Dry Edible Beans: Mexican Bean Beetle

Excerpt from Agronomy Guide for Field Crops (Chapter 7)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Description
2. Life History
3. Damage
4. Scouting Technique
5. Action Threshold
6. Management Strategies
7. Updates on Dry Edible Beans: Mexican Bean Beetle
8. Related Links

Description

This is the only member of the lady beetle family in Ontario that eats plants. All the other lady beetles are beneficial predacious insects. The adult beetles are oval in shape, approximately 6 mm (1/4 in.) in length with 16 small black spots on their coppery-red backs, resembling a lady beetle (see Plate 129 on page 297). The heads of the Mexican bean beetle, however, are the same coppery-red colour as their backs, whereas lady beetlesí heads are typically black and white. Adults will darken with age to an orange-brown colour. The larvae are yellow and have six rows of long, branching, black-tipped spines along their backs.

Plate 129. Mexican bean beetle adult and larva. The beetles have 16 small black spots on their coppery-red back. Larva have black-tipped spines along back.

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Life History

This pest overwinters as adults in grassy fencerows and wooded areas. The adults emerge in May and begin feeding on young bean plants. Eggs are laid on the underside of the bean foliage. Larvae hatch, and young larvae feed in clusters. As they mature, larvae will scatter more. Feeding continues for approximately 1 month before larvae attach themselves to the plant and pupate. Adults emerge approximately 2 weeks later. There are one to two generations per year.

Damage

Both the adult and larvae feed on the leaves, flowers and pods, but most injury is concentrated on the leaves. First symptoms are large holes, but as feeding continues, the entire leaf is consumed except for the leaf veins. Damage is usually localized.

Scouting Technique

Examine 10 plants at five locations across the field. Monitor border rows.

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Action Threshold

Substantial yield loss does not occur until up to 35% defoliation occurs before bloom and 15% after bloom. If border plants show evidence of Mexican bean beetle, a border spray may be necessary.

Management Strategies

Insecticide is only recommended if pest populations and damage are extremely high, as insecticide will kill the natural enemies as well. Refer to OMAFRA Publication 812, Field Crop Protection Guide, for insecticide recommendations. (Order OMAFRA Publication 812)

Control weeds along borders and fencerows to expose adults in their overwintering sites. Several natural enemies keep this pest under action thresholds.

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Updates on Dry Edible Beans: Mexican Bean Beetle

• Field Crop Protection Guide - Publication 812
  • Dry Edible Beans: Mexican Bean Beetle

Related Links

• Insects
• Dry Edible Beans

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