Insect review: The bean leaf beetle
The latter first and second generation of adult beetles can occasionally present a problem for well-developed soybean fields. Adults can chew on leaf material and thus can reduce photosynthetic area or they can chew on pods allowing disease to enter the pod chamber. Before bloom, U of I recommends treatment when defoliation reaches 30 percent and five or more beetles are detected per foot of row. From bloom to pod fill, U of I recommends treatment when defoliation reaches 20 percent and there are 16 or more beetles per foot of row. A threshold of 5 to 10 percent pod damage with 10 or more beetles per foot of row (while plants are still green) is recommended for commercial soybean production. However, those involved in seed production will often manage beetles at much lower densities to avoid seed discoloration and other issues that may precipitate pod injury
Early season injury, as noted earlier, tends to present the most severe threat from the bean leaf beetle. While also exceptionally rare, a seedling treatment is recommended when 16 beetles exist per foot of row. After the first two leave are visible, about 39 beetles are needed per foot of row.