Insect Note #28 (Revised)

Prepared by:
K. A. Sorensen, Extension Entomologist, NCSU

Dated 5/94
Placed on the Web 4/95 by the Department of Entomology, NCSU

The Bean Leaf Beetle

Certoma trifurcata (Forster), Coleoptera: Chrysomelidae

Introduction

Bean leaf beetles are most destructive to small bean plants where they chew round holes in the leaves. Larvae contribute to the damage by feeding on roots, nodules and stems.

Biology

Adult bean leaf beetles pass the winter near fields previously planted in beans. They are found on the underside of leaves as soon as the plant emerges. Females descent to the ground and lay eggs in the soil near the base of plants. Eggs hatch and slender white larvae feed on the roots and underground stems for about 30 days. Full-grown larvae form an earthen cell and pupate. About one week late, adults appear and the life cycle is completed. There are at least 2 generations a year in North Carolina.

Description

Adult - Adult beetles are reddish to yellowish in color, about 1/4 inch long, usually with 4 black spots on the back surrounded by a black band near the outer margin of the wing covers. They also have a black triangle in the anterior margin of the wing. When disturbed, they drop to the ground.

Egg - Eggs are lemon shaped, orange in color and may be found in small clusters around the base of the plants.

Larva - Larvae are whitish, dark brown at both ends, segmented and have 6 very small legs near the head.

Pupa - The pupae are exposed, white and soft bodied.

Host Plants

Both adults and larvae feed on beans, peas, cowpeas, soybeans, corn and related weeds.

Damage

Adults feed on the undersides of leaves causing rounded holes. They also chew upon stems of young plants. Larvae may girdle the plant by chewing on the roots, nodules and stems. Reduced stands and yields often result as this insect feeds on seedling plants. Adult feeding on pods results in roughened lesions and reduced quality.

Distribution

Bean leaf beetle occurs primarily in the southeastern states. It ranges north to Canada and west to Kansas and New Mexico.

Management

Planting beans so that germination occurs between the 2 generations of adult beetles is helpful. Rapid growth compensates for some damage. Chemical control consists of applying foliar insecticides or using a granular insecticide in furrow at planting. Control practices for the Mexican bean beetle will also control the bean leaf beetle.

One good control procedure for commercial plantings is to apply carbaryl (Sevin) 80WP at 5/8 to 1/2 lbs. of formulation per acre. Use low rate of Sevin on small plants. In home gardens, use 1/2 tablespoons of Sevin per gallon of water. There is not a waiting period with Sevin on beans.

The use of Admire of Platinum as a drench at planting will control beetles, thrips, whitefly, aphids, and leafhoppers. Seed treatments with a neonicotinoid insecticide also will give early season insect control.

The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsements by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. All pesticide recommendations provided in this publication apply to pest control in North Carolina. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain information about product usage in your locale and examine a current product label before applying any chemical. The information presented on pest biology is based on conditions in North Carolina and may not be applicable to your locale. For further information, we recommend that you contact the Cooperative Extension Service office in your county.