Sitophilus oryzae (Linnaeus, 1763)

Rice weevil

The granary weevil's smaller, flying cousin, with four pale spots on its back and a taste for warmer grain.

Order: Coleoptera
Family: Curculionidae
Group: Stored-product beetles
Ontario status: Established, the warm-climate grain weevil; in Ontario mainly in heated storage, mills, and imported product

Why it matters

One of the world's most important grain pests and a research celebrity: its intracellular bacterial symbiont is a model for how insects and microbes co-evolve.

Origin

Tropical Asia, where rice has been stored for millennia; now worldwide.

Behavior

Flies well, especially in warm weather, it can infest grain in the field before harvest as well as in storage. Otherwise the same hidden-larva lifestyle.

Living conditions

Whole grains, rice, wheat, corn, in warm storage. Like the granary weevil, the larva grows inside a single kernel.

Signs of presence

Small weevils (2.5-3.5 mm) with four pale spots, sometimes seen flying in warm storages
Exit holes in kernels

Weaknesses

Killed by freezing and cooled grain
Needs whole kernels; absent from flour
Field infestations are minor in Ontario's climate, storage is the battleground

Myths and facts

Myth The spots mean it's a different, harmless beetle.
Reality The four pale spots are the rice weevil's ID badge, it is the more mobile and warm-adapted of the two grain weevils.

Research notes

Morphology

2.5-3.5 mm, brown, rostrum ~1 mm. Four distinct pale (reddish-yellow) spots on the elytra. Fully winged and a capable flier. Smaller and more active than S. granarius.

Diagnostics

Four elytral spots + flight + small size = rice weevil. Spots can be faint in worn specimens, check flight and size.

Easily confused with

Granary weevil, Sitophilus granarius Rice weevil: smaller, spotted elytra, flies. Granary weevil: larger, unspotted, flightless.
Maize weevil, Sitophilus zeamais Nearly identical; separated reliably only by genitalia or molecular markers, a known cryptic pair.

Life cycle

Adult: Emerges, feeds, flies; lives several months
Egg: One per kernel, sealed; hatch in under a week at 30 °C
Egg to adult: 4-5 weeks at 30 °C
Generations per year: 4-6 in warm storage
Nymph or larva: Internal feeding ~2-3 weeks

Development

Complete metamorphosis inside the kernel: egg → larva → pupa → adult.

Reproduction

~200-400 eggs per female, one per kernel.

Distribution

Global: Cosmopolitan; dominant Sitophilus in warm regions
Native range: Tropical Asia
Ontario: Established in heated storage; field activity limited by climate

Ecology

Field-to-storage continuum in warm climates, infests maturing grain pre-harvest. Carries the intracellular endosymbiont Sodalis pierantonius, which supplies B vitamins missing from the grain diet; the symbiosis is a model system in insect-microbe evolution.

Resistance

Phosphine resistance documented in several regions; monitored in the grain trade.

IPM implications

Flight plus pre-harvest infestation means incoming grain cannot be assumed clean, receiving inspection and monitoring matter more than for the flightless granary weevil. Cooling and sanitation apply equally.

Chemical ecology

Male-produced aggregation pheromone sitophilure ((4S,5R)-5-hydroxy-4-methyl-3-heptanone); symbiont-mediated nutritional ecology is the deeper chemical story.

Genetics and DNA

No reference genome published as of 2026 (endosymbiont genome and host-symbiont interaction well studied at the molecular level).

Taxonomy

Order Coleoptera, family Curculionidae. Sister species to S. granarius; the pair separates on flight, size, and elytral maculation.

Open questions

How the endosymbiont modulates host thermal tolerance and insecticide susceptibility.

References

Hagstrum, D.W. & Subramanyam, B. (2006). Fundamentals of Stored-Product Entomology. AACC International.
Vigneron, A. et al. (2014). Insects recycle endosymbionts when the benefit is over. Science, 344:1010-1012.