. After mating, females disperse oviposition among kernels and detritus, laying single eggs and, at times, small clusters of 2–3. Eggs eclose to mycetophagous larvae that pass through successive instars while moving among fines and frass. Larvae agglutinate food particles to form protective chambers; the final instar pupates within this self‑made cell. Teneral adults emerge, sclerotize, and become reproductively active shortly thereafter. With continuous food and moisture, generations overlap year‑round, maintaining persistent infestations in grain and processed products.

Mycetophagous and synanthropic, it favors damp, moldy grain and residues in mills, warehouses, and granaries; high humidity and warmth sustain populations, often introduced from fields at harvest into storage cells.

Rusty grain beetle (Cryptolestes ferrugineus). The grain beetle is distinguished from the rusty grain beetle by its clubbed antennae and the projections on the front margin of the pronotum. Flat grain beetle (Cryptolestes pusillus) Longhorned flat grain beetle (Cryptolestes turcicus) Sawtoothed grain beetle (Oryzaephilus surinamensis) Merchant grain beetle (Oryzaephilus mercator) Longheaded flour beetle (Latheticus oryzae) Small-eyed flour beetle (Palorus ratzeburgii) Hairy fungus beetle (Typhaea stercorea)