Flour mite

Acarus siro (flour mite) is a minute acarid, visible without magnification, measuring about 0.5–1.0 mm. The body appears unsegmented: a soft, translucent idiosoma with a small gnathosoma, clothed in short setae that lend a faintly dusty aspect. Color is white to hyaline. Adults carry four pairs of legs (8 total), whereas the larva has three pairs (6). In heavy infestations, aggregations look like a living film of white dust, slowly shifting across the substrate.

Rasping–sucking feeding abrades the embryo (germ), aleurone, and adjacent endosperm, often eliminating the scutellum and completely destroying seed viability. Kernels become mealy at the germ end, with reduced test weight and loss of germination and malting quality. Infestations heavily contaminate lots with acarid dust (exuviae, fecal material, and carcasses), causing taint, off-odors, and visible caking or clumping of meal and flour. Activity elevates localized moisture and favors secondary mold development, accelerating spoilage. The result is downgraded quality and product that may become unfit for human or animal consumption.

Signs of infestation indicating the flour mite (Acarus siro L., Acari: Acaridae) in stored grain: - “Crawling dust”: grain dust appears to ripple or move as dense mite populations migrate. - A fine grey‑grayish bloom on products, with a bitter, acrid off‑taste. - A distinct sweet, sickly odor from infested lots. - Stock deterioration: downgraded grain and product damage, sometimes accompanied by larvae, pupae, or silk webbing from co‑infesting storage insects. - External evidence on packaging: pinholes, stray larvae, or webbing on/around bags, cartons, and seams.

Flour mite (Acarus siro) has a rapid, multistage life cycle: egg → larva (hexapod) → protonymph → deutonymph (heteromorphic hypopus under stress, non-feeding) → tritonymph → adult. After mating, females oviposit up to 250 eggs per month in grain masses and crevices. Neonates and feeding nymphs graze preferentially on the grain germ, but can subsist on organic detritus lodged in cracks and recesses of storage structures. The hypopus is a highly resistant, phoretic stage that endures adverse conditions and aids dispersal. Populations thrive at high relative humidity, enabling continuous oviposition and rapid buildup in stored grain.

Flour mite (Acarus siro) is a hygrophilous storage mite, favoring warm, food‑rich microhabitats: RH ≥70%, 25–30°C, low air exchange. Foci include grain silos, pantries, and processing rooms; heated, poorly ventilated sites sustain year‑round infestations.

It is easy to confuse the flour mite with many species of mites, including some predatory species. Some psocids are similar in size, but their body structure is different.