The thistle caterpillar, Vanessa cardui, is in the Nymphalidae family and order Lepidoptera. The adult is also known as the painted lady butterfly. This species is widespread throughout the world. Larvae have a wide host range that includes soybean, but can feed on over 300 plants.
Adults have orange-brown wings with a span that reaches 2-2 7/8 inches. Body length is around 1 inch. Their forewings have a black patch and white bar on leading edge, and hindwings have a row of five small black spots. The undersides of the wings are black, brown, and gray pattern with four eyespots.
Eggs are pale green, barrel shaped and have 14-19 vertical ribs (Fig. 2). Thistle caterpillars range in body color from creamy white to gray-brown and can also have a yellow stripe running down the top of the body (Fig. 3). Mature thistle caterpillars range from 1/5-1.75 inches in body length. The body is covered with obvious and numerous branching spines. Pupae, or chrysalids, can be a various colors, including metallic green, brown and blue (Fig. 4).
Thistle caterpillars are not known to overwinter in Iowa. They migrate north annually from the southern US and Mexico. The butterflies arrive in Iowa in early to middle JUne to lay eggs. This insect is bivoltine in Iowa, or has two generations. The painted lady butterfly will lay eggs singly on soybean where they will develop about seven days. In addition to various thistle species, females will lay eggs on soybean. Caterpillars will feed for 2-6 weeks until pupation, which will last about 7-17 days. New painted lady butterflies begin to emerge in late July and August, and will begin to migrate south in September.
Injury and Management
Adult butterflies feed on plant nectar and aphid honeydew. Plants with large nectar resources are more attractive egg-laying sites. Thistle caterpillars cause the most injury during V3-V4 soybean stages in Iowa. The larvae web together the leaves of soybean and will consume about 37.5 in2 of tissue per larva. 97% of this tissue consumption occurs during the last two larval instars (Figs. 5-6). Along with soybean, thistle caterpillar have also been common in sunflower and can cause similar amounts of injury to those crops. Thistle caterpillar tend to aggregate toward the field edges, especially if there is Canada thistle is present along the edges. Populations rarely build up to treatable levels in these other crops. Treatments recommendations would be warranted if fields reach 30 percent defoliation (vegetative) or 20 percent defoliation (reproductive).