Thistle caterpillar on soybean

Identification of Thistle caterpillar (Vanessa cardui)

Adults

The upper side of the front wings are red-orange and brown, with black and white spots.
Hindwings are red-orange and brown, with four black eye spots along the edge.

Eggs

Eggs are light green and barrel-shaped.

Caterpillar/larva

The body is 1-1/4 inch long and brown–black with yellow striping on both sides.
Branched, spiny hairs covering the body give the caterpillar a prickly appearance.
Caterpillars are often found in webbed leaves where they feed.

Chrysalis (pupa)

Brown chrysalis with a bumpy appearance can be found hanging from plants.

butterfly with wings spread out on a flower. — Painted lady butterfly. Photo: Kevin D. Arvin, Bugwood.org

thistle caterpillar clinging to a plant leaf. — Thistle caterpillar. Photo: Daren Mueller, Iowa State University, Bugwood.org

thistle caterpillar in webbing on a plant leaf. — Thistle caterpillar and webbing. Photo: Adam Sisson, Iowa State University, Bugwood.org

Natural history

Painted lady butterflies do not overwinter in Minnesota, but migrate from southern states each spring.
Eggs are laid singly on the upper side of leaves of host plants, including Canada thistle, sunflower, and soybean.
Caterpillars hatch and form webs by tying leaves together with silk, which creates a protected area for them to feed.
After two to four weeks, they form a chrysalis, which hangs from the host plant.
After seven to 10 days, the butterfly emerges.
There are one to two generations per year in Minnesota.

Impacts

Chart of defoliation of soybean leaves by bean leaf beetle, Japanese beetle, and green cloverworm. Percent of defoliation ranges from 5% to 50%. Illustrations of soybeans leaves show the different patterns made by each insect. The number of holes in the leaves increases the higher the percent of defoliation. — Defoliation chart. Percent defoliation of soybean leaves by 3 different insects.

Thistle caterpillar is a sporadic pest of soybean.

Caterpillars feed on leaves causing defoliation in the upper canopy of the soybean plant.

Scouting and management

Deciding if a rescue treatment is necessary requires an estimate of the percentage of defoliation.

Scouting

To estimate percent defoliation, examine a minimum of 10 plants:

From each plant, select a leaf from the top, middle and bottom third of the plant.
Use the defoliation chart to estimate percent defoliation for each leaf and calculate the average percent defoliation across the three leaves from each plant and across multiple plants.
This average percent defoliation for the field's canopy can be compared to treatment thresholds.

Thresholds

For vegetative plants (before flowering), treat if caterpillars are present and defoliation reaches 30 percent.
For reproductive plants (flowering to pod fill stage), treat if caterpillars are present and defoliation reaches 20 percent.

Labeled rates of insecticides can be used to manage this pest. Follow the directions on the product label.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.

Author: Robert Koch, Extension entomologist, and Suzanne Burkness

Reviewed in 2024