Beware, Hungry Caterpillars Could Be Eating Your Profit

3 tips for dealing with soybean defoliation

Then the very hungry caterpillar ate through one field of the farmer’s soybeans, but he was still hungry. That might sound like a passage from a children’s book, but it’s actually a true story taking place in soybean fields across the country.

Many insects, not just caterpillars, can impact soybean farmers’ crops by chewing holes in the plants’ leaves.

“Insect defoliation is the most common insect injury we see in soybeans,” said Thomas Hunt, Ph.D., entomology extension specialist at the University of Nebraska in a recent Focus on Soybean webcast about insect damage. “Partially because so many insects do this type of injury, but also because it’s just so physical.”

Leaves are critical to soybean development because they intercept sunlight, which the plant uses as fuel. The more leaf area a plant has, the more fuel it can capture to develop yield.

Knowing defoliation can cause significant economic damage, Hunt provided three important tips for dealing with the insects that cause it.

 1.   Catch the culprits. The blame can’t be placed solely on just caterpillars. There are also very hungry yellow woolly bears, differential grasshoppers, bean leaf beetles and more out in your fields. These insects cause substantial damage, so knowing about these bugs can help save your crop. When scouting your field, Hunt recommends checking all parts of the soybean plants because different insects feed in different locations in the canopy and on different tissues, such as stems, leaves and pods. In addition, some insects prefer specific locations in your field, like the outside rows, so checking your field thoroughly is essential. For tips on scouting your field for insects, check out



2.  Estimate the damage. Hunt says the two most common problems farmers face when determining defoliation are overestimation and inaccuracy. Measuring the damage incorrectly could cost you money and even your crop. Knowing farmers struggle with this, the figure below to helps farmers know how to identify the correct amount of damage on their leaves. Additionally, check out the full webcast for exercises you can use to determine the extent of leaf defoliation.

3.  Understand the thresholds. Knowing whether to treat for insects depends on both the level of defoliation and the crop’s growth stage. Hunt recommends considering treatment when defoliation is expected to exceed 30 percent during the vegetative stages and 20 percent in the reproductive stages. Check with your state’s extension service for local thresholds, particularly if a single species is causing the defoliation. Treating too early could be a waste of money, but treating too late could mean yield loss.



There are a few important things to keep in mind when determining a strategy for your field. Remember the correct thresholds to know when to treat, make sure insects are present and feeding and be sure that you are properly estimating the damage. However, if your crop is close to maturity, Hunt advises not to treat.