Soybeans - Bean Leaf Beetle (BLB)

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Description
2. Life History
3. Damage
4. Scouting Technique
5. Damage Thresholds
6. Management Strategies
7. Updates on Soybeans: Bean Leaf Beetle (BLB)
8. Related links...

Description

The bean leaf beetle adult is around 5 mm in length (small) with four black spots usually found on the wing covers. Adult beetles can vary in colour but are most often yellow-green, tan or red. A small, black triangle is visible at the base of the wing covers (the prothorax - behind the head). The margins of the wing covers have a black border.

Plate 61. Bean leaf beetle adults vary in colour, but always have a small black triangle visible behind the head. They may or may not have four spots.

The bean leaf beetle is often confused with the spotted cucumber beetle. A small black triangle is visible at the base of the wing covers (behind the head) of the bean leaf beetle.

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Life History

There is only one generation of BLB per year. The BLB has recently been found to overwinter in Ontario after mild winters. It overwinters in the adult stage in woodlots, leaf litter and soil debris. In late April, the overwintering adults become active and begin feeding on nearby alfalfa fields until the first cutting of alfalfa or soybeans emerges. Mated females then lay lemon-shaped, orange-coloured eggs in small clusters in the soil at the base of the soybean plants. Egg-laying occurs until late June. Newly hatched larvae feed on roots and other underground plant parts for about 30 days before pupating. Adults emerge from the soil in late July and begin feeding on the soybean foliage and pods. Feeding continues until plants dry down for harvest, when BLB then migrate back to alfalfa fields, if available, or move to overwintering sites.

Damage

Defoliation injury by bean leaf beetle adults is generally not serious in Ontario. The exception is damage caused by overwintering adults to young soybean plants (V1 to V2). Adult feeding appears as small round holes between the major leaflet veins. Pod feeding is another concern. BLB feed on the surface of the pod, leaving only a thin film of tissue to protect the seeds within the pod. These pod lesions increase the pod's susceptibility to secondary diseases such as Alternaria. The most important concern is that BLB vectors bean pod mottle virus. The virus causes the plant and seed to become wrinkled and mottled, reducing the quality of the seed. For more information on this disease, see the section Bean Pod Mottle Virus.

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Scouting Technique

During the seedling stage, select at least five sampling sites from across the entire field at random. At each sampling site, slowly walk down 4.5-6 m (15-20 ft) of row and carefully count all beetles. Do not disturb the plants, but check closely enough that you can see the underside of the leaves. Calculate the average number of beetles per foot of row.

For fields beyond the seedling stage, select at least five sampling sites from across the entire field at random. Determine the percent defoliation that has occurred, using Figure 4-5, Defoliation Chart for Soybean Leaf-Feeding Insects.

Damage Thresholds

In seed fields and beans grown for Asian export, BLB should be controlled to avoid bean pod mottle virus. Thresholds for bean leaf beetle range from 16 adult beetles per foot of row in early seedling stages to over 39 per foot of row at V2+ stages. If the defoliation exceeds the thresholds in Table 4-20, Standard Damage Thresholds for Soybean Insect Defoliation, then a rescue treatment may be warranted to prevent excessive yield loss.

Management Strategies

See OMAFRA Publication 812, Field Crop Protection Guide, for insecticide recommendations. Spraying for BLB on seedlings may be necessary in a seed or export crop. Rescue treatments for bean leaf beetles later in the season are usually not economical due to the soybean plant's tolerance to defoliation and the great number of beetles needed to reach thresholds. The field planted earliest in a local area is usually the most attractive. Consider planting export or seed soybeans a few days later than crushing soybeans. (Order Publication 812)

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Updates on Soybeans: Bean Leaf Beetle (BLB)

• Field Crop Protection Guide - Publication 812
  • Bean Leaf Beetle

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Related links...

• Insects
• Soybeans

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