Continue Monitoring Soybean Fields for Bean Leaf Beetles and Alfalfa for Stubble Injury

June 2, 2000
Don't forget to monitor soybean fields for bean leaf beetle feeding. As we indicated in previous issues (no. 7 and no. 8) of the Bulletin, large densities of bean leaf beetle adults are required to deliver an economic blow to seedling soybeans (16 per foot of row in the early seedling stage, 39 per foot of row at stage V2+). Thresholds based on the market value of beans, cost of the insecticide rescue treatment, and the growth stage of soybean plants are presented in Table 1 of issue no. 8.

Also, don't ignore alfalfa stubble that fails to green up adequately within a few days following harvest. Alfalfa weevil densities have been very impressive this spring throughout Illinois. In addition, potato leafhoppers are becoming more evident in some fields. Rescue treatments may be required after a cutting when alfalfa weevil larvae and adults are feeding on more than 50% of the crowns and regrowth is prevented for 3 to 6 days. Very low densities (0.2 per sweep) of potato leafhoppers can result in damage to alfalfa stubble. So don't blame a poor-looking stand on dry conditions: insects also may be hard at work! In an upcoming issue of the Bulletin, we'll provide a more complete description of the potato leafhopper life history and also offer more complete management tips.--Mike Gray

Author: Mike Gray