Grain Weevil

The grain weevil, also called the granary weevil or wheat weevil, is a tiny beetle with a long snout that lives inside stored grain. It is found all over the world wherever wheat, barley and other cereals are kept. Because its young grow up hidden inside the kernels, it can do a lot of damage before anyone notices.

Grain weevil Quick Facts

Common name	Grain weevil
Scientific name	Sitophilus granarius
Kingdom	Animalia
Phylum	Arthropoda
Class	Insecta
Order	Coleoptera
Family	Dryophthoridae
Genus	Sitophilus
Type	Insect (weevil, a kind of beetle)
Diet	Whole stored cereal grains
Habitat	Grain stores, mills and pantries
Native range	Worldwide, spread by trade
Size	3-5 mm long
Lifespan	Adults about 7-8 months
Special feature	Cannot fly; larvae grow inside single grains

What is a weevil?

Weevils are a huge group of beetles, with tens of thousands of species. Most of them have a long snout, called a rostrum, with tiny chewing jaws at the tip, and bent “elbowed” antennae halfway along it. Nearly all weevils eat plants.

The grain weevil’s scientific name is Sitophilus granarius. Its close cousins, the rice weevil (Sitophilus oryzae) and the maize weevil (Sitophilus zeamais), are also serious pests of stored grain. Some other pantry pests, such as the biscuit beetle (drugstore beetle), are sometimes called weevils, but they are not true weevils.

What does the grain weevil look like?

Adult grain weevils are about 3 to 5 mm long, but most are 3 to 4 mm, smaller than a grain of rice. They are slim and shiny, and their color ranges from reddish-brown or chestnut-brown to almost black.

Look closely with a magnifying glass and you will see the long, thin snout and the elbowed antennae. The thorax is covered in small, oval pits. Unlike the rice weevil and maize weevil, the grain weevil has no flying wings under its wing covers, so it cannot fly. It walks from place to place instead.

Eggs, larvae and pupae

You almost never see the young stages of the grain weevil, because they all grow up inside a single grain kernel. The larva is white, fat and legless, with a small brown head. It stays curled up in its kernel, eating and growing.

What does it eat?

Grain weevils eat whole cereal grains and some seeds. Their favorites are wheat, barley, oats, rye, corn (maize), rice, millet, sorghum and triticale, a cross between wheat and rye. They can also infest dried beans, pasta and other hard, dry foods.

Life cycle

• Eggs. The female chews a tiny hole in a grain kernel, lays one egg inside, and seals the hole with a sticky plug. Over her life she can lay around 150 to 300 eggs. The eggs hatch in about 4 to 14 days, depending on temperature and humidity.
• Larvae. The larva eats the inside of the kernel, molting four times as it grows. Only one weevil develops in each kernel.
• Pupae. The larva turns into a pupa inside the hollowed-out grain.
• Adults. The new adult chews its way out, leaving a large, ragged exit hole.

In warm weather, the whole cycle takes about 4 to 6 weeks. In cool weather it slows down a lot and can take several months. Adults usually live for about 7 to 8 months, and they can survive for weeks without food in cool conditions.

Clever behaviors

When a grain weevil is disturbed, it pulls its legs in close to its body and plays dead. This trick can fool predators that are looking for moving prey.

Females are also choosy about where they lay their eggs. A female can tell whether a kernel already has an egg inside, probably by sensing the plug left by another female, and will usually move on to an empty kernel. This means each larva has a whole kernel to itself.

Why is it a pest?

Grain weevils are pests of stored grain all over the world. They eat the inside of the kernels, leaving behind hollow shells and powdery dust. Infested grain loses weight and food value, and it can heat up, get damp and grow mold.

Nobody knows exactly how much grain weevils destroy each year, because losses are hard to measure, especially on small farms. But in many countries they are among the most damaging insects in grain stores. The good news is that they do not bite people or carry diseases.

How to spot grain weevils

• Adult weevils crawling on grain, sacks, shelves or containers.
• Kernels with round or ragged holes in them.
• Fine, flour-like dust in the bottom of a container.
• Grain that feels warm or damp, or smells musty.
• Kernels that float when you put a handful of grain in water. Hollow kernels with larvae inside are lighter.

How to prevent and control grain weevils

Good storage habits

Prevention is the best control. Clean grain stores and cupboards well before new grain arrives, and throw away old, spilled or infested grain. At home, buy only what you will use soon and keep it in airtight containers. Farmers can help by harvesting crops as soon as they are ripe and drying them well before storage.

Physical methods

Sieving grain removes many adult weevils, though not the young ones hidden inside kernels. Some farmers mix fine powders such as wood ash, clay or diatomaceous earth into stored grain. These dusts damage the insects’ waxy outer layer, so they dry out and die.

Freezing and heating

Cold and heat are safe ways to kill weevils at every stage. At home, freezing grain, flour or pasta for about a week will do the job. In large stores, grain can be cooled or heated under careful control.

Controlled atmosphere

Weevils need oxygen to survive. Big grain stores sometimes use sealed bins or bags where the oxygen is replaced with carbon dioxide or nitrogen. Without oxygen, the insects die, and no chemicals are left on the food.

Natural enemies

Scientists are testing natural helpers against grain weevils, such as a fungus called Beauveria bassiana that infects and kills insects, and tiny parasitic wasps that lay their eggs in weevil larvae.

Chemical treatments

Large commercial grain stores may be treated with insecticides or fumigant gases. These are dangerous and should only ever be used by trained professionals. At home, they are never needed: good storage, cleaning and freezing are enough.

Related: broad bean

Grain weevil FAQs