By Adam Varenhorst
Populations of green cloverworm, a caterpillar pest of soybean, have been increasing over the last couple of weeks in South Dakota. Although green cloverworm caterpillars feed on soybean leaves, defoliation rarely results in economic loss. However, reports of increasing populations of green cloverworm and increasing amounts of defoliation suggests that scouting for this insect pest is warranted.
Green Cloverworm Identification
There are several species of green caterpillars found in soybean fields that are similar in appearance to green cloverworm. All caterpillars have three pairs of thoracic or true legs that are near the head (Figure 1).
In addition to their true legs, caterpillars also have abdominal prolegs that vary in number depending on the species. Abdominal prolegs are present in the middle of the caterpillar’s body. Caterpillars also have one pair of anal prolegs that are present at the very end of their bodies.
Figure 1. General organization of a caterpillar’s body, including the types of legs. The thoracic legs are shaded green, and the abdominal prolegs are blue.
Positive identification of green cloverworm involves matching multiple characteristics. The most important characteristic used to identify green cloverworm caterpillars are their three pairs of abdominal prolegs (Figure 2).
Additional characteristics used for green cloverworm identification include size and coloration, although these characteristics alone may not provide a definitive identification. Green cloverworm caterpillars are approximately 1 inch in length when they reach the final larval growth stage. Younger caterpillars may appear yellow to pale green, but as they mature, they will have a darker green appearance. Green cloverworm caterpillars have a white stripe present on each side of their body that originates near the head and runs the length of their body (Figure 2).
Figure 2. Green cloverworm caterpillar. Notice the three abdominal prolegs and white stripe.
The moths of the green cloverworm are a dark brown color with mottling. When at rest, its wings resemble a triangle.
In addition, the mouthparts of the green cloverworm moth protrude forward when at rest and resemble a snout or short trunk (Figure 3).
Figure 3. Green cloverworm moth.
Scouting for Green Cloverworms
There are two methods that may be used to scout for green cloverworm caterpillars; the sweep net method and the drop cloth method. For the sweep net method, while walking in a “Z” or “W” pattern through the field swing the net for a total of 20 pendulum swings. A pendulum swing consists of swinging the net across the body and then back again. To use the drop cloth method, place a white sheet onto the ground between soybean rows. Bend one foot of the row of soybeans over the cloth and shake them to dislodge any insects present. Repeat this process at ten locations throughout the field. Count the total number of green cloverworms observed to determine if an economic threshold has been exceeded.
Other soybean defoliating insects may be present within soybean fields; therefore, the cumulative defoliation can also be used to determine when management is necessary. For soybean, scout a total of 20 plants from throughout the field, and determine how much defoliation is present as a percent on each plant.
The diagram in figure 4 can be used to assist with this estimation. The threshold in South Dakota for soybean defoliation is 30% defoliation prior to flowering, and 20% defoliation after flowering. Even when using the defoliation method, it is necessary to observe the insects present in the field to determine the appropriate insecticide.
Management of Green Cloverworms
Although green cloverworm caterpillar populations are increasing in South Dakota soybean, this insect is not usually associated with yield loss. Green cloverworm populations are normally kept in check by a naturally occurring fungus. However, if thresholds are exceeded refer to the current edition of the South Dakota Pest Management Guide: Soybean for additional information on the insecticides that are available for managing green cloverworm populations.
Figure 4. Defoliation associated with green cloverworm caterpillars.