Thistle Caterpillars Chewing up Soybeans: Round 2! Back »

Figure 1. Soybean field with a heavy infestation of thistle caterpillars in the upper canopy of soybean. Credit: E. Byamukama

Written collaboratively by Adam Varenhorst, Patrick Wagner, and Amanda Bachmann.

About a month ago, we were seeing thistle caterpillars in soybean, but they were not causing enough defoliation to be of much concern. A couple of weeks ago, we saw very large populations of painted lady butterflies in parts of South Dakota. This week, we have been receiving reports of thistle caterpillars in soybean again, except this time the populations are denser and causing a lot more defoliation (Figure 1). The highest populations observed thus far have been in the northeast counties of the state. This coincides with where the dense painted lady butterfly populations were seen. Thistle caterpillars have been showing up throughout the state, which means there is potential for other areas to experience heavier infestations as well.

What to look for?

Here is a quick refresher on what to look for when determining if you have a thistle caterpillar problem in your fields. Thistle caterpillars feed on soybean leaves by initially rolling one of the three leaves of a trifoliate. They use a silken webbing to pull all of the leaves together when they are rolling them. Once the leaves are rolled, the caterpillars will begin to feed on these leaves and cause heavy defoliation (Figure 2). Upon inspection of rolled leaves, it is often possible to find the thistle caterpillar inside (Figure 3). Thistle caterpillars are the immature stage of the painted lady butterfly (Figure 4 and Figure 5). Although these caterpillars are generally not present in high numbers, they can cause severe defoliation when large populations are present within a field.

Fig. 2. Rolled soybean leaves that are defoliated. Credit: A. Varenhorst

Fig. 3. Thistle caterpillar on soybean. Credit: A. Varenhorst

Fig. 4. Painted lady butterfly with wings spread out. Credit: A. Varenhorst

Fig. 5. Painted lady butterfly with wings up. Credit: A. Varenhorst


Scouting & Management

To scout for thistle caterpillars, 10 random soybean plants should be scouted and the process repeated for five areas within a field. For each of the selected plants, evaluate the amount of defoliation that is present and record the growth stage of the soybean. Soybeans that are in the flowering and pod-filling developmental stages are less tolerant to defoliation. The threshold for these growth stages is 20%. If defoliation exceeds 20% during reproductive stages, the soybean yields will be reduced by 3-7% (Figure 6).

When evaluating defoliation, remember that the entire plant must be considered instead of just a few trifoliates. In addition, if other defoliating insects are present, there is the potential for defoliation to reach and exceed the threshold much faster. For a list of foliar insecticides that are labeled for thistle caterpillar management, please refer to the current edition of the South Dakota Pest Management Guide: Soybean.

Figure 6. Soybean defoliation chart.
Developer: M. E. Rice

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