Many of our treatment thresholds for defoliating pests in soybean are based on percent defoliation and growth stage. This is an important component of our IPM recommendations because we are often dealing with a complex of pests, many of which nibble on leaves. This includes bean leaf beetles, Japanese beetles, and several caterpillar species. Soybean plants are very tolerant to levels of defoliation less than 40-50%. You would be hard pressed to find research demonstrating yield loss, regardless of growth stage, where percent defoliation did not exceed 40%. Treatment thresholds are set lower to prevent defoliation from exceeding these levels.

Recommended Defoliation Thresholds in Tennessee:

• Treat at 30-35% defoliation between emergence and R1 (early bloom)
• Treat at 20% defoliation from R1 through pod filling (R6, full seed)
• After R6 to R6 + 7-10 days, treat if defoliation exceeds 30-35%
• Do not treat after R6 + 7-10 days

It takes some practice to judge percent defoliation. The tendency is for people to overestimate the amount of defoliation. Below, you will find an image from the UT insect control recommendations for soybean. It will help you calibrate judging percent defoliation, but you must be sure to judge the entire canopy and not just the top leaves.