Pest Expert Overview: Stored Product Weevils in Bryan & College Station
Granary weevils, rice weevils, and maize weevils are closely related stored product pests that infest whole grains and seeds. They are commonly found in kitchens, pantries, food storage facilities, grain bins, bird seed, and bags of wildlife feed.
These insects are often called grain weevils or pantry weevils. Unlike many insects casually referred to as weevils, these three species are true weevils with a distinct elongated snout.
Adults are small, measuring approximately 1/8 inch long. Their bodies are cylindrical, and their heads extend forward into a narrow snout. The chewing mouthparts are located at the end of this snout.
The granary weevil, Sitophilus granarius, rice weevil, Sitophilus oryzae, and maize weevil, Sitophilus zeamais, are in the Order Coleoptera and the Family Curculionidae.
All three species are primary stored product pests. This means their larvae can develop inside whole, undamaged kernels. Many other pantry beetles are secondary pests that can only feed effectively on grain that has already been cracked, milled, or damaged.
Stored product weevils are usually brought into a home inside an infested food product. They do not normally enter from the yard and then search the house for a bag of rice.
Identifying Granary, Rice, and Maize Weevils
The three species are similar in size and overall appearance.
Granary weevils are usually dark reddish-brown to nearly black. Their wing covers lack the four lighter spots commonly visible on rice and maize weevils.
Granary weevils also have fused wing covers and cannot fly.
Rice and maize weevils are usually reddish-brown to nearly black and commonly have four lighter reddish or yellowish spots on their wing covers. Both species have functional wings and can fly.
Rice and maize weevils are extremely difficult to distinguish from one another without magnification and a careful examination of anatomical features. Maize weevils are generally somewhat larger, but their sizes overlap enough that body size alone is not reliable.
Fortunately, separating rice weevils from maize weevils usually does not change the control recommendations inside a home. It is more important to confirm that the insects are grain weevils and locate the product in which they are developing.
Life Cycle
Granary, rice, and maize weevils undergo complete metamorphosis and develop through four stages: egg, larva, pupa, and adult.
The female uses her snout to chew a small cavity into a whole kernel of grain. She deposits a single egg inside the cavity and seals the opening with a waxy or gelatinous material.
After hatching, the small, legless larva remains hidden inside the kernel. It feeds internally, eventually hollowing out much of the grain.
The larva also pupates inside the kernel. Once development is complete, the adult chews a round exit hole through the outer surface and emerges.
Because the egg, larval, and pupal stages are concealed inside the grain, an infestation may not be obvious when a product is first purchased. The first sign may be adult weevils crawling through the food or appearing around the pantry several weeks later.
Development time varies with temperature, moisture, grain type, and species. Warm conditions generally allow populations to develop more quickly. Multiple generations can occur inside a product that remains stored for an extended period.
What Do Stored Product Weevils Eat?
Granary, rice, and maize weevils primarily infest whole grains and seeds.
Commonly infested products include:
- Whole kernel corn
- Rice
- Wheat
- Oats
- Barley
- Rye
- Sorghum
- Millet
- Bird seed
- Grass seed
- Garden seed
- Wildlife feed
- Decorative corn
- Dry beans and other large seeds
Despite their common names, rice weevils are not limited to rice, maize weevils are not limited to corn, and granary weevils can infest several types of stored grain.
These weevils require a relatively intact kernel for successful development. They are much less likely to originate in finely milled flour, although adults may be found crawling through flour or other processed foods after leaving the original source.
This distinction is useful during an inspection. When true grain weevils are present, whole grains and seeds should receive the most attention.
Why They are Pests
Stored product weevils do not bite, sting, or damage the structure of a home. Their importance comes from their ability to destroy and contaminate stored food.
Each developing larva consumes the inside of a grain kernel. Heavy infestations can result in significant weight loss, reduced food quality, excessive grain dust, unpleasant odors, and contamination with insects, shed skins, and fecal material.
Damaged kernels may appear hollow and frequently have a small, round exit hole where an adult emerged.
Infested grain can also become warmer and more humid because of insect activity. In large commercial or agricultural grain storage systems, this can contribute to mold growth and additional deterioration.
Inside a home, the most common problem is not a massive quantity of damaged grain. It is a single overlooked bag of rice, bird seed, decorative corn, or animal feed that continually produces adult weevils.
Rice and maize weevils can fly and may be found around windows, walls, ceilings, or light fixtures. Granary weevils cannot fly but may crawl a considerable distance from the infested product.
Finding adults away from the pantry does not mean they entered through the window or are living inside the walls. It usually means they dispersed from an infested grain product somewhere in the building.
Stored Product Weevil Prevention and Control
The most important step in controlling stored product weevils is locating and removing the infested grain.
Spraying the adult weevils will not eliminate the eggs, larvae, or pupae concealed inside individual kernels.
Inspect all whole grain and seed products in the pantry and nearby storage areas. Look for:
- Living adult weevils
- Small round exit holes in kernels
- Hollow or damaged grains
- Grain dust collecting in the package
- Insects trapped inside clear packaging
- Unusual odors or moisture
Pay particular attention to products that have been stored for a long time.
Do not limit the inspection to food intended for people. Some of the most persistent infestations originate in large bags of bird seed, deer corn, grass seed, dry animal feed, or decorative corn stored in a garage, closet, utility room, or attic.
Infested products should be placed in a sealed bag and removed from the house.
After removing the source:
- Vacuum pantry shelves, corners, cracks, and shelf pin holes.
- Remove spilled grain and seeds from beneath shelving and appliances.
- Dispose of the vacuum contents outside.
- Inspect nearby packages before returning them to the pantry.
- Store whole grains and seeds in tightly sealed glass or heavy plastic containers.
- Avoid storing large quantities longer than necessary.
- Rotate older products to the front so they are used first.
Products that appear uninfested but were stored beside the source should be inspected carefully. Adult females may have laid eggs inside additional kernels before the original infestation was discovered.
Questionable products can sometimes be frozen to kill developing insects. However, freezing does not remove insects, feces, damaged kernels, or other contamination already present. In most residential situations, inexpensive infested products should simply be discarded.
Heating may also kill stored product insects, but it can damage food quality and requires the entire product to reach a lethal temperature. Placing a bag of grain in a hot garage or leaving it in the sun is not a dependable treatment.
Insecticide applications inside food storage cabinets are rarely necessary. Products must be removed before any crack and crevice treatment, and pesticides should never be applied directly to food, food packaging, or food contact surfaces unless the label specifically permits that use.
The treatment that matters is finding the grain in which the insects are developing.
It is normal to find a few adult weevils shortly after an infested product has been removed. Adults that had already emerged may continue wandering through the home. Continued or increasing activity usually indicates that another source has been overlooked.
Preventing Stored Product Weevils
Inspect grain and seed products before purchasing them whenever possible. Avoid packages that are damaged, leaking grain dust, or have insects visible inside.
Once products are brought home:
- Store rice, whole grains, bird seed, and similar products in sealed containers.
- Clean up spills promptly.
- Avoid combining new grain with an older product already in storage.
- Wash and dry containers before refilling them.
- Purchase quantities that can be used within a reasonable period.
- Inspect decorative corn and seed arrangements before bringing them indoors.
Sealed containers may not prevent an infestation that was already inside the product when purchased. However, they will contain the insects and prevent them from spreading into other foods.
Professional Stored Product Weevil Control
Stored product weevil infestations can be difficult to locate because most of the life cycle occurs inside individual kernels. The original source may also be stored outside the kitchen in bird seed, animal feed, decorative corn, garden seed, or another whole grain product.
A professional inspection can help confirm the identification, distinguish true grain weevils from other pantry beetles, locate overlooked sources, and determine whether additional products have become infested.
If you are dealing with an infestation of granary weevils, rice weevils, or maize weevils, Venus Pest Company can help. Founded by Aggie Entomologist Dr. Scott Lingren, we provide expert, eco-friendly Integrated Pest Management solutions in Bryan and College Station. To get started, fill out the contact form, call/text 979.774.0486 or email [email protected]. Don’t let nuisance pests take over your home!
