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Figure 1. Soybean defoliation from multiple pests. (Photo by Nathan Mueller)

‘Tis the season for soybean defoliation. With frequent rain comes lush vegetation, and with lush vegetation comes defoliators. This seems particularly true for lepidopterous defoliators (caterpillars). Whenever we have a wet year with lush vegetative growth, one or several species of caterpillars seems to thrive. No one specific species seems to stand out in Nebraska soybean yet, at least in the north; however, several species of caterpillar appear to be relatively abundant (e.g., woollybear caterpillar, thistle caterpillar, green cloverworm).

We are beginning to receive reports of soybean defoliation in Nebraska. In almost all cases a complex of soybean defoliators is being reported. The most common single pest reported is first generation bean leaf beetle, but there are also numerous grasshoppers and as mentioned above, a variety of caterpillars present. This is typical in Nebraska, where several defoliators are present at the same time.

Because several species of defoliators are in most fields, we do not typically use insect counts to determine whether to treat. We use percent defoliation.

Insect Defoliation of Soybean
Soybean plants can tolerate significant defoliation. In fact, when holes are chewed in the top of the canopy, light gets deeper in the canopy and the inner leaves can increase their photosynthetic rate. Even with defoliation, if the remaining leaves are still intercepting at least 90% of the incident light, the soybean can compensate for loss of leaf tissue.

Canopy size is also important when making treatment decisions. Small soybean canopies cannot tolerate as much defoliation as large canopies.

Timing of defoliation should also be considered. Unless severe, defoliation during vegetative stages usually doesn't cause significant yield loss. The reproductive stages are more sensitive. Nebraska soybeans are currently in the reproductive stages, so it is important to check fields now.

Growing conditions are a final factor. When environmental conditions are favorable for soybean development (e.g., adequate water), plants have a greater capacity for regrowth and compensation.

Making a Management Decision
Because Nebraska soybeans are in the reproductive stages, consider treatment if insect defoliators are present and defoliation is expected to exceed 20%. This percentage (20%) can vary 5% or more according to
• the stage or type of insect(s) present,
• environmental conditions,
• the specific stage of the soybean, and
• the size and condition of the canopy.

Experience will have to be your guide when making a final decision. Since conditions are favorable for soybean in most Nebraska fields, 20% is likely the most appropriate threshold.
Defoliation is difficult to estimate. It is almost always overestimated. This is because the injury is so dramatic and all parts of the canopy often are not considered when making defoliation estimates. Some insect species primarily feed in the upper part of the canopy (e.g., bean leaf beetle), and some feed lower in the canopy (e.g., looper caterpillars). Different portions of the canopy will suffer different levels of injury. Therefore, when estimating defoliation, consider the entire canopy, not just the injured portion.

Figure 1. Compare field defoliation with these sample leaves to help guide your treatment decision.

To predict if defoliation will exceed 20%, the current injury must be estimated. The following steps are suggested:
1. Remove a trifoliolate leaf from the top, middle, and lower third of 10 randomly selected plants.
2. Discard the most and least damaged leaflet from each trifoliolate leaf. This will leave you with 30 leaflets.
3. Compare the 30 leaflets with the leaflets in Figure 1 and determine the average level of defoliation.
4. Repeat steps 1-3 at four or more randomly selected locations in the field.

If defoliation approaches 20% in reproductive stage soybean, treatment may be warranted if insects are actively feeding and defoliation is expected to increase.
If treatment is warranted, identify the primary defoliating insect(s) and use the insecticide guides found on the Entomology Department website at here. Most of our commonly used foliar insecticides are effective against most soybean defoliators.

This article originally appeared on the University of Nebraska-Lincoln Crop Watch blog.