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It’s total leaf loss that matters, not holes

Photos by Tom J. Bechman soybean leaves with insect holes
FORGET ABOUT IT: These insect holes are big, but in the scope of things, leaf loss averaged across the entire plant is extremely small. There would be no yield impact.
Soybean Watch: Big insects make big holes of little significance.

The old saying goes that if you pay too much attention to minor details, you can get caught up in the trees and lose sight of the forest. Adapted to soybeans and insects, you could get all wrapped up worrying about unsightly insect holes and lose sight of what matters most — whole plants.

“You’re going to see some insect feeding by this time of year on soybeans,” says Steve Gauck, a regional agronomy manager for Beck’s, sponsor of Soybean Watch ’22. “Some of the feeding may be unsightly. For example, Japanese beetles can make fairly large holes in leaves.

Related: How competition affects soybean plant structure

“What you need to do is keep it in perspective. Even if you see large holes, what you really need to evaluate and think about is total leaf area lost to insects or disease. The key is whether you still have enough green leaf area left for adequate photosynthesis to occur, creating enough sugars and other plant materials to feed the plant for maximum yield.”

Size up entire plant

When walking a field and the grower notices Japanese beetle feeding damage, Gauck likes to pull his copy of the Purdue University Corn & Soybean Field Guide out of his back pocket. He turns to Page 131 in the 2021 edition and shows the grower an illustration depicting what various percentages of leaf loss look like on a single leaf.

“You can have several holes like a Japanese beetle might make and still only have 5% leaf loss,” Gauck says. “In fact, at 50% loss, the leaf looks pretty well riddled. Then, remember that is it not a single leaf that counts, but leaf loss over the entire plant. It takes a lot of defoliation to amount to a sizable percentage of overall leaf loss on a plant.”

Japanese beetles feeding on soybean leaves

Japanese beetles get your attention because they can chew big holes into soybean leaves. However, they usually feed on top leaves, and overall, are of little to no consequence in soybeans.

The other major factor determining when defoliation is a problem is growth stage of soybeans. According to the Purdue field guide, leaf loss is a much bigger issue from R2 to R5, when pods are forming and filling, than before or after those stages. At R4, 10% defoliation of the whole plant could lead to 3% yield loss, or a couple of bushels per acre.

However, that refers to defoliation of the entire plant, not one leaf, Gauck says. It usually takes major damage, like hail or a very large insect infestation, to produce that amount of defoliation.

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