Thistle Caterpillar

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.

Identification

Adult: Adults have orange-brown wings and are easily confused with other butterflies. The forewings (closest to head) have a black patch and white bar on the leading edge (Photo 1). The hindwings have a row of five small black spots. Their body is about 1 inch in length while their wingspan reaches two to three inches. The undersides of the wings are black, brown, and gray with four eyespots.

Egg: Eggs are pale green and barrel-shaped, and have 14-19 vertical ribs (Photo 2).

Larva: The larvae are called thistle caterpillars, and they range in body color from creamy white to gray-brown and can also have a yellow stripe running down the top of the body (Photo 3). The body is covered with obvious and numerous branching spines. Mature thistle caterpillars range from 1.5 to 1.75 inches in body length.

Pupa: Pupae, or chrysalids, can be various colors, including metallic green, brown, and blue (Photos 4 and 5).

Biology

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 mid-June to lay eggs. This insect typically has two generations in Iowa, but may occasionally have a partial third generation. Thistle caterpillar hosts include thistle species, soybean, and sunflower.The painted lady butterfly lays eggs singly, and they develop over seven days. Caterpillars feed for two to six weeks before pupation, which lasts about 7-17 days. New painted lady butterflies begin to emerge in late July and August and begin to migrate south in September.

Adult painted lady butterflies feed on plant nectar and aphid honeydew, choosing plants with large nectar resources as preferred egg-laying sites.

Plant Injury

Thistle caterpillars cause the most injury during the V3 to V4 soybean stages in Iowa. The larvae web together the leaves of soybean (Photo 6) and consume about 37.5 in² of tissue per larva. 97% of this tissue consumption occurs during the last two larval instars. Thistle caterpillars tend to aggregate toward the field edges, especially if Canada thistle is present (Photo 7).

Scouting

Because thistle caterpillar is a migratory pest, specific scouting for this pest is often not required. Instead, look for evidence of thistle caterpillars as you scout soybean early in the season for other issues. Keep an eye out for defoliation in thistles near field edges and throughout the field, and look for webbed leaves in soybean. Thistle caterpillar is not the only insect that will web together soybean leaves, so make sure to look inside the webbing for the caterpillar and dark, brick-like frass (Photo 8). Also, look for thistle caterpillars that do not look healthy and note any predation that may be occurring.

Populations rarely build up to treatable levels in Iowa. Treatment may be warranted if fields reach 30 percent defoliation during the vegetative stages or 20 percent defoliation during the reproductive stages (Photo 9). You can learn more about assessing defoliation in the Japanese beetle encyclopedia article and train your eye using the Soybean Insect Defoliation Training Tool from the Crop Protection Network.

Management

Because thistle caterpillar is rarely an economic issue in Iowa soybeans, it is not recommended to apply insecticides as a precaution or alter normal field operations for this pest alone. Instead, plan to scout and assess defoliation across the field.

Cultural: Effective termination of thistles within and around fields early in the season may make the field less attractive to painted lady butterflies looking to lay eggs. However, soybean is a perfectly acceptable host for thistle caterpillar, so this may not completely deter them.

Biological: Like other caterpillars, thistle caterpillar may be attacked by entomopathogens (Photo 10) such as fungi, bacteria, or nematodes, or preyed upon by beneficial insects and spiders (Photo 11).

Chemical: If an insecticide application is warranted based on the amount of defoliation present, there are many products labeled for use on thistle caterpillars. In general, insecticides tend to perform best when applied when caterpillars are small (less than 1 inch) and when sufficient coverage is achieved to penetrate the webbed leaves. Be sure to follow all label requirements. If defoliation is worse at the field edge, a perimeter treatment may be sufficient and reduce application costs.