Illinois Field Crop Applicators Practice Test

During what stage do bean leaf beetles cause about 20% leaf defoliation?

1. Pod fill

2. Vegetative growth

3. Emergence

4. Seedling

The correct answer is: Vegetative growth

Bean leaf beetles tend to cause approximately 20% leaf defoliation during the vegetative growth stage. This is a critical time for soybeans as the plants are actively growing and developing their leaves, which are essential for photosynthesis. High levels of defoliation at this stage can significantly impact the plant's ability to produce energy and, consequently, yield. At other stages, such as during emergence or seedling, the plants are less affected because they have fewer leaves, and their overall ability to withstand defoliation is higher. During the pod fill stage, the plants are focused on fruit development rather than leaf production, and while defoliation is still harmful, it typically has less impact on yield compared to the vegetative stage where leaf area is crucial for growth. Therefore, the vegetative growth stage presents the most opportunity for the beetles to inflict noticeable damage to the leaf canopy, hence the significant impact of defoliation during this time.