Photo Gallery
Thistle caterpillar feeding on soybean
leaf.
View of thistle caterpillar showing the stout, spiny hairs on its
body.
Thistle caterpillars spin a web, tying up the leaf where they are feeding.
Thistle caterpillar feeding injury on
Canada thistle.
Thistle caterpillar killed by a pathogen (top).
Chrysalis of the thistle caterpillar.
The adult is known as the Painted lady butterfly.
Larva / caterpillar: The full grown larvae are about 11/4 inches (32 mm) in length. The bodies are brown to black with yellow striping along each side of the body. The caterpillars are covered with stout, spiny hairs which give the caterpillar a prickly appearance. The larvae are found in webbed leaves where they feed.
Pupa / chrysalis: The chrysalis, an exposed pupa, is a lavender-brown color, and is bumpy. They can be found hanging upside down on plants.
Adult / butterfly: The adult is known as the Painted lady. The butterfly is colorful. The topside of the wings are red-and-orange with black-and-white spots. The bottom side of the front wings is rose-pink with olive, black , and white markings. Bottom side of the rear wings is olive with white markings and blue spots.Life cycle: The thistle caterpillar / Painted lady does not overwinter in temperate regions, but overwinters in in tropical and subtropical areas. The butterflies migrate northward in the spring. They reach North Dakota from May through June. Upon arrival, females lay pale green, barrel-shaped eggs singly on the tops of host plant leaves. The caterpillars feed for 2 to 4 weeks, then pupate. The butterfly emerges from the chrysalis in 7 to 10 days. Two generations are expected in North Dakota. This may be influenced by time of year when butterflies arrive and weather conditions that occur afterwards.
Host(s):
This insect is found worldwide. The butterfly is very common and easily recognized. The caterpillar feeds on over 100 species of plants. The most common plants are Canada thistle, sunflower, soybean, and vegetable plants in the garden. The Painted Lady prefers nectar from composites 3-6 feet high, especially thistles; also aster, cosmos, blazing star, ironweed, and joe-pye weed. Flowers from other families that are visited include red clover, buttonbush, privet, and milkweeds.
Damage/Symptoms:
Thistle caterpillars injure plants by defoliation. The webbing has little effect. A common question is whether the caterpillar's feeding will kill Canada thistle, a colony-forming perennial. The answer is no. The extensive horizontal root structure of Canada thistle makes this weed pest very hardy. Consuming the leaves may set back individual plants, but the root and its energy reserves will allow the plant to recover.
Comments:
Occasionally, large migrations of Painted lady butterflies occurs in North Dakota. When these occur, several crops may be affected. The two most commonly infested crops are sunflower and soybean. Other crops where problems have developed include canola and dry edible beans. Normally, infestations are not extensive enough to warrant control. However, there are times when caterpillar populations are large and defoliation can be extensive. Observations in infested fields during the 2001 growing season found that populations were definitely greater in and around where the thistle was established. Weed control directed at thistle resulted in movement of caterpillars to the healthy crop plants. In these cases, spot spraying or the addition of an insecticide to the herbicide can be done to limit crop damage.
Soybeans: In general, treatment of soybean is recommended when there is 25 to 30% defoliation prior to bloom; 20% after bloom or pod set. This normally requires about 4 to 8 caterpillars per row foot. If plants are still small, then best judgement may have to be made. One reference suggests a treatment guideline of 3 larvae per row foot in young soybeans, another when damage is severe and plants are being killed. (Insecticide recommendations)
Sunflower: In sunflower, the threshold is 25% defoliation provided that most of the larvae are still under 1¼ inch long. If the majority of the larvae are 11/4 to 1 1/2 inch long, most of the feeding damage will have already occurred and treatment is not advised. (Insecticide recommendations)
