Weather N/A
Set your location
Futures
  • Thistle caterpillars are named for their feeding preference on thistle or nettle, not for their spiny appearance.
  • These caterpillars occasionally feed on soybeans, defoliating and webbing leaves at the top of the plant.
  • Heavy thistle caterpillar feeding was observed on soybeans in 2019, and several reports of soybean feeding have already been made in 2020.
  • Control is seldom needed but the same soybean defoliation thresholds used for other defoliating pests can be used; 30% before bloom and 20% after bloom.

Background

Thistle caterpillars are named for their feeding preference on certain species of nettle or thistle, but because these caterpillars happen to have spines on their bodies, many people believe they got their name for this reason. They can also feed on soybeans, sunflowers and other vegetable and ornamental plants. These caterpillars will occasionally feed on the upper leaves of soybean plants, defoliating and webbing the edges of soybean leaves together. 

Two migratory butterflies produce larvae called thistle caterpillars, the painted lady butterfly, Vanessa cardui, and the red admiral butterfly, Vanessa atalanta.  These colorful butterflies are common migrants into our area, with cyclical high populations developing from time to time. Most likely, the overwintering environment will directly affect thistle caterpillar populations and the number of painted ladies that make it up north in late spring. Dr. A. Shapiro at UC Davis indicated that the size of northern migrations is tied to wildflower blooms in the desert. Cool wet weather favors the abundance of their overwintering hosts and that can drive population size. These insects generally have two seasonal broods but can potentially have a third generation if they arrive early enough and weather is favorable for their development.

Description
Painted lady butterflies are medium sized (2 ½" wingspan) with wings that are mostly black, brown and orange or pink on the upper side and mottled, on the underside, with shades of brown, gray and pink (Image 1).  They can often be seen visiting flowers, including thistles, for nectar.  The red admiral is about the same size as the painted lady butterfly, but they are black with orange or red bands across the forewings and along the rear edge of the hind wings (Image 2).  The forewings also have white spots.  Red admirals and painted ladies are actually related, in the same genus.  The larvae of these butterflies, the thistle caterpillars, are usually dark colored, spiny with yellow to orange bands, spines or spots (Image 3). 

Butterflies_CAPTIONS.JPG

Thistle caterpillars_CAPTION.JPG

Management
Control of thistle caterpillars in soybeans is rarely necessary but may occasionally be justified with defoliation levels that are the same as for other insects.  In the bloom to pod-fill stage of growth, insecticide use may be justified when there is 20% defoliation. Prior to bloom the defoliation threshold is 30%. The tendency for these caterpillars to feed right at the top of soybean plants causes defoliation to be overestimated. When scouting, be sure to evaluate defoliation on a trifoliate from the top, middle and low portion of the plant (total of 3 trifoliates per plant).  This would provide a more accurate assessment of average plant defoliation. Several insecticide options are available. Pyrethroids, Chlorpyrifos products or premixes (Endigo, Lorsban Advanced, Hero etc.) will provide effective control. For more information on products and defoliation thresholds contact your FS Crop Specialist.


Related Articles

Related Videos

Thistle Caterpillars & Soybean Feeding

06/23/2020