Bean Leaf Beetle
Cerotoma trifurcata (Forster), Chrysomelidae, COLEOPTERA

 


DESCRIPTION

Adult - Though the adult varies greatly in color and markings, it is typical reddish-brown to yellow with black margins and about 5 to 6 mm long. Each wing cover usually, but not always, is marked with three black spots. All bean leaf beetles, however, have a black triangular-shaped spot on the forward margin of the wings.

 Egg - The lemon-shaped egg is orange and about 0.85 mm long.

 Larva - The larva is basically whitish with both ends colored dark brown. Conspicuously segmented, it has three pairs of legs near the head. It grows to a length of about 10 mm.

 Pupa - The pupa is exposed, white, soft-bodied, and about 5 mm long.

 


BIOLOGY

Distribution - The bean leaf beetle is abundant in the southeastern states particularly in the coastal counties. Its range, however, extends into Canada, New York, Minnesota, Kansas, Texas, and New Mexico. The insect appears to prefer poorly drained clay and organic soils.

 Host Plants - Hosts of the bean leaf beetle include bean, clover, corn, cowpea, soybean, peanut, and several leguminous weeds.

 Damage - Damage to bean, pea, and cowpea is caused primarily by the foliar-feeding adults. Bean leaf beetles prefer the youngest plant tissue available; when vegetative growth terminates, they will consume tender pod tissue. Pod damage is usually limited to outer layers of pod, developing beans themselves being infrequently attacked. In North Carolina, these beetles damage leaves and stems from late May through September. In addition to the beetles' direct attack, adults are also known vectors of the bean pod mottle, cowpea mosaic, and southern bean mosaic viruses. Bean leaf beetle larvae do a little damage by feeding on roots.

 Life History - Adults overwinter in leaf litter or other vegetation, primarily in wooded areas. They become active in April and move to the earliest host plants available. In the southeastern U.S., beetles usually do not attack beans or peas until mid-May. They feed voraciously for several days and then mate. Each female lays 175 to 250 eggs in clusters of 12 to 24 in the soil at the plant's base. Eggs hatch in 1 to 3 weeks, depending upon temperature. Larvae find their way to the base of the stem or roots and feed there for 3 to 6 weeks. Mature larvae form earthen cells within which the pupae form. In southern states, peak periods of adult activity generally occur the last of May, the last of July, the second and third weeks in August, and the second and third weeks of September. Second generation beetles overwinter in North Carolina.

 


CONTROL

Chemical control consists of applying foliar insecticides or using a granular insecticide in furrow at planting. Control practices for the Mexican bean beetle will control the bean leaf beetle.

 For up-to-date recommendations, consult the current North Caroldina Agricultural Chemicals Manual.

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