Seed weevils occasionally become pests of stored beans, cowpeas, and peas. Damage consists of complete or partial destruction of infested seeds by numerous round holes or destruction of all but the outer shell. Many bean weevils may develop from a single seed and later be discovered on windows and doors. They do not bite or sting humans or pets, spread disease, or feed on or damage the house or furniture.
Seed weevils are oval-like, have a triangular pronotum (top plate-like segment on middle body part), small head, 11-segmented antennae arising in front of the eyes, and shortened wing covers exposing the tip of the abdomen. They are less than 1.9-cm long and are covered with short hairs or scales that are brown, grey, black, or a combination of these colours. Larvae are white, curved, thick-bodied, wrinkled, about 3.17-mm long at maturity, and feed inside the seeds. Eggs are oval and white.
Adult bean weevils are light olive brown, thickly covered with darker brown, greyish-yellow hairs, but with dark cross bands on the wing covers. They are about 3.17-mm in length with reddish-brown legs and black antennae. The body narrows evenly toward the small head.
LIFE CYCLE & HABITS
The life cycle of seed weevils may be completed in 21 to 80 days, depending on the temperature. If seeds are stored in a warm place, the bean weevil and cowpea weevil will breed continuously in dried seeds throughout the year. Bean weevils infest kidney beans, lima beans, and cowpeas in the field and all varieties of beans, peas, lentils, and certain other seeds in storage. Adults do not feed on the beans, but may fly to windows and doors to escape. They often feign death. Females lay white eggs on pod beans in the field or on beans in storage, and larvae emerge in 5 to 20 days. Tiny legless grubs enter the beans and eat out a cavity, becoming mature in 11 to 42 days. They pupate in the beans for 5 to 18 days and, when adults emerge, they cut round holes 2.54-mm in diameter through the seed and crawl out. Usually one to two generations are produced in the field each year and six or more generations are produced in storage each year. The pea weevil attacks peas only in the field and does not lay eggs on dried seeds. Under heavy infestations, as many as a dozen or more weevils may develop from a single seed. When a pair of bean weevils were placed in a bag containing 39 kilograms of red kidney beans, 250,000 adults were produced in 14 months.
The homeowner frequently sees seed weevils for the first time on windows and doors as they emerge from stored seeds, are attracted to light, and attempt to escape. Usually, there is little concern for their presence until a sack of dried beans or peas, especially homegrown, is emptied and found full of holes.
The simplest and most effective prevention 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. As soon as seed weevils are correctly identified, they can then be quickly discovered in stored beans or peas. Other foods, such as cereals, cornmeal, flour, nuts, grains, pet food, birdseed, etc. will not be infested by the bean weevil. If practical and regulations permit, wrap infested foods securely in heavy grocery bags and discard in a dumpster to be taken to a sanitary landfill. Also, if permitted, burning or burying infested seeds can be effective.
Apply control measures soon after harvest and before storage. Examine seeds carefully at the time of purchase or harvest. Homegrown beans and peas are first infested in the garden or field before harvest and storage. Store only clean, dry seeds with a moisture content of 12 percent or less to reduce damage. All insect life stages can be killed by super-cooling in a deep freeze at -17 degrees C for 4 days, cold storage at 0 degrees C for 58 days, or super-heating in an oven at 62 degrees C for 2 hours or in a microwave oven for 5 minutes. However, seeds saved for planting may have the germination reduced by super-cooling, super-heating, or microwave methods. After treatment, seeds should be stored in containers of glass, heavy plastic, or metal with screw-type, airtight lids. Refrigeration or deep freeze storage is helpful.
Storage of grains for a month or more during the warm summer months may lead to infestations. Purchase in small quantities for early use.
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 pyrethrin, 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.
William F. Lyon
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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