Bean weevil (bean bruchid)

Adult bean weevil (Acanthoscelides obtectus) is a small, globose bruchid, 3–5 mm long, with relatively long legs. The prothorax is short, olive‑yellow to greenish; the elytra are gray‑mottled and shortened, not reaching the abdominal apex, leaving the last tergites exposed. The abdominal apex is rust‑colored, and the dorsal surface of the terminal tergite bears yellowish setae. The metafemora present 3–4 denticles along the ventral inner margin. Larva white, up to 4 mm, setose and virtually apodous—several may share a single seed.

Damage in stored beans is primarily internal and cryptic. After oviposition on pods or seeds, first instars penetrate the testa, leaving pin-size entry marks, then develop endophagously within the seed. Larvae tunnel through cotyledons and the embryo, converting tissue to frass, causing weight loss, hollowed (excavated) kernels, and severe loss of germination. Typical signs during storage include round emergence holes of larger diameter with a detached operculum, cotyledon galleries packed with fine powder, and accumulations of fines and cast skins. Infestations often remain unnoticed at intake because infested seeds appear sound until the final instar cuts an opercular “window” before pupation. Heavy populations can reduce lots to powder, contaminate with frass, promote hot spots and secondary molds, and accelerate quality downgrades.

Signs of infestation by the bean weevil Acanthoscelides obtectus (Say) in stored bean grain include: - Beans perforated with clean, round entry or emergence holes in the testa (seed coat). - “Windowing”: a thin, translucent patch on the seed coat marking the preformed exit site. - Internal feeding: hollowed cotyledons, larval galleries, and a pupal chamber beneath the testa. - Fine, powdery frass and seed dust accumulating in bags, bins, and at seams. - Light, brittle, shriveled beans; loss of weight and germination capacity. - Presence of adults (small mottled bruchids) and shed exuviae in or near the lot. - Adhered eggs or oviposition scars on the seed surface in early infestations.

Acanthoscelides obtectus (bean weevil) is a polyvoltine bruchine. Adults feed mainly on bean foliage and pods. Females oviposit 40–50 eggs, placed individually on stored seeds or in clusters on green pods. After ~10 days, mobile neonates penetrate the pod and become endophagous within developing or dry seeds. All larval instars develop inside a single seed; pupation (nymphosis) occurs in situ. The imago emerges by perforating a circular hole in the testa. In temperate climates, development takes 6–8 weeks. In heated storage, adult emergence and reproduction occur year‑round, producing overlapping generations. Spring cohorts disperse to fields to oviposit on green pods, while in storage larvae also invade dry beans, sustaining several generations annually.

Preferred environment: warm, heated storage sites, granaries and warehouses with stored dry beans. Acanthoscelides obtectus, a stored-product bruchid, thrives at elevated temperatures within seed lots, and may occasionally attack field plants.

Pea weevil (Bruchus pisorum).