Ohio State University Extension Fact Sheet

Ohio State University Extension Fact Sheet

Entomology

1991 Kenny Road, Columbus, OH 43210-1000


Granary and Rice Weevils

HYG-2088-97

William F. Lyon

Common Name Scientific Name
Granary Weevil Sitophilus granarius (Linnaeus)
Rice Weevil Sitophilus oryzae (Linnaeus)
Maize Weevil Sitophilus zeamais Motschulsky

Both granary and rice weevils, often known as "snout weevils," penetrate and feed on the internal portions of whole grains during the larval (immature) stage, making early detection of infestations difficult. They are usually found in grain storage facilities or processing plants, infesting wheat, oats, rye, barley, rice, and corn. Although not often found in the home, sometimes they infest table beans, acorns, chestnuts, birdseed, sunflower seeds, and ornamental corn. They are rarely found in macaroni and spaghetti. Homeowners sometimes refer to infested foods as "weevilly." Granary and rice weevils do not bite or sting humans or pets, spread disease, or feed on or damage the house or furniture.

Identification

Both weevils have chewing mouthparts at the end of their snouts or prolonged heads, and are about 1/8- to 3/16-inch long, depending on the size of the grain kernel. In small grains, such as millet or milo maize, weevils are small in size; they are larger in corn. The adult granary weevil is a shiny reddish-brown with elongated pits on the thorax, whereas the adult rice weevil is a dull reddish-brown with round or irregularly shaped pits on the thorax and four light spots on the wing covers. These deep round punctures and light spots are lacking on the granary weevil. Also, the granary weevil cannot fly, whereas the rice weevil can fly. Both weevils in the larval stage are legless, humpbacked, white to creamy white, with a small, tan head. Weevils in the pupa stage have snouts like the adults. The maize weevil is similar to the rice weevil, but larger.

Granary Weevil  Rice Weevil
Granary Weevil Rice Weevil

Life Cycle and Habits

The egg, larva, and pupa stages of both weevils occur in the grain kernels and are rarely seen. Feeding is done within the grain kernel, and adults cut exit holes to emerge. Emergence holes of the granary weevil are larger than those of the rice weevil, and tend to be more ragged than smooth and round. Females drill a tiny hole in the grain kernel, deposit an egg in the cavity, then plug the hole with a gelatinous secretion. The egg hatches into a young larva which bores toward the center of the kernel, feeds, grows, and pupates there. New adults bore emergence holes from the inside, then leave to mate and begin a new generation.

Female granary weevils lay from 36 to 254 eggs. At 80 to 86 degrees F, 75- to 90-percent relative humidity, eggs hatch in wheat with a moisture content of 13.5 to 19.6 percent in 3 days. Larvae mature in 18 days, and the pupa in 6 days. The life cycle is about 30 to 40 days during the summer, and 123 to 148 days during the winter, depending on temperature. Adults live 7 to 8 months. Female rice weevils lay between 300 to 400 eggs, with the life cycle requiring about 32 days for completion. Rice weevil adults live 3 to 6 months, infesting grain in the field, especially in the South. Two larvae can develop in one wheat kernel, but only one larva of the granary weevil can develop per wheat kernel. Both granary and rice weevils feign death by drawing up their legs close to the body, falling, and remaining silent when disturbed.

Non-Chemical Control Measures

The simplest and most effective measure is to locate the source of infestation and quickly get rid of it. Use a flashlight or other light source to examine all food storage areas and food products carefully. If practical and regulations allow, dispose of heavily infested foods in wrapped, heavy plastic bags or in sealed containers for garbage removal, or bury deep in the soil. If you detect an infestation early, disposal alone may solve the problem.

Storage of grains for a month or more during the warm, summer months may lead to infestations. Purchase grains in small quantities for early use, and store in containers of insect-proof glass, heavy plastic, or metal with screw-type, airtight lids. For longer storage, refrigerate or deep freeze.

At the time of purchase, carefully examine whole grains, such as wheat, oats, rye, buckwheat, barley, corn, rice, birdseed, nuts, table beans, etc. for weevil infestations. Especially check grains purchased from grain storage facilities, processing plants, and stores. Fortunately, all stages of these weevils can be killed easily by super heating or cooling. Heat in a shallow pan in the oven at 120 degrees F for 1 hour or at 130 degrees F for 30 minutes, place in a deep freeze at 0 degrees F for 4 days, or heat in the microwave for 5 minutes. However, seeds saved for planting may have the germination reduced by super heating, cooling, or microwave methods. Properly ventilate the storage area to discourage these moisture-loving stored product pests. Be sure to store only clean, dry grain with a moisture content of 12 percent or less to reduce weevil problems.

Insecticides

The use of insecticides is discouraged around food materials. Insecticides are supplementary to sanitation and proper storage. Household insecticides have no effect on insects within food packages. For extra protection, some treat seeds or grains before storage with dusts or sprays of synergized pyrethrins, labelled for this use. (Follow label directions and safety precautions.) If the problem becomes severe and widespread, contact a reputable, licensed pest control operator who has the training, experience, equipment, and insecticides to get the control job accomplished safely.


All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.

TDD No. 800-589-8292 (Ohio only) or 614-292-6181



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