What’s Eating the Canada Thistle? Back »

Figure 1. Thistle caterpillars feeding on Canada thistle. Courtesy: J. Blastick

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

Thistle Caterpillars

While at Dakotafest last week, we were brought a sample of insects that were feeding on Canada thistle. This week, we received a few reports of a worm-like insect feeding on Canada thistle in prairie patches and pastures (Figure 1). What is this mystery insect? Well, it is none other than the thistle caterpillar! During a typical year, the thistle caterpillar may go undetected for the entire growing season due to very low populations. However, 2017 has been a great year for this insect. We have seen it feeding on sunflower, soybean, and now Canada thistle.

It isn’t much of a surprise to observe these caterpillars feeding on multiple hosts, as the thistle caterpillar is a polyphagous insect. Thistle caterpillars are known to readily feed on over 100 species of plants. The most common hosts include the plants that we have been observing them on (sunflower, soybean, and Canada thistle). The thistle caterpillar is often considered a pest because of its polyphagous nature. However, when it is feeding on weeds such as Canada thistle, it may be considered a beneficial insect.

Behavior & Identification

Similar to its other hosts, the thistle caterpillar feeds on thistle leaves by rolling them and using silken webbing to hold them together. In Figure 1, the caterpillars can easily be seen feeding on top of the plants and not in a protective leaf covering. The caterpillars appear to have silken hideouts made on the plants, but are foraging outside of them. These hideouts provide shelter to the caterpillars (Figure 2). Thistle caterpillars can vary in coloration, but are typically brown or black with a lighter colored line running the length of their bodies on each side (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 still cause severe defoliation. In the case of weed management, they may greatly reduce the aboveground biomass of Canada thistle.

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

Impact on Canada Thistle

Unfortunately, thistle caterpillars will not remove Canada thistle from an area. This is due to the extensive horizontal root structure of Canada thistle. Although the plants may not flower and set more seeds, they will be back the following year. In addition, reports of trying to use thistle caterpillars as biocontrol agents for Canada thistle resulted in high infestation rates of surrounding crop fields. Therefore, the thistle caterpillar is not an ideal biocontrol agent for the management of Canada thistle.

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