By Stephanie Karhoff
As you scout soybean fields this August, you will likely come across plants with holes in the leaves or along the leaf margin. The culprit(s) behind this damage are mid-season defoliators like bean leaf beetles, Japanese beetles, grasshopper nymphs, and caterpillars. Defoliation in soybean is typically observed twice during the growing season. First, following plant emergence, and then during early reproductive stages in July and continuing through August and early September. Damage is often from multiple insect species, and collective defoliation levels are used to determine the need for an insecticide application. Soybeans can withstand a significant amount of insect feeding, but how much is too much?
The economic threshold for soybean defoliation is 30% up until the R2 (full bloom) growth stage, 10% during pod development and fill, and 15% at R6 or full seed (Table 1). Keep in mind, these numbers apply to the entire canopy and the entire field. Insect damage usually appears worst at the top of the canopy and at field edges, so it is important to assess overall damage before determining if a rescue insecticide treatment is needed.
Table 1. Soybean defoliation economic thresholds by growth stage.
| Growth Stage | Description | Threshold |
| V1-R2 | vegetative – bloom | 30% |
| R3-R5 | pod development – fill | 10% |
| R6 | full seed | 15% |
When estimating defoliation levels, inspect several areas throughout the field. If economic injury is suspected, collect three trifoliates from the top, middle, and bottom parts of the canopy from 40 different plants. On each trifoliate, remove the most and least damaged leaf. Then estimate defoliation of the remaining leaves and calculate the field average.
It is common to overestimate defoliation, so refer to a visual guide such as the one available at go.osu.edu/defoliation to ensure accurate defoliation ratings. Other scouting tools include 3D-printed leaves depicting 30%, 10%, and 15% defoliation developed by the Agronomic Crop Insects Team available at various Ohio State University Extension programs and county offices. Another helpful tool to train your eyes before scouting is the Crop Protection Network “Disease Severity and Insect Defoliation Training” available online at https://severity.cropprotectionnetwork.org/.
State Specialist Dr. Kelley Tillmon has found that accuracy of estimating defoliation improves on average 35% when using a visual guide. Properly estimating defoliation levels helps avoid unnecessary insecticide applications, saving time and input costs.
Should you test for SCN this fall?
By Greg LaBarge
Soil sampling to understand nutrient levels is economically and environmentally smart. When collecting a soil sample for nutrient analysis with a hand probe, we collect more soil in the bucket than needed. We take a subsample, then dump the rest. Before you dump that excess soil this fall, think about the extra agronomic information you could get from what you already have in your hand. One test I suggest is a Soybean Cyst Nematode (SCN) test.
SCN is a parasitic pest of soybean. Think of a tick on you or an animal. Ticks take blood from the host, swell, and develop eggs. Like a tick, an SCN female latches onto the soybean plant root using plant sugars to feed and grow eggs. Classic plant symptoms are stunted plants with chlorotic (yellowing) foliage. Unfortunately, just because we do not see these symptoms does not mean we are not experiencing yield loss from SCN.
A soil test is the most reliable way to monitor this important soybean pest since the yield-limiting SCN population may not result in noticeable plant symptoms. Knowing that you have SCN is vital since management is our best control method. Our goal in SCN management is to keep the population low. Soil tests provide a monitoring tool and direct us to the correct management. I have an excellent example of the value of SCN testing from a West Central Ohio field.
Over the past 10 years, I have spent much time in this 80-acre field. Through three soybean crops, I did not notice any SCN symptoms. I’ve pulled nutrient soil samples regularly at eight points in the field. For the spring 2022 soil sampling before soybeans, I split the sample. I sent one out for standard nutrient testing. I sent the other half of the sample to Dr Horacio Lopez-Nicora, our state soybean pathologist, for SCN testing.
Figure 1 shows spring and fall SCN results from three representative zones in my West Central Ohio field for 2022 to 2024. The rotation was soybean (2022), corn (2023) and soybean (2024).
You see a few things in the figure. First, my expectation was no in-season plant signs, so no SCN. Instead, all eight zones had some level of SCN. Second, not every zone has the same population. You will find spatial differences across the field. Third, the fall 2022 sample shows dramatically increased populations in zones A and C even when planting a resistant variety (PI88788). Fourth, rotation is an effective tool, with the spring 2023 population below 1,000 for all zones.
We categorize the risk and management using values reported as eggs per 100cc. Here are a couple of key categories. Below 200 is good. Expect no yield loss on SCN susceptible varieties. At moderate populations of 2000 to 5000, soybean yield losses of 25% to 50% on susceptible varieties occur. Rotate to a non-host crop next year and return with SCN-resistant soybeans the following year.
Are you interested in testing a field for SCN? One opportunity is through Dr Lopez-Nicora’s lab, which provides two free samples per grower this fall through the Ohio Soybean Council and The SCN Coalition funding. Sample submission information is at go.osu.edu/scnsample. More Ohio SCN information is at go.osu.edu/scn. Commercial soil testing labs often can provide SCN testing. Check with your lab.
By Stephanie Karhoff, OSU Extension Field Specialist, karhoff.41@osu.edu or 567-376-4019 and Greg LaBarge, OSU Extension Field Specialist, labarge.1@osu.edu or 740-956-5047
