First, no one likes to see a soybean canopy devoured by Japanese beetles, grasshoppers, or various caterpillars. Second, most people overestimate the amount of defoliation that has occurred. Lastly, most folks do not like scouting several locations within a field in July/August because it is tough walking. We should tolerate some low level of defoliation simple because there is a low probability of return on the input cost. For all of these reasons, we are going to discuss current economic thresholds, scouting guidelines, training, and a new scouting tool to help us determine the current level of defoliation in a soybean field.
The recommended economic threshold for soybeans in full pod to beginning seed stage is around 20 percent defoliation of the canopy. 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, the economic threshold can vary 5 percent, so a range of 15 to 20 percent. This 20 percent defoliation threshold is a weighted-average of the entire soybean plant or canopy, not just the top leaves or worst leaves.
The Estimating Insect Defoliation in Soybean Infographic from UNL 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 at https://cropwatch.unl.edu/2017/managing-soybean-defoliators. 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. I do plan to put an order in to print and laminate the infographic, so please contact me at or 402-821-1722 if you would like one.
It is common for the defoliation pattern on the leaflets not look like the reference pictures that makes it a little challenging to determine the percent defoliation. There are two new tools available to you. First, there is a new online (https://severity.cropprotectionnetwork.org/) tool from the Crop Protection Network called Severity and Defoliation Training. This training helps your mind and eyes better estimate defoliation before you head to the field for scouting. The online tool allows you to change the leaflet percent defoliation prior to taking a practice quiz.
Luckily, there is a new free app, called LeafByte, for your Apple phone/tablet to estimate defoliation instead. The 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. Thresholds, training, and tools are available to make informed decisions on managing defoliating insects. Know your crop, know your tech, know your bottom line.
