Granary Weevil (Sitophilus granarius): Identification, Damage & Control

The Granary Weevil (Sitophilus granarius) is one of the oldest known pests of stored grain. Evidence of its presence has been found in ancient Egyptian tombs and Roman granaries, proving that humans have battled this insect for thousands of years.

Unlike secondary pests such as flour beetles or merchant grain beetles that prefer processed products, the Granary Weevil is a primary pest. It can infest whole kernels of wheat, corn, barley, and rice, boring inside them to feed and reproduce.

Because of this hidden lifestyle, infestations often go unnoticed until significant damage has occurred. Today, Granary Weevils remain a global problem, causing huge economic losses in stored grains and threatening food security.

Identification

Adults:

• Length: 3–5 mm.

• Color: reddish-brown to black.

• Body: elongated, with a characteristic long snout (rostrum).

• Wings: fused wing covers, meaning Granary Weevils cannot fly (unlike rice weevils).

• Behavior: slow-moving but highly destructive inside kernels.

Larvae:

• White, legless, soft-bodied grubs.

• Develop entirely inside the grain kernel, invisible from the outside.

Eggs:

• Tiny, white, laid inside holes chewed into kernels by the female.

• Sealed with a gelatinous plug.

Pupae:

• Form inside the kernel before the adult emerges.

Key distinction: Unlike Lesser Grain Borers, which create round exit holes, Granary Weevils’ development is hidden until adults chew their way out.

Biology and Lifecycle

• Egg laying: Females lay 200–400 eggs during their lifespan.

• Hatching: Eggs hatch within 3–5 days.

• Larvae: Feed inside kernels for 3–4 weeks.

• Pupation: Takes about 1–2 weeks inside the kernel.

• Adults: Live 4–8 months, sometimes up to a year.

Generations per year:

• 4–6 generations annually under favorable conditions.

Optimal conditions:

• Temperature: 25–30 °C (77–86 °F).

• High humidity supports rapid reproduction.

Ecology and Distribution

• Originated in the Middle East but now found worldwide.

• Prefers temperate regions, but with global grain trade, infestations occur almost everywhere.

• Common in silos, bins, warehouses, and shipping containers.

Unlike Lesser Grain Borers, Granary Weevils cannot fly, so spread occurs through the movement of infested grain.

Risks and Damage

The Granary Weevil is one of the most destructive pests of stored grain.

• Direct Kernel Damage: Larvae feed entirely within kernels, hollowing them out.

• Weight Loss: Infestations can reduce grain weight by 20–30%.

• Quality Loss: Infested kernels are downgraded, lowering trade value.

• Contamination: Grain dust and insect fragments reduce marketability.

• Secondary Effects: Infestations encourage mold, mites, and other pests.

• Economic Impact: Global losses from weevils, including the granary species, amount to billions annually.

Signs of Infestation

• Holes in kernels where adults emerged.

• Grain dust or frass in storage bins.

• Musty odor from infested grain.

• Reduced grain weight and quality.

• Live adults visible in samples.

Because larvae develop hidden inside kernels, infestations may remain invisible for weeks or months.

Household Control

Granary Weevils are not just a warehouse problem—they can enter households through infested rice, wheat, or pasta.

• Inspection: Check stored grains for holes, dust, or live adults.

• Removal: Discard infested food in sealed bags.

• Freezing: Store grains at –18 °C for 5–7 days.

• Heating: Treat grains at 55–60 °C for a few hours.

• Airtight Storage: Use sealed containers to prevent spread.

Industrial and Professional Control

Large-scale control requires a multi-layered approach:

Sanitation:

• Remove old residues and dust from silos, bins, and floors.

Aeration & Cooling:

• Lowering grain temperature below 15 °C halts reproduction.

Fumigation:

• Phosphine is widely used to kill all life stages in silos and warehouses.

• Must be applied by licensed professionals.

Modified Atmosphere Storage (MA):

• Reducing oxygen and raising CO₂ suffocates insects inside kernels.

• Leaves no chemical residues.

Grain Monitoring:

• Sampling and sieving to detect adults.

• Probe traps to measure hidden populations.

Integrated Pest Management (IPM):

• Combines sanitation, monitoring, fumigation, and alternative methods.

Case Study: Infested Wheat Shipment

A shipment of wheat bound for Europe was rejected after inspectors found Granary Weevils. The infestation likely began in storage bins at the port.

Control measures included:

• Fumigating the entire shipment with phosphine.

• Cleaning and sealing storage bins.

• Implementing stricter monitoring before export.

The case highlights how even small unnoticed populations can cause costly international trade problems.

FAQ

Q: Do Granary Weevils fly?
A: No, unlike rice and maize weevils, granary weevils cannot fly.

Q: What foods do they infest?
A: Whole grains such as wheat, corn, barley, rice, and stored cereals.

Q: Can I eat food with weevils?
A: No, infested grains are contaminated and unsafe.

Q: How much can they damage?
A: Infestations can reduce grain weight by up to 30%.

Q: What’s the best control in silos?
A: Fumigation and temperature management, combined with sanitation.

Final Thoughts

The Granary Weevil is one of the oldest and most persistent enemies of stored grain. Its history stretches back thousands of years, proving just how closely linked it is with human civilization and agriculture. Even though it cannot fly like its rice and maize weevil relatives, its destructive feeding behavior makes it just as dangerous.

What makes this pest especially problematic is its hidden development inside kernels. By the time holes and frass are visible, infestations are already well established. For households, the solution is usually simple—airtight storage, freezing, and discarding infested food. But for silos, warehouses, and international grain trade, the stakes are higher: rejected shipments, export bans, and massive financial losses.

The best approach is always Integrated Pest Management (IPM): combine sanitation, temperature control, monitoring, and fumigation when needed. In recent years, modified atmosphere technologies have also provided a cleaner, residue-free solution, especially important for organic and export grains.

In short, the Granary Weevil may be small, but it has earned its reputation as one of the most damaging stored product pests in the world. With proper knowledge, vigilance, and professional methods, it can be managed—but never ignored.

Read more in our blog and explore guides on many other enemies. Stay informed.

Disclaimer

This article is for informational purposes only. Pest control laws and approved chemicals vary by country. For best results and legal safety, we strongly recommend contacting a licensed pest control professional in your local area. Always make sure that the pest control technician is properly certified or licensed, depending on your country’s regulations. It’s important to confirm that they only use approved products and apply them exactly as instructed on the product label. In most places in Europe, UK, or USA, following label directions is not just best practice—it’s the law.

Author

Nasos Iliopoulos
MSc Agronomist & Certified Pest Control Expert
Scientific Director, Advance Services (Athens, Greece)
Licensed Pest Control Business – Ministry of Rural Development & Food (GR)

References

• Wikipedia– Wheat weevil

• Britannica – Grain weevil

• Penn State Extension – Weevils on Stored Grain