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Extension > Agriculture > Crops > Soybean Production > Insects & mites > Thistle caterpillar in Minnesota soybean

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Thistle caterpillar in Minnesota soybean

Robert Koch, Extension entomologist and Suzanne Burkness, Research assistant
2015

Name

5469468-PPT.jpg

Photo: Daren Mueller, Iowa State University, Bugwood.org

5175025-PPT.jpg

Photo: Kevin D. Arvin, Bugwood.org

Figure 1. Thistle caterpillar (top) and Painted lady butterfly (bottom).

Thistle caterpillar (Vanessa cardui)

Identification

Adults (Figure 1)

Eggs

Eggs are light green and barrel–shaped.

Caterpillar (larva)

Chrysalis (pupa)

Brown chrysalis with "bumpy" appearance can be found hanging from plant.

Natural history

549464-PPT.jpg

Photo: Adam Sisson, Iowa State University, Bugwood.org

Figure 2. Thistle caterpillar and webbing.

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 (Figure 2). After about 2–4 weeks, they form a chrysalis, which hangs from the host plant. After 7–10 days, the butterfly emerges. There are 1–2 generations per year in Minnesota.

Impacts

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 estimation of percent defoliation.

Scouting

defoliation guide

Photo: Robert Koch, University of Minnesota

Figure 3. Percent defoliation of soybean leaves.

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

  1. From each plant, select a leaf from the top, middle and bottom third of the plant.
  2. Use Figure 3 to estimate percent defoliation for each leaf and calculate the average percent defoliation across the three leaves from each plant and then across the multiple plants (also see Visual guide for estimation of soybean defoliation).
  3. This average percent defoliation for the field's canopy can be compared to treatment thresholds.

Thresholds

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

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