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.
Both weevils have chewing mouthparts at the end of their snouts or prolonged heads, and are about 3 – to 4-mm long, depending on the size of the grain kernel. In small grains, such as millet or milo maize, weevils are small; 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.
LIFE CYCLE & 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 centre 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.
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 superheating or cooling. Heat in a shallow pan in the oven at 48 degrees C for 1 hour or at 52 degrees C for 30 minutes, place in a deep freeze at -17 degrees C for 4 days, or heat in the microwave for 5 minutes. However, seeds saved for planting may have the germination reduced by superheating, 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.
William F. Lyon
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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