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Thistle Caterpillar in Soybeans


Thistle caterpillars, the relatively well-known larvae of the painted lady butterfly, can wreak havoc on soybean fields early to mid-season if left unchecked. Here are some insights to keep in mind as the annual migration begins.


  • Eggs: The barrel-shaped eggs are white to light green in color.
  • Larvae: Caterpillars are around 1¼” in length and are covered in branching spines with small hairs protruding from them. Their colors can range from a light, creamy white to dark brown and even black, often accompanied with a long yellow stripe along the top of their body.
  • Adults: Closely resembling monarch butterflies, painted lady butterflies are known for their unique black and orange “splatter” pattern with distinct white eyespots on the edges of the wings. Body length is usually around 1” long and wingspan ranges from 2-3” wide.

Life Cycle

Painted lady butterflies do not overwinter in the corn and soybean belt states. Instead, they migrate north from the southern U.S. and Mexico. When they arrive in the Soybean Belt, they lay their eggs on soybean plants. Eggs typically take a week to develop and hatch. Feeding of the larvae (thistle caterpillars) can last anywhere from 2-6 weeks until they reach the pupation period, which lasts around 7-17 days. Each season usually sees 2 generations of the butterfly until they migrate back south in September.

Management Thresholds
According to the University of Minnesota Extension, if caterpillars are currently present in the field, it is justifiable to consider treatment solutions, i.e. an insecticide application, at the following recommended guidelines. Be sure to inspect at least 10 plants at several areas of the field for feeding symptoms before making a final decision. 

  • Vegetative stages (any time before flowering): leaf defoliation at 30% or greater
  • Reproductive stages: leaf defoliation at 20% or greater

Observed feeding from thistle caterpillar on soybean leaves - 35% defoliation on the left and right; 25% defoliation in the middle: Iowa State University Extension, Marlin E. Rice.

​​​​​​​Additional Considerations

Iowa State University provides some additional insight to keep in mind around the management of this insect.

  • Thistle caterpillars typically congregate more toward field edges – especially if Canada thistle plants are present.
  • Most soybean plant injury is caused by caterpillar feeding in the V3-V4 stages.
  • Sunflowers are another significant attractant for thistle caterpillars; if a field is present nearby, be sure to pay extra attention to any soybeans planted in close vicinity to it.

Some regions may experience increased populations of thistle caterpillars, so it is important to scout for damage. Please contact your local Golden Harvest Seed Advisor or agronomist with any questions or with help identifying pests.

Photos are either the property of Syngenta or used under agreement.
Syngenta hereby disclaims liability for third-party websites.


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