The bean leaf beetle (Cerotoma trifurcata) adult is 1/4 inch long, yellowish-green to red or brown, with or without four black spots and two marginal stripes on the back. Despite the variability in color and pattern, the adult always has a black triangular spot at the base of the wing covers, behind the "neck". The larvae are whitish and worm-like.

Identification of Bean Leaf Beetle

The bean leaf beetle adult is 1/4 inch long, yellowish or reddish, with or without four black spots and two marginal stripes on the back. Despite the variability in color and pattern, the adult always has a black triangular spot at the base of the wing covers, behind the "neck".

The adult beetles eat holes in the leaves and pods of beans. They feed heavily on the seedlings and may kill or severely stunt them. The larvae feed on the roots and can cause damage by girdling the stem just below the soil line.  

Damage caused by the Bean Leaf Beetle

The adult beetles eat holes in the leaves and pods of beans. They prefer young, tender tissues and may feed heavily on the seedlings and may kill or severely stunt them. Older plants can tolerate more feeding damage, but extensive feeding will reduce yields. Feeding on pods can create roughened lesions, which reduces quality and can provide an entry point for diseases.

The larvae feed on the roots and can cause damage by girdling the stem just below the soil line.  

Life Cycle of the Bean Leaf Beetle

Adults overwinter in the soil or in plant debris. They will emerge in late spring and will begin feeding. After feeding, they will mate, and females will lay orange eggs in clusters near the base of plants. After hatching, larvae will feed on the roots of plants before pupating. After about a week, adults will emerge and begin feeding. There are two generations a year.

Management of Bean Leaf Beetles

Peak numbers of beetles occur in late May to early June and then again in August to September.

• Treat seedlings if defoliation is severe.
• After establishment, plants can withstand moderate defoliation without suffering a loss in yield. Control even light infestations after pods form to prevent cosmetic damage from feeding scars.

Contact your county Extension office for current pesticide controls.