Agriculture: Dry Beans Pest Management Guidelines

Weevils

  • Bean weevil: Acanthoscelides obtectus
  • Broad bean weevil: Bruchus rufimanus
  • Cowpea weevil: Callosobruchus maculatus
  • Description of the Pest

    Adult weevils are relatively small beetles, 0.13 to 0.2 inch (3.5–5 mm) in length, somewhat teardrop or triangular in shape, and dull-colored with white, reddish, or black markings. The cowpea weevil is perhaps the most common of the weevils in California. Infestations of cowpea weevils, broad bean weevils, and bean weevils can originate from adults moving to bean fields from trash beans left in sacks, harvesters, planters, or feed areas. In addition to being field pests, the cowpea and bean weevils are also serious storage pests, damaging dried beans after harvest. The broad bean weevil does not damage stored beans.

    The location of eggs varies depending on the weevil:

    • Cowpea weevil: eggs glued to the bean or the pod
    • Broad bean weevil: eggs glued to green pods
    • Bean weevil: eggs laid loosely among beans or through cracks in the pods

    The larval and pupal stages are spent inside the bean.

    Damage

    The larval stage of the weevil pests of dry beans tunnel and develop within bean seeds. They may consume nearly the entire bean contents. Pupation occurs in the beans and adults emerge through a round hole in the seed coat. Damage is a combination of the feeding and contamination of stored product (presence of insects or feeding damage in stored beans).

    Management

    Sanitation is the most practical means of control. Eliminate potential sources of weevils in production areas:

    • Broken sacks of seed beans left over from planting
    • Seed beans left in planting hoppers
    • Cull beans used in animal feed programs in a production area
    • Small collections of beans remaining on or in a harvester following harvest
    • Small piles of beans remaining in or around the field after harvest or in a warehouse area

    Monitoring and Treatment Decisions

    Currently there are no field survey methods or treatment thresholds available for weevils. Applying insecticides during the production season has not provided effective weevil control. Watch for and control weevils and other insect pests in storage operations to prevent insect infestations in stored products.

    Text Updated: 06/18