Acanthoscelides obtectus Say Coleoptera Bruchidae

Natural History

Distribution. Bean weevil is believed to have originated in Central America. It is now widespread in the United States and southern Canada, most of Central and South America, Africa, southern Europe, and New Zealand.

Host Plants. This insect is a pest of several legumes including bean, chickpea, cowpea, faba bean, lentil, lima bean, mung bean, pea, and soybean. Many cultivars of bean, Phaseolus vulgaris, are susceptible, including kidney bean, navy bean, pink bean, red bean, wax bean, and white bean. Not all legumes are equally suitable, with faba bean, lima bean, pea, and soybean often cited as being relatively poor hosts.

Natural Enemies. Surprisingly little is known about the parasitoids of bean weevil. Two species known from North America are Dinarmus laticeps (Ashmead) (Hymenoptera: Pteromalidae) and Eupel-mus cyaniceps Ashmead (Hymenoptera: Eupelmidae).

Life Cycle and Description. A life cycle of bean weevil can be completed in just 21 days during summer months in California, but also may extend up to six months under less ideal conditions. At the latitude of Washington D.C., as many as six generations have been reared annually.

Egg. The eggs of bean weevil are white, and measure about 0.5-0.8 mm long and 0.2-0.4 mm wide. They are elliptical in shape with one end broader than the other. Eggs are deposited in small clusters of 3-30

on legume seeds, with fecundity of 40-60 eggs per female. They are not flattened against the seed, as is the case of the cowpea weevils, Callosobruchus spp. Also, they are not tightly glued to the seed, and can easily be removed. Duration of the egg stage is highly variable and determined principally by temperature. When reared under warm condition (30°C), eggs may hatch in five days, whereas during the winter nearly 30 days may be required.

  1. There are four instars. On hatching, the young larva is whitish with a black head, and measures about 0.5 mm long. It is cylindrical in shape with the posterior end markedly tapered. The young larva is equipped with well-developed thoracic legs and bears stout bristles along the back. Bristles are also located near the thoracic legs. In the second and subsequent instars the legs are reduced. After attaining the fourth instar, the larva remains white, but measures about 4 mm long, is very stout in appearance, and has a small head with stubby, fleshy thoracic legs. Head capsule widths are about 0.15, 0.22, 0.38, and 0.46 mm, respectively, for instars 1-4. Duration of the larval stage may be as little as 12 days, or as long as six months, depending not only on temperature but also on humidity and type of food. When reared at 27°C, duration of the instars was reported to be about six, seven, seven, and six days, respectively. Larvae feed entirely within the seed, and near the completion of larval development they burrow to the seed surface where they transform to pupae.
  2. The pupa greatly resembles the adult in general appearance, but the wings are reduced in size. The body initially is white but darkens as the pupa completes its development. The pupa measures about 2.5-4.0 mm long. Duration of the pupal stage is about
Acanthoscelides Obtectus
Bean weevil larva.

8-20 days, but probably can be even longer under cool conditions.

Adult. The adult is oval in shape, brownish gray, and covered with short, golden brown pubescence. The head is small. Antennae are serrate, with the basal four segments and the distal segment red, and intermediate segments black. Antennal segments tend to become larger toward the tip. The thorax is quite narrow behind the head, but widens rapidly. Elytra are dark with irregular yellowish-white transverse lines. The tips of the elytra are broadly rounded. The tip of the abdomen is exposed, and reddish when viewed from above. The beetle measures 2.6-3.8 mm long. The adult cuts a circular hole in the epidermis of the bean seed and escapes to seek mates.

The biology of bean weevil was given by Garman (1917) and Larson and Fisher (1938). Some elements of developmental biology were provided by Howe and Currie (1964). Description of the larval instars was given by Pfaffenberger (1985).

Damage

The bean weevil is principally a pest of stored beans. It breeds continuously in stored beans, reducing them to dust. Up to 28 weevils have been reared

Bruchidae Identification

from a single seed. However, the adults also fly to the field and oviposit on bean pods that are green but close to maturity. In the field, the female normally gnaws a hole through the pod wall and oviposits within. The hatching larvae then move within the pod to seek an appropriate feeding site, and burrow into the seed. The larva creates a cavity in the seed, reducing its nutrient content, germination potential, and commercial value. Damage potential is much greater in southern states.

Management

Sanitation is very important. Bean weevils overwinter in stored beans, and production areas should be located distant from storage. Also, insect-free seed must be planted or problems will persist. (See the section on cowpea weevil, Callosobruchus maculatus (Fab-ricius), for discussion of disinfestation of seed.) Wild beans have been identified with resistance to bean weevil, but this resistance has not yet been incorporated into commercially available varieties (Schoonho-ven et al., 1983; Cardona et al., 1989).

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