Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 21/August 15, 1997

Soybean Defoliators Require Attention

Everyone is so focused upon spider mites in soybeans that some of the defoliators that occur at this time of year may be overlooked. Several recent telephone calls suggest that soybeans in some areas of Illinois are being defoliated by grasshoppers. The numbers of grasshoppers in affected areas apparently are significant. We should not lose sight of the fact that defoliators attacking soybeans when the pods are filling out can cause noticeable reductions in yield.

The defoliators you might find in soybeans right now include grasshoppers, Japanese beetles, bean leaf beetles, woollybear caterpillars, and green cloverworms. In some areas of eastern and central Illinois, western corn rootworms also might be feeding on soybean foliage. The injury they cause is similar to injury caused by bean leaf beetles, so accurate identification and diagnosis are important. Other less frequently occurring defoliators are blister beetles and thistle caterpillars.

Economic thresholds for all defoliators are based upon the percent of defoliation and the stage of soybean development. In general, soybean plants can withstand up to 35 or 40 percent defoliation during vegetative growth before bloom. During the critical blooming through pod-filling stages, the threshold is 20 to 25 percent defoliation. These "working" thresholds can be altered, depending upon growing conditions and soybean prices. For example, thresholds might be lower if soybeans are suffering from a lack of moisture and soybean prices are high.

Soybeans usually can tolerate considerable defoliation without yield reduction. Tolerance varies with the stage of plant growth, overall plant vigor, and the adequacy of growing conditions. The general relationship between defoliation at four growth stages and probable yield reductions is shown in Figure 1. The four stages are vegetative growth (VC to beginning of R1), blossom development (R1 to R2), pod development (R3 to R5), and seed maturation (R6 to R8).


Figure 1. Relationship between defoliation and yield reduction at four stages of soybean plant growth.

While the plants are growing and producing new leaves, and again after the seeds are completely filled, soybeans can tolerate considerable defoliation without yield loss. But during the early part of the reproductive stage, the plants become more sensitive to defoliation. They are most sensitive during pod development. Even at this stage (R4 to R6), however, soybean plants normally can lose 20 percent of their leaf area before yield is affected.

An accurate estimate of the percent of defoliation requires scouting the entire field. Don't estimate defoliation from just a few plants near the field edge and assume they represent the percent of defoliation for the field. Scan the plants from top to bottom as you walk through a field. Inexperienced observers tend to overestimate defoliation.

A standard procedure for estimating the percent of defoliation follows:

  1. Without looking at the plants, stretch out your arm and collect at random 20 leaflets each from the top, middle, and bottom thirds of scattered plants in the field, for a total of 60 leaflets. At this time of year, you probably can focus on the top and middle portions of the plants.

  2. Compare the leaflets with the set of diagrams in Figure 2, which illustrates insect-produced defoliation at six increments.


    Figure 2. Examples of levels of insect defoliation of soybean leaflets.

  3. Record your estimates of the percent of defoliation for each of the 60 leaflets, and take the mean (add up the estimates and divide the total by 60). The result is the overall level of defoliation in the field.

A few years ago, entomologists at the University of Nebraska developed a grid system for estimating defoliation. Use the grid in Figure 3 to measure the length (in grid units) of each leaflet, and estimate the number of missing squares in each sampled leaflet.


Figure 3. Grid for estimating soybean leaflet length (in grid units) and defoliation.Collect leaflets from several areas of the field. Determine the length and number of missing squares for each leaflet, then refer to Table 3 to determine the percent of defoliation for each leaflet. Record your results for each leaflet. Calculate the average percent of defoliation by dividing the sum of percentages by the number of leaflets examined.

Table 3. Estimates of the percent of defoliation, using the "grid system" developed at the University of Nebraska.

Leaf length (in grid units)Number of squares missing
123468101520253040
Percent of defoliation
825791418223445566790
1023571013172533425067
121245810121925313850
14123468101520253040
1612235781217212533
1811234671114182129
201122456912161925

Although these thresholds are standard for all insect defoliators, remember to identify the insect causing the injury. Different insecticides are labeled for different insects, and the rates of application vary as well.

One final note regarding insects in soybeans. Although I have focused upon defoliation in this article, some of these insects can cause economic damage to soybean pods. Grasshoppers chew chunks from pods, and bean leaf beetles scrape off the pericarp of pods, creating an entry site for disease organisms to infect the beans. In addition, green and brown stink bugs, which usually show up in soybean fields quite late in the season, also can cause significant injury to soybean pods. They insert their piercing­sucking mouthparts through the pod wall and feed on developing seeds. They inject digestive enzymes into seeds, and the feeding wound provides an avenue for disease organisms to gain entry into the pod. Seed quality also is reduced by feeding stink bugs, and beans are more likely to deteriorate in storage. As you scout for soybean defoliators, be aware of the injury that some of them might cause to pods.
Kevin Steffey, Extension Entomology, (217)333-6652