Bean leaf beetle (Cerotoma trifurcata), Coleoptera

Bean leaf beetle biology & life cycle

Adults overwinter in leaf litter or other vegetation, primarily in or near wooded areas. They become active in April and move to the earliest host plant available. They feed for several days, then mate. Each female lays 175 to 250 eggs in clusters of 12 to 24 in the soil at the base of plants. Eggs hatch in one to three weeks, depending on the temperature. The larvae find their way to the base of the stem or roots and feed for 3 to 6 weeks. Mature larvae form earthen cells within which pupae form. Damage to soybeans is due primarily to 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 the outer layers of the pod. Damage is usually greatest in July and August. In addition to this beetle's direct attack, these adults are also known vectors of the bean pod mottle, cowpea mosaic, and southern bean mosaic viruses. The larvae can also cause some damage, as they feed on the root and stem tissue.

Identifying Bean leaf beetle

Bean leaf beetle: eggs

Eggs are lemon-shaped, orange in colour, and about 0.85 mm long.

Bean leaf beetle: larvae

Larvae are whitish with dark brown markings at both ends. The larvae also have conspicuous segments and six tiny legs near the head. These larvae/worms grow to a length of about 10 mm. The pupae are soft-bodied, white, and only about 5 mm long. Once hatched, the larvae remain underground for about a month, feeding on roots.

Bean leaf beetle: adult

The adult bean leaf beetle varies greatly in colour (red, orange, tan or even yellow) and markings. It is typically reddish-brown with black margins and about 5 to 6 mm long. Each wing cover usually, but not always, is marked with three or four black spots. All bean leaf beetles, however, have a black triangular-shaped spot on the forward margin of the wings.

Bean leaf beetle: management

Early-season protection against overwintering bean leaf beetle adults can be achieved with a dual-purpose seed treatment that includes a systemic insecticide recommended for their control. By reducing the number of adults early in the spring, you may cut down the next generation of beetles emerging in July and see less foliage and pod feeding.

Recommended management practices include:

In fields with a history of injury, delay planting to the end of May/beginning of June after the emergence of the overwintering beetles. Be cautious as later-planted fields may be susceptible to late-season pod feeding from first generation adults.
Use insecticide seed treatments in those fields with a history of early-season seedling infestations or to reduce vector abundance when planting food-grade soybeans to reduce bean pod mottle virus incidence.
Well-timed foliar insecticides are warranted when defoliation or pod feeding thresholds are reached.

Source
http://www.omafra.gov.on.ca/english/crops/pub811/pub811.pdf