Growers in northern Illinois are keeping careful watch in their fields for the presence of the soybean aphid. And although it is indeed wreaking havoc during these summer months, don't forget those pesky soybean defoliators. Defoliation during the critical time of pod fill can reduce yield. Japanese beetles, bean leaf beetles, grasshoppers, woollybear caterpillars, and green cloverworms all fall into the category of "soybean defoliators." In areas of central and east-central Illinois, the western corn rootworm beetle may also fall into this category.
The percentage of defoliation and stage of soybean development are both factors considered in economic thresholds for soybean defoliation. Soybean plants can generally withstand 35% to 40% defoliation during vegetative growth stages (or before bloom). The threshold is only 20% to 25% during the critical stages of bloom through pod fill. These thresholds can be adjusted depending on growing condition and even soybean prices. An example of this is to lower thresholds when soybeans are moisture stressed or soybean prices are high.
During vegetative stages, when plants are growing and producing new leaves, soybeans can tolerate considerable defoliation without any yield loss. The same is true after pods are completely filled. As soybean plants enter reproductive stages, they become more sensitive to defoliation. Yet even during the most critical period, pod development, soybean plants can still lose 20% of their leaf area before yield is affected. In general, defoliation tolerance varies with the stage of plant growth, overall plant vigor, and the adequacy of growing conditions.
Figure 1 illustrates the general relationship between defoliation at four growth stages and probable yield reduction. The four stages are vegetative growth (VC to the beginning of R1), blossom development (R1 to R2), pod development (R3 to R5), and seed maturation (R6 to R8).
To accurately assess the percentage of defoliation, be sure to scout the entire field! Don't estimate defoliation from a few plants near the field edge. They may not necessarily represent the amount of defoliation in the rest of the field. As you walk through the field, scan the plants from top to bottom. A simple procedure to estimate the percentage of defoliation follows:
- Without looking at the plants, stretch your arm out and randomly collect 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 can probably focus on the top and middle thirds.
- Compare the leaflets against Figure 2, which illustrates insect-produced defoliation at six different percentages.
- Record your estimates of the percentage of defoliation for each of the 60 leaflets, and take the average (add all the estimates and divide the total by 60). The result is the overall level of defoliation in your field.
Another method to estimate defoliation is the grid system developed by entomologists at the University of Nebraska. Use the grid (Figure 3) to measure the length in grid units of each leaflet, and estimate the number of missing squares in each sampled leaflet. With this method, collect leaflets from several areas of the field. Determine the length and number of missing squares for each leaflet, and then refer to Table 1 to determine the percentage of defoliation for each leaflet. Record results for each leaflet. Calculate the average percentage of defoliation by dividing the sum of percentages by the number of leaflets examined.
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 rates of application also can vary.--Kelly Cook