Thistle Caterpillar in Soybean
Crop Damage: Thistle caterpillars and their adult stage, painted lady butterflies (Vanessa cardui), occur throughout the globe. Larvae damage soybeans by feeding on the leaf tissue in the upper canopy. Larvae web leaves together, providing a protective area to feed. Young larvae skeletonize leaves while older larvae consume entire leaves except for the midrib.
Thistle caterpillar and leaf webbing.
Adam Sisson, Iowa State University
Thistle caterpillar and skeletonized leaf.
Adam Sisson, Iowa State University
Pest Description: Full grown larvae are 1 ½ inches long with brown to black bodies. A yellow stripe runs along each side of the body. Distinct cream to yellow branched spines give larvae a prickly appearance. Adults have brightly colored wings with orange-red-brown-black coloration. The front wing tip has white markings and a row black spots can be found along the outer hindwing edge.
Thistle caterpillar.
Adam Sisson, Iowa State University
Painted lady butterfly.
Adam Sisson, Iowa State University
Scouting: Painted lady butterflies are attracted to thistle plants and other hosts in the sunflower and mallow families where they lay eggs. When larvae emerge, they first feed on these plants before moving to soybean. Inspect field edges, especially where thistle plants are present. If thistle caterpillars are present, randomly select plants throughout the field and estimate the level of defoliation. Use the Crop Protection Network’s Insect Defoliation Tool to help you correctly assess the level of defoliation.
Management: Spot treatments may be warranted if larvae are localized on thistle plants on field edges or in isolated areas within a field. If defoliation is widespread, treatment thresholds are 30% in vegetative growth stages and 20% in reproductive growth stages. If additional defoliators are present, assess their impact before making management decisions.
Developed and reviewed by Carol Pilcher, Iowa State University and Robert Wright, University of Nebraska-Lincoln, with support from the United Soybean Board.
Thistle caterpillar. Photo by Adam Sisson, Iowa State University.
Thistle caterpillar and skeletonized leaf. Photo by Adam Sisson, Iowa State University.
Painted lady butterfly. Photo by Adam Sisson, Iowa State University.
Thistle caterpillar and soybean leaf webbing. Photo by Adam Sisson, Iowa State University.
Gallery images courtesy Adam Sisson, Iowa State University.
This publication was developed by the Crop Protection Network, a multi-state and international collaboration of university/provincial extension specialists and public/ private professionals that provides unbiased, research-based information to farmers and agricultural personnel. This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the USDA, its Agencies, offices, and employees, and institutions participating in or administering USDA programs are prohibited from discriminating based on race, color, national origin, religion, sex, disability, age, marital status, family/parental status, income derived from a public assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident.
Persons with disabilities who require alternative means of communication for program information (e.g., Braille, large print, audiotape, American Sign Language, etc.) should contact the State or local Agency that administers the program or contact USDA through the Telecommunications Relay Service at 711 (voice and TTY). Additionally, program information may be made available in languages other than English.
To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online at How to File a Program Discrimination Complaint and at any USDA office or write a letter addressed to USDA and provide in the letter all of the information requested in the form. To request a copy of the complaint form, call (866) 632-9992. Submit your completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Mail Stop 9410, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: program.intake@usda.gov.
This work is supported by the Crop Protection and Pest Management Extension Implementation Program from the U.S. Department of Agriculture’s National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and should not be construed to represent any official USDA or U.S. Government determination or policy.
USDA is an equal opportunity provider, employer, and lender.
©2026 by the Crop Protection Network. All rights reserved.
