Bean Leaf Beetle: Essential Information for Plant Care

The bean leaf beetle, scientifically known as Cerotoma trifurcata, is a pest that affects soybean crops. Belonging to the leaf beetle family Chrysomelidae and the order Coleoptera, this insect is notorious for causing damage to soybean foliage and pods. In some cases, they have been known to spread the bean pod mottle virus, which can significantly affect crop yield.

With a diverse range of colors, adult bean leaf beetles measure about 1/4 inch in length. They normally have four black spots on their backs, along with a black triangle on their wing covers and a black band surrounding the outer margins of their wings Wisconsin Horticulture. These beetles overwinter near the soil surface, typically hidden under leaves or debris close to bean plantings Wisconsin Horticulture.

As temperatures rise in mid-May to early June, adult bean leaf beetles emerge and start feeding on nearby plants like alfalfa and clover before eventually migrating to their preferred host—soybean UMN Extension. After colonizing soybean fields, they mate and lay eggs at the base of soybean plants, thus beginning a damaging cycle affecting both the foliage and developing pods of soybean crops.

Bean Leaf Beetle Overview

Identification and Physical Characteristics

The bean leaf beetle (Cerotoma trifurcata), belonging to the Chrysomelidae family, is a small oval-shaped insect often found in North America. They are typically around 1/4 inch long, displaying various colors such as red, yellow, green, or tan with distinct black spots on their wings. A unique feature is the black triangular mark located on their wing covers, just behind the thorax ¹.

Some physical characteristics include:

• Oval-shaped body
• 1/4 inch in size
• Black spots on wings
• Black triangular mark on wing covers

Habitat and Distribution

Bean leaf beetles are native to the United States, particularly in states such as Minnesota, Iowa, and Nebraska ². They overwinter under leaf litter and emerge around mid-May to early June ³. Initially, they feed on alfalfa and clover, but around mid-June, they switch to their preferred host – the soybean plant ³.

Comparing their distribution in different states:

Bean Leaf Beetle’s Life Cycle

Overwintering Process

Bean leaf beetles (Cerotoma trifurcata) overwinter as adults in various locations, such as:

• Leaf litter
• Plant debris
• Weeds
• Grassy fields near bean fields¹

During winter, beetles remain inactive until temperatures rise above 50°F².

Eggs and Larvae

When warmer weather arrives, female beetles lay eggs in the soil near bean plants³. Key features of eggs and larvae include:

• Eggs: yellow to orange, oval-shaped⁴
• Larvae: slender, white with dark areas on both ends⁵

Larval development can take approximately 3-5 weeks⁶.

Adults and Mating

Upon reaching adulthood, bean leaf beetles exhibit the following characteristics:

• 1/4 inch long¹
• Oval-shaped body⁷
• Color ranges from yellow-green to red⁸
• Four black spots and a black triangle on the wing covers⁹

Adults feed on soybean foliage or pods, mate, and lay new eggs¹⁰. High-value soybeans, like food-grade soybeans, often face the majority of economic injury from these beetles¹¹.

Host Plants and Feeding Habits

Soybean and Other Legumes

Bean leaf beetles, scientifically known as Cerotoma trifurcata, are primarily attracted to soybeans as their preferred host plant. However, they are also known to feed on various other legume plants such as:

• Snap and lima beans
• Peas
• Alfalfa
• Clover

Adult beetles emerge from overwintering sites around mid-May to early June and initially feed on alfalfa and clover before finding soybean plants¹.

Leaf Damage and Defoliation

When the beetles infest soybean plants, they can cause significant damage to the plant’s leaves and pods. Feeding on soybean foliage and pods can lead to:

• Holes in the leaves
• Scarred pods
• Reduced yield

In the case of the Mexican bean beetle, larvae and adults feed on the undersides of leaves, removing the lower epidermis and causing a lacy, transparent appearance.

Please note that in certain conditions, defoliation caused by bean leaf beetles can be significant, reducing yields and affecting the overall health of the plant. Keeping an eye on the presence of these pests and taking appropriate measures to control their population can help maintain healthy crops. ²

References:

Impact on Crop Yield and Diseases

Impact

Detection and Monitoring

Scouting Methods

Scouting for bean leaf beetles is crucial to ensure early detection and minimize infestations. Regularly examining soybean plants, focusing on the underside of leaves, helps in detecting adult beetles and their feeding damage (Iowa State University). Some key points to consider while scouting:

• Begin scouting when plants emerge
• Check for beetles or feeding damage signs
• Look for skeletonized leaves and holes in the foliage
• Focus on field edges as the beetles often start there

Pest Thresholds and Economic Injury Levels

Pest thresholds are essential in determining the need for control measures. For bean leaf beetles, pest thresholds depend on various factors like the crop stage, market value, and management costs. The University of Maryland Extension provides examples of thresholds:

Economic injury levels reflect the point at which the cost of controlling the pest equals the expected crop loss caused by the pest. Keep in mind that these figures are guidelines and may vary based on factors such as the cost of control measures or the crop value.

Management and Control Strategies

Cultural Control Methods

• Rotate crops to disrupt the life cycle of the bean leaf beetle
• Remove plant debris that could harbor overwintering beetles
• Choose bean varieties with white or light-colored flowers, which are less attractive to beetles

Biological Control Agents

• Ladybugs are natural predators of bean leaf beetles
• Introduce other beneficial insects, such as lacewing larvae and parasitic wasps

Chemical Control Options

• Apply neem oil or other botanical insecticides like pyrethrins and spinosad
  • Pros: Natural and less toxic to beneficial insects
  • Cons: May require multiple applications

• Use synthetic insecticides as a last resort
  • Pros: Effective and fast-acting
  • Cons: Harmful to beneficial insects and may lead to resistance

To catch beetles in small gardens, use a drop cloth to collect and dispose of them. Place the cloth under the bean plants, shake the plants, and beetles will fall onto the cloth.

Conclusion

In summary, the bean leaf beetle is a pest affecting soybean plants. These beetles cause significant damage to soybean foliage and developing pods. They have a distinct appearance, with colors ranging from yellow-green to red and unique markings on their backs¹.

Some key features of the bean leaf beetle include:

• Adult size of approximately 1/4 inch long
• Colors ranging from yellow-green to red
• Four black spots on their backs
• Black triangle on wing covers behind the head
• Black band surrounding their outer wing margins²

When comparing the bean leaf beetle to other pests, certain characteristics make them particularly harmful to soybean plants:

Managing bean leaf beetles is important for soybean crop protection. Timely detection and monitoring, along with effective control methods, can help reduce damage and minimize the impact on yield.