Alfalfa Caterpillar in Soybean
Published: 10/10/2025
Alfalfa Caterpillar in Soybean
Crop Damage: Alfalfa caterpillar (Colias eurytheme) occurs throughout the U.S., Mexico, and southern Canada. Unlike other foliar feeders that may leave the soybean’s midrib intact, larvae can consume the entire leaf.
Alfalfa caterpillar feeding on a soybean leaf.
Tristan Mueller
Pest Description: Full-grown larvae are 1 ½ inches long and have a velvet green body with a distinct white stripe running along both sides of the body. One key characteristic is the presence of four pairs of prolegs and three pairs of thoracic legs. Adults, commonly known as “sulfur butterflies” or “alfalfa butterflies,” are medium-sized and easily recognized by their white or yellow wings with distinct black borders.
Scouting: Begin early season scouting when the adult butterflies are flying above fields. Randomly select plants throughout a soybean field and estimate the average defoliation over the whole crop canopy. The Crop Protection Network’s Insect Defoliation Tool can train to assess defoliation levels.
Alfalfa caterpillar. Orange arrows indicate characteristic prolegs and black arrows indicate thoracic legs.
Whitney Cranshaw, Colorado State University, Bugwood.org
The alfalfa caterpillar adult is known as the sulfur butterfly.
Frank Peairs, Colorado State University, Bugwood.org
Management: The treatment thresholds for alfalfa caterpillars in soybean are 30% defoliation in the vegetative stages and 20% in the reproductive stages. If additional defoliators are present, assess their combined impact before making a management decision. In addition, it is important to note that alfalfa caterpillars in alfalfa are often managed by natural parasitism and pathogens.
Please contact your local or state extension specialist for information on the most current management options.
Green caterpillar on a soybean leaf with feeding injury. Image by Tristan Mueller.
An alfalfa caterpillar adult is yellow with black fringed wings, as shown here.. Frank Peairs, Colorado State university Bugwood.org
An alfalfa caterpillar is green with three front true legs and four hind prolegs, as shown in this image. The caterpillar is on a green plant stem. Image by Whitney Cranshaw, Colorado State University, Bugwood.org.
Gallery images by Tristan Mueller; Frank Peairs, Colorado State University, Bugwood.org; and Whitney Cranshaw, Colorado State University, Bugwood.org.
Developed and reviewed by Carol Pilcher, Iowa State University and Robert Wright, University of Nebraska-Lincoln, with support from the United Soybean Board.
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