Bean Weevil

Bean Weevil minor pest in New Jersey, the bean weevil, Adults are small snout beetles, 1/10 to 1/8 inch long,and brownish-black mottled with a lighter brown.

The body narrows towards the head, and the tip of the abdomen extends beyond the wing covers. Eggs are translucent white, elongate-oval, and about 1/40 inch in length. Mature larvae are about 1/8 inch long, white, legless, and wrinkled in appearance. When first hatched from the egg, larvae have short legs but lose them at the first molt. Pupae are white and are usually found in the beans.

The larval and pupal stages are spent inside the bean. The cowpea weevil is perhaps the most common of the weevils in California. Infestations can begin in the field. Adults move to bean fields from trash beans left in sacks, harvesters, planters, or feed areas. The cowpea weevil readily attacks dried beans; thus this weevil can be a serious storage pest. Bean weevil infestations can also start in the field and may also originate from trash beans. As with the cowpea weevil, bean weevil will attack dried beans and can be a serious pest in stored beans. Broad bean weevil infestations also start in the field, but this pest is not a storage problem.

Life Cycle: Weevils breed continuously if infested beans are stored at about 50°F or higher. Adults pass the winter indoors in storage or outdoors in protected areas, becoming active about the time bean plants are in bloom. When overwintering indoors, adults may escape from stored beans and infest the garden. Eggs are deposited in loose groups of a dozen or more in natural openings or in holes chewed in the pod by the female, hatching in 20–30 days.

Larvae migrate throughout the pod and burrow into seeds. Entrance holes seal over making infested beans difficult to detect. From 2–6 weeks later, larvae pupate within the bean, depending on temperature and moisture content of the bean. Within 5–18 days after pupation, adults emerge and exit by neatly cutting a circular lid through the seed coat.

The entire life cycle takes from 21–80 days, depending on moisture and temperature of the bean (infested seeds produce heat). Adults emerging outdoors seek other bean plants. As long as beans are available and the temperature is sufficiently warm, breeding continues. Usually 6–8 generations are produced each year.

Damage: The bean weevil is a major stored product pest because of the damage it causes to bean seeds. It becomes a pest in heated granaries where it can breed continuously on dried seed. The damage is characteristic of seed weevils in the sub-family Bruchinae.

Adult bean weevils feed on bean plant foliage and seed pods. The female chews small areas in the pod for oviposition sites. The larva develops in growing or dry stored beans. It bores through the pod to the seed in growing plants. The larva destroys the seed as it forms a chamber inside the bean and then feeds on the bean. A bean seed can contain more than a dozen chambers. After pupating, the adult weevil chews a round hole in the seed coat and exits.

Management: Sanitation is one of the most practical means of weevil control. Plow under all field trash and bean residues in the fall. In spring, do not plant seed known to be infested with weevils. Avoid planting from packets or sacks of seed left over from the previous year, and avoid storage of seed during the warm, summer months.

Keep seed in containers of glass, heavy plastic, or metal with airtight lids.

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