It is that time of year when Japanese beetles (Figure 1), grasshoppers, or various caterpillars consume the soybean leaf canopy. It is very easy to overestimate the amount of defoliation that has occurred. Many do not like scouting several locations within a field this time of year either but is it important. We should tolerate some low level of defoliation simply because there is a low probability of return to the application and insecticide cost. For all these reasons, we are going to discuss economic thresholds, scouting guidelines, training, and a new scouting app to help us determine the current level of defoliation in a soybean field.
The recommended economic threshold for soybeans is 30 percent defoliation up to beginning flowering, then changes to 20 percent defoliation of the canopy once soybeans are in reproductive stages (flowering, pod set, seed fill) this time of year. The economic threshold is simply the point where we are more likely to see an economic return by controlling defoliating insects. This assumes the defoliating insects are still present and expected to continue feeding and exceed 20 percent defoliation. However, due to the type of insects present, environmental conditions, canopy size, and soybean reproductive growth stage, and soybean price the economic threshold can vary 5 percent, so a range of 15 to 20 percent. This 15 to 20 percent defoliation threshold is a weighted average of the entire soybean plant or canopy, not just the top leaves or the worst leaves.
The Estimating Insect Defoliation in Soybean Infographic from Nebraska Extension is a great image to have saved on your phone or printed out to leave in your truck as a reminder of the scouting process and reference pictures of levels of defoliation. View the infographic online at https://cropwatch.unl.edu/2020/management-defoliating-insects-nebraska-soybeans. As a reminder of the scouting process, it is recommended get a field average, which means assessing defoliation in four random locations in the field. At each location in the field, collect a trifoliolate (also called trifoliate, which is all three leaflets) from the top, middle, and bottom of a plant. Then discard the highest and lowest defoliated leaflet on each of those three trifoliate leaves, leaving you with three leaflets per plant. Repeat this step on nine more plants in that location resulting in 30 leaflets per location. Compare these leaflets to the reference pictures.
Estimating the percent defoliation can be challenging when the defoliation pattern of the leaflet does not look like the reference pictures. There is an online training from the Crop Protection Network called Severity and Defoliation Training at https://severity.cropprotectionnetwork.org. This training helps your mind and eyes better estimate defoliation before you head to the field for scouting. The online training allows you to change the leaflet percent defoliation prior to taking a practice quiz. I really encourage you to try a phone app, called LeafByte, for your Apple phone/tablet that estimates defoliation. This app utilizes your camera to measure the total leaf area, consumed leaf area, and percent consumed or defoliated. You can learn more about how the app works at https://zoegp.science/leafbyte.
Use these thresholds, training, and app to make informed decisions when managing defoliating insects. For inquiries about these and other agronomic resources from Nebraska Extension, contact me at or 402-821-1722. Know your crop, know your tech, know your bottom line.
