Bruchus rufimanus Boheman Coleoptera Bruchidae

Natural History

Distribution. Broadbean weevil is widespread in Europe, Asia, and northern Africa, and is established in Australia and North America. It was first observed in North America near San Francisco, California, in 1888. Despite spreading southward in California, it has not become a national problem, largely owing to the limited culture of faba beans elsewhere in North America. Thus, it is a pest only in California, although it is occasionally transported to eastern states and to Canada along with dried faba beans.

Host Plants. This insect attacks only faba bean (Middlekauff, 1951). Although there are reports of this species attacking pea, this probably stems from the similarity of adults with pea weevil, Bruchus pisorum (Linnaeus). Broadbean weevil adults emerge from dried seeds but cannot reinfest dried beans, so they are not a threat to beans in storage.

Natural Enemies. Several wasp parasitoids were reported to be associated with broadbean weevil by Chittenden (1912e), but only one species, Triaspis stic-tostiba Martin (Hymenoptera: Braconidae), was listed by Krombein et al. (1979). Additional parasitoid species were imported from Europe and released, but did not establish successfully. An interesting natural enemy is a predatory mite, Pediculoides ventricosus

Newport. Although it is not known to inflict high levels of mortality, the gravid females of this mite are greatly swollen and easily observed within the cells caused by the feeding of the weevil larvae.

Life Cycle and Description. There is a single generation per year, with the adult stage overwintering. In California, egg laying occurs principally in March and April. The larval period typically extends from March to mid-October. Pupae occur from August to late October. Adults are found from August until the following June.

  1. The whitish eggs are deposited singly on the outside of the bean pods, although each pod may receive several eggs. Oviposition occurs at night. There is no relationship between the number of eggs deposited and the availability of food. Sometimes, many more eggs are deposited than can develop within a pod. Females prefer larger pods. The egg is elliptical in shape, and measures 0.55-0.60 mm long and 0.25-0.28 mm wide. One end of the egg is more pointed than the other. The chorion lacks significant sculpturing and appears quite smooth. Duration of the egg stage averages 12.5 days, with a range of 9-18 days.
  2. The hatching larva chews a hole through the underside of the egg, burrows directly into the pod, and then into the seed. Initially the young larva is pale yellow except for the blackish brown head and mouthparts. As the larva matures, it becomes cream colored, but retains the dark head. At maturity, it measures 4.5-5.5 mm long. As the larva approaches maturity, it creates a cavity just beneath the seed covering. Therefore, when the weevil is ready to emerge, it has only to break through a thin covering. Duration of the larval stage is about 90 days, with a range of 65-105 days.
  3. The pupa greatly resembles the adult beetle in form. Initially it is light-yellow or cream-colored, but gradually turns darker until it attains a light brown. It measures about 5 mm long. Mean duration (range) of the pupal stage is 10 (7-16) days.
  4. Emergence of adults from seeds in the autumn is more likely in warm weather or if the bean seeds are handled. Adults emerging from seed seek sheltered overwintering sites such as under bark of trees, and remain in reproductive diapause until spring. Increasing day length and consumption of plant pollen terminate reproductive diapause (Tran et al., 1993). During cool weather, many adults remain within the seeds up to eight months. Adults may emerge from beans planted as seed, thus resulting in infestation of new, and sometimes remote, fields. The adult is blackish, with white marking on the elytra and tip of the abdomen. The white markings form an irregular white line across the distal portion of the elytra, giving the beetle a mottled appearance. The four basal segments of the antennae are reddish brown, with the remaining segments black. The forelegs are reddish brown and black, while the middle and hind pairs are black.

Broadbean weevil is easily confused with pea weevil, as the adults are very similar in appearance. In pea weevil, the terminal abdominal segment, when viewed from above, bears a pair of distinct black spots; in broadbean weevil these spots are lacking or poorly defined. In pea weevil, the posterior femora are equipped with sharp spines; in broadbean weevil spines are absent from the hind femora or blunt when they occur.

The biology of broadbean weevil was best summarized by Campbell (1920), although Chittenden (1912e) provided a few additional details.

Damage

Larvae feed within growing bean seed, pupate, and the adults emerge from the seed. Adults may emerge soon after completion of development, or remain in the seed for several months. The weight of infested seeds is reduced and germination potential decreased, but the most important consequence of infestation is the marked reduction of seed value. Even low infestation levels preclude sale of the product for human consumption. The amount of bean tissue consumed is surprisingly low, with each weevil consuming only about 3% of the bean seed's weight. However, several weevils may develop in a single seed.

Bruchus Rufimanus Boheman

Management

Insecticides. Chemical suppression of beetles in the field is not very satisfactory unless residual insecticides are used. It is essential that insecticides be present when adults invade the field so that they can be killed before oviposition (Middlekauff, 1951). Sanitation is frequently stressed, and if carefully implemented, greatly reduces the need for insecticides. As infestations often originate with beans, it is important that clean, insect-free seed be planted. Chemical fumigation of seed is very effective and economically practical.

Cultural Practices. Beetles within the bean seeds are killed by soaking in a water bath at 65°C for 20 minutes. Also, if seed is held for at least a full year before planting—a period that does not reduce germination—the weevils contained within the seeds will emerge and perish before planting. Seed in storage, spilled seed, and culled seed that is destined to be fed to livestock are the principal sources of infestation, so care must be taken to eliminate these sources or fumigate the seed.

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