Thistle Caterpillar

• Thistle caterpillar (Vanessa cardui), also known as the painted lady butterfly, is a sporadic pest of soybeans in the U.S.
• Vanessa cardui does not overwinter in the Corn Belt, but migrates north from southern states and Mexico each year.
• Thistle caterpillar larvae have a wide host range that includes up to 300 plant species.
• Larvae feed on soybeans for 2-6 weeks before pupating, feeding preferentially on earlier planted soybeans.
• In most of the northern Corn Belt, Vanessa cardui is bivoltine, meaning it has two generations per year.
• Vanessa cardui is predated on by ants, spiders, parasitic wasps, birds, and bats.

• Characteristic webbing and leaf rolling indicate the presence of thistle caterpillar (Figure 1 and Figure 2).
• Higher populations can be found near field borders because of closer proximity to plants with a high nectar content.
• Most feeding occurs in the final two larval instars.
• Larvae feed primarily on softer tissue in the upper canopy but can be found anywhere on the plant.
• The economic threshold for thistle caterpillar defoliation is 30% in vegetative soybeans and 20% in reproductive soybeans (Rice & Hodgson, 2017).

Figure 1. Thistle caterpillars feeding on the upper canopy.

Figure 2. Thistle caterpillar inside of unrolled soybean leaf.

Dark frass (droppings) inside of the webbing can be another indicator of this pest. In Figure 2 above, frass is seen to the left of the caterpillar.

Larvae

• Coloration of larvae is incredibly variable ranging from gray brown (Figure 3) to creamy white (Figure 4).
• A yellow strip running down the back may be visible on some caterpillars, but will not be present on every one.
• Branching spines along the body are easy to spot and characteristic of this species.

Figure 3. Darker colored thistle caterpillar.

Figure 4. Lighter colored thistle caterpillar.

Moths

• Adults can vary in coloration but have a few distinguishing features.
• Forewings have a black patch and a white bar on the leading edge.
• Hindwings have a row of five small black spots.
• Undersides of wings are mottled with brown, gray, and black, and have four eyespots (Figure 6).

Figure 5. Painted lady butterfly displaying forewings and hindwings.

Figure 6. Painted lady butterfly displaying wing underside.