Throughout South Dakota we are beginning to notice a lot of small green caterpillars in soybean fields. Currently, these caterpillars aren’t causing too much defoliation due to their small size, but as they grow their feeding may become more noticeable.
Common Green Caterpillars
The three most commonly observed green caterpillars in soybean are the green cloverworm (Figure 1), cabbage or soybean looper (Figure 2 and Figure 3) and alfalfa caterpillars (Figure 4). In some fields, two or three of these species may be observed simultaneously.
Although these caterpillars are generally not severe pests of South Dakota soybean, their presence is important when other defoliators may also be present within the field. If only one defoliator species is present in high populations it may be possible to determine thresholds for it, however, when multiple defoliator species occur in a field we recommend looking at the average cumulative defoliation within the field.
Fig. 1. Green cloverworm caterpillar. Notice the three pairs of abdominal prolegs present in the middle of the body. Credit: A. Varenhorst
Fig. 2. Cabbage looper caterpillar. Notice the two pairs of abdominal prolegs present in the middle of the body. Courtesy: A. M. Sparks, Bugwood.org
Fig. 3. Soybean looper caterpillar. Notice the two pairs of legs present in the middle of the body. Courtesy: R. Ottens, Bugwood.org
Fig. 4. Alfalfa caterpillar. Notice the four pairs of abdominal prolegs present in the middle of the body. Credit: A. Varenhorst
The three caterpillar species are all very similar in appearance. That is, all three species are green and have white stripes that run the length of their bodies. The easiest way to discern the green cloverworm, soybean/cabbage looper and alfalfa caterpillars from one another is to look at the number of abdominal prolegs they have.
Not familiar with abdominal prolegs? All caterpillars will have three pairs of true legs, which are present near their heads. In addition, caterpillars have one pair of legs at the end of their bodies, which are referred to as the anal prolegs. The remaining legs in the middle of their bodies are the abdominal prolegs (Figure 5). The number of pairs of abdominal prolegs differs depending on the species of caterpillar.
The green cloverworm caterpillars have three pairs of abdominal prolegs. The soybean looper and cabbage looper caterpillars each have two pairs of abdominal prolegs. The alfalfa caterpillars have four pairs of abdominal prolegs. Of the three species, the green cloverworms have the greatest potential for being serious defoliators in South Dakota, but all three species may be present and will feed on leaf tissue.
Figure 5. Caterpillar body diagram. Courtesy: K. Zumach
Thresholds & Management
Because there is the potential for numerous defoliators to be present in soybean in the late summer, we recommend looking at the average cumulative defoliation that is present within a field. To obtain an average, look at 10 plants on each leg of a “Z” walking pattern. Remember that the total defoliation of the plant should be recorded and not individual trifoliates. For soybean during the reproductive stages (i.e., after flowering) 20% defoliation is the recommended threshold. Defoliation that exceeds this will result in a 3-7% yield loss per acre. For reference of defoliation caused by caterpillars and the recommended threshold please refer to Figure 6. If the 20% threshold is exceeded please refer to the current edition of South Dakota Pest Management Guide: Soybean for a list of insecticides that are labeled for caterpillar pests.
Figure 6. Soybean defoliation chart.
Developer: M. E. Rice