University of Illinois

No. 20/August 7, 1998

Time to Watch for Soybean Defoliators

Who knows what to expect with many soybean defoliators? Thus far, the only insect we have observed defoliating soybeans is the Japanese beetle in some east-central Illinois soybeans. In this same area, we may see western corn rootworm adults feeding on soybeans soon, too. However, we have received very few reports of any other common soybean defoliators (bean leaf beetles, grasshoppers, green cloverworms, yellow woollybears, etc.) this season. Nevertheless, August usually is a critical month for soybeans. Defoliators that attack soybeans when the pods are filling out can cause noticeable reductions in yield.

Bean leaf beetle injury to a mature soybean leaf.

Economic thresholds for all defoliators are based upon the percentage 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.

Green cloverworm larva on soybean leaf.

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.

Grasshopper on soybean pod.

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

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

A standard procedure for estimating percentage 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.

  3. Record your estimates of the percentage 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. Collect leaflets from several areas of the field. Determine the length and number of missing squares for each leaflet, then refer to Table 2 to determine the percentage defoliation for eachleaflet. Record your results for each leaflet. Calculate the average percentage defoliation by dividing the sum of percentages by the number of leaflets examined.

FIgure 3. Grid for estimating soybean leaflet length (in grid units) and defoliation.

Table 2. Estimates of percentage defoliation using the "grid system" developed at the University of Nebraska.

(in grid
Number of squares missing
Percentage defoliation

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.

Kevin Steffey (, Extension Entomology, (217)333-6652