Seed beetles are short, stout beetles belonging to the beetle Family Bruchidae. The main seed beetles that are stored product pests are the bean weevils, Acanthoscelides bruchid, cowpea weevils, Calosobruchus spp, and groundnut bruchid, Caryedon serratus. The bean weevil and cowpea weevils closely resemble each other. These two small beetles are short ( 1/8″, 3 mm), stoutly shaped insects nearly round in shape which have elytra (wing covers) that do not fully cover the abdomen. The pronotum of both beetles tapers (narrows) toward the head.
The bean weevil is coloured yellow green to dark olive with darker mottling patches on the wing covers and some specimens can appear darker, especially dead specimens. Cowpea weevils are brownish in color, and some specimens may appear greenish brown in color.
The main difference between the two is the presence of a single dark spot along the outer margins of each wing cover of the cowpea weevil. Bean weevils are further distinguished by the presence of one large and two small teeth at the uppermost point of the femur on the hind leg. Cowpea weevils have only one such tooth.
The groundnut bruchid is longer and thinner than the bean or cowpea weevils. It is also larger (1/4″, 4-7mm). It is colored reddishbrown and has numerous smudgy black spots on the wing covers.
Biology and life cycle
The female bean weevil lays its small white eggs on bean pods in the field or on beans in storage. These eggs are easily visible on the outside of the beans and numerous eggs can be found on a single bean. Each female lays up to 6o eggs and the larvae emerge in about 5 to 20 days. The tiny, grub-like larvae bore their way into the bean and eat out a cavity. Several larvae feed on each bean and end up consuming a considerable portion of its insides. Following their last molt after 11 to 42 days of feeding, the larvae pupate near the surface of the bean and then emerge from the bean in 5 to 18 days, leaving numerous holes in the hole. The entire life cycle can take as little as 21 days or as long as 80 days depending on environmental conditions.
Bean weevils are strong fliers and the first indication of infested stored beans is often the presence of flying weevils. The larvae of the bean weevil do all of the damage as the adults do not feed. Bean weevils feign death (play dead) when disturbed and may take up to five minutes to resume movement.
Cowpea weevils develop best in cowpeas (black-eyed peas), but may develop in a few other types of beans, such as chick-peas and lentil beans. The female cowpea weevil lays its eggs on the outside of the bean. The larvae bore into the bean and several, usually around three. The life cycle is similar to that of bean weevils and huge populations can quickly develop within months under ideal conditions and 90 % humidity.
The female deposits her eggs on the outside of the seed pod after harvest when drying in the sun or when the groundnuts are in storage. Upon hatching, the grub-like larvae bore through the shell of the pod and begin feeding on the seeds inside. The larvae leaves the pod after its last molt and pupates inside a thin cocoon on the pod’s outer surface. The life cycle from egg to adult is completed in about 40 days under optimum conditions.