Grain Weevil vs Rice Weevil vs Maize Weevil

📐 FIELD GUIDE ILLUSTRATION

Stored product beetles identification illustration with labeled anatomical features — PestControlBasics.com

Original illustration by PestControlBasics.com. Use the labeled features above to confirm your identification.

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PestControlBasics Editorial Team

Reviewed by Derek Giordano · Updated 2026

🔍 Identification

Granary Weevil (S. granarius): 3-5mm; dark reddish-brown to black; no spots on wing covers; CANNOT FLY; elongated snout; attacks wheat, barley, rye in cooler climates.

Rice Weevil (S. oryzae): 2.5-3.5mm; reddish-brown; FOUR reddish/orange spots on wing covers; CAN FLY; attacks rice, corn, wheat; most common in warm climates.

Maize Weevil (S. zeamais): Similar to rice weevil but larger (3-4mm) and more distinctly spotted; attacks corn and grain sorghum; most destructive weevil in tropical areas.

🧬 Biology & Behavior

All three: females bore into individual grain kernels, lay one egg per kernel, and seal the hole. Larvae develop completely inside the kernel — development invisible without cutting kernels open. Adults emerge through a round exit hole. Infested grain may look intact while harboring hundreds of developing weevils.

⚠️ Damage & Health Risk

Hollow, ruined grain kernels; adult emergence from stored grain; grain warming from metabolic activity; grain clumping from moisture produced by insect respiration; economic losses in grain storage.

🔧 DIY Treatment

Freeze at 0°F for 4+ days kills all stages. Heat (140°F for 15 minutes) also kills all stages. Store all grain in sealed airtight containers. Diatomaceous earth mixed with grain (approved rates) provides long-term protection. For bulk storage: grain protectants (malathion, pyrethrins) applied at label rates before storage.

👷 When to Call a Pro

For commercial grain elevators, licensed pest management professionals provide Integrated Grain Management programs.

❓ FAQ

How did weevils get into my sealed grain?

Most grain weevil infestations start in the field or during storage at the farm or processing facility — eggs are laid before you purchase the grain. Even commercially packaged products can be infested. This is why rotating grain stock and inspecting regularly matters.

Does freezing kill weevil eggs in grain?

Yes — 0°F for 4 days kills all life stages including eggs in grain. This is a practical household treatment for small quantities of grain, flour, or cornmeal. Allow the package to return to room temperature in a sealed bag before opening to prevent condensation damage.

🪲 Stored Grain Weevils — 3-Species Guide