You may instantly recognize the most common pantry and kitchen cupboard pests, such as ants, houseflies, cockroaches, or even pantry moths. But your food-storage areas can also be plagued by a strange-looking insect that's less well known—the weevil.
Weevils comprise a very large group of more than 95,000 species of insects, but all are characterized by long snouts not found in other types of insects. The most common type of weevils found in pantries and cupboards are the rice weevil (Sitophilus oryzae), the granary or wheat weevil (Sitophilus granarius), and the maize weevil (Sitophilus zeamais). These are not the same weevils that can damage outdoor landscape plants and agricultural crops, but they can create a serious problem in a kitchen food-storage area. These species feed primarily on grains and they are often found in boxes or bags of stored whole-grain cereals or grains. They are most likely to attack raw seed grains; highly processed grain products are not as susceptible.
Weevils are fairly small insects, thus it is easy to overlook an infestation in your food storage areas. They typically enter a home through products that were infested at a food processing or packaging center. Food products containing raw wheat, oats, rye, barley, rice, and corn are most susceptible to weevils.
The Lifecycle of Weevils
The three types of weevils that common indoor kitchen pests range in size from 1/8 to 3/16 inch long. Rice weevils are flying beetles, dull red-brown in color with four lighter spots. They are most common in slightly warmer climates. Granary weevils cannot fly and are shiny red-brown in color; they are found in cooler climates. Maize weevils look very much like rice weevils; only a magnified examination of the reproductive parts will distinguish the two species.
Beyond this, these insects exhibit similar behavior and breeding habits. All three types of weevils lay eggs by boring a small hole in a grain seed and laying a single egg. The tiny larvae, legless, humpbacked worm white in color, develop entirely inside the shell of the seed, transforming into the pupa which then emerges and leaves the seeds as an adult insect. Because so much of the lifecycle is spent inside the seed, and because the adult insects themselves are quite small, an infestation of weevils can go overlooked for quite a while until it becomes widespread. The most obvious sign is often the presence of many empty seed husks in the stored grain product, which creates a dusty residue.
Thus, it requires careful examination to spot an infestation of weevils in your stored grain products, and getting rid of the bugs involves several strategies employed together.
Identifying a Weevil Infestation
Because the insect's reproductive cycle requires whole seeds, it is bulk whole-grain products that are most likely to experience an infestation of weevils. Bags or boxes of almost any cereal grain can be susceptible to weevils.
A weevil infestation is most often spotted when a cereal grain product is poured out for use in cooking. If you are lucky, you may spot active adult beetles. Often, a container of the grain-based product will be teeming with tiny adult beetles when you open the container. But sometimes the only symptom is the presence of a dry, dusty residue from the hulls of seeds that have served as the home for eggs, larvae, and pupae. If the grain is poured into a container of water, this dusty residue often floats on the surface. A container of grain product that seems exceptionally dry and dusty may well be one that has been infested with weevils, and many of the whole seeds may still be actively harboring hidden larvae.
3 Ways to Get Rid of Weevils in Food-Storage Areas
Throw Away Affected Foods
Any open containers of cereal-grain products for which you suspect weevil infestation should be sealed and thrown away immediately. Also, throw away any nearby open containers. Even unopened cardboard containers may be suspect if they don't have sealed inner liners that are intact. When it comes to weevils, it's better to be aggressive about disposing of suspected foods.
Apply Heat or Cold to Stored Foods
If you find a food that is infested, or if you suspect it may be and you want to try to kill the weevils instead of discarding the food, you can usually kill the adult weevils, as well as the eggs, larvae, and pupae, by heating the product to 140 degrees Fahrenheit for at least 15 minutes or by freezing the product at 0 degrees or lower for three days.
Clean to Eliminate Adult Insects
After emptying out cupboards and pantry, and after inspecting and discarding any suspect foods, thoroughly vacuum out the food-storage areas and clean them with hot soapy water or a disinfecting spray cleaner. Individual food cans and glass containers should also be cleaned before being returned to the cupboards. The goal here is to make sure that no adult weevils remain to establish a new infestation. These are very small insects, so they may escape your efforts unless you clean quite diligently.
What Causes Weevils in Food Storage Areas?
Weevils generally gain entry into your home through purchased containers of whole-grain cereal products that have been infested at food processing and packaging centers. Once in your home, weevils can spread to whatever cereal grains are available to them. Storage areas with a lot of spilled foods are more susceptible to weevil infestations, as well as those in which many partially used containers are stored. It may be best to set a limit on how long you store partially used containers of grain products, throwing old containers away and buying new products regularly.
How to Prevent Weevils in Food-Storage Areas
Preventing weevil infestations requires routine inspection of any purchased foods coming into your house, along with periodic inspection of cereal-grain products being kept in your cupboards or pantries. Any foods that are suspect in any way should be discarded.
Some people like to routinely freeze containers of cereal-grain products for four days upon purchase, moving them into cupboard storage only after they have been disinfected in this way. Freezing will kill any weevils that are hitchhiking in commercial packaging. You can even store some of these products permanently in the freezer if you have the room. Grain products that are merely refrigerated will be less susceptible, though not immune, from weevil problems.
Transferring grain products to sealed metal or glass containers can also prevent infestations. Before each use in cooking, carefully inspect the grain to ensure there's no insect activity.
Traditional chemical insecticides can never be applied to foods or surfaces, utensils, or other items that can or will contact raw foods. Always thoroughly read and carefully follow all label directions, even for products labeled as "organic" or "non-toxic."
Follow these additional tips to prevent weevils:
- Do not add new, fresh food to old; and always clean out containers before refilling.
- Store pet foods, birdseed, etc. in buildings away from the house or basement areas well away from other foods.
- Do not purchase bagged or boxed foods with torn or damaged packaging.
- Do not buy bagged or boxed foods that are past their expiration dates.
- Do not overbuy food—just buy what you expect to use quickly. Or store excess in the refrigerator or freezer.
- Keep pantry and other food areas clean and free of crumbs and loose foods. Clean up spills immediately.
- Keep food storage areas dry; weevils are attracted to moisture.
Do weevils carry diseases?
Weevils are not known to carry or spread any diseases. Even if a few weevils are overlooked in a grain product used in a recipe, there is no health hazard associated with eating those foods.
Do weevils bite?
Weevils do not have the mouth parts necessary to inflict bites. Other than the damage to stored cereal grains, these insects do not pose any risk to humans.
How long do weevils live?
Adult insects emerge from the juvenile phase about four weeks after the eggs are laid and the entire lifespan lasts on an average 7 to 13 months; each female lays 50 to 250 eggs in that time. This means that a pantry or cupboard can be seriously infested in a matter of a few months unless the insects are spotted.
Are there any organic repellents?
Some people swear by the practice of hanging bay leaves in pantries—or even placing a couple of bay leaves in packages of cereal grain products. Studies have shown bay leaf can be an effective means of repelling insects such as weevils or pantry moths.
What about pesticides?
While it's best to avoid pesticides in any areas where foods are prepared or stored, there are some non-toxic pesticides that are reputed to repel and kill adult weevils. Some sources suggest spraying cupboards and pantries with a non-toxic insecticide, such as pyrethrin—a compound formulated from substances found in chrysanthemum flowers.
Buhler, W. Using Pesticides Safely and Correctly. Pesticide Environmental Stewardship.
Rice Weevil and Granary Weevil. Iowa State University Horticulture and Home Pest News.
Batool, Saima et al. Bay Leaf. Medicinal Plants of South Asia: Novel Sources for Drug Discovery, edited by Muhammad Asif Hanif et al., Elsevier, 2020, pp. 63–74. doi:10.1016/B978-0-08-102659-5.00005-7
Kareru, Patrick et al. Use of Botanicals and Safer Insecticides Designed in Controlling Insects: The African Case, Insecticides - Development of Safer and More Effective Technologies. IntechOpen, 2013. doi:10.5772/53924