The thistle caterpillar is the larva of the painted lady butterfly. While it may sound beautiful, the damage it can cause to Midwestern fields is not. 

Thistle caterpillars can feed on more than 300 plant species, including soybeans. These pests are not the most dangerous to a soybean crop compared to other pests. But in some fields, they can occasionally cause extensive enough damage to warrant treatment.

Here’s an idea of what to look for and how to deal with a potential thistle caterpillar problem:

What do thistle caterpillars look like?

Depending on which phase it’s in, a thistle caterpillar will look completely different from month to month — or even week to week. 

True to its “painted lady” moniker, an adult thistle caterpillar has orange, brown and black wings. Look for black “eye spots” along the edges of their hindwings, as well as black and white spots on the front wings. Meanwhile, bodies are about 1-inch long, while their wingspans stretch to 2 – 2.8 inches. 

Adults will lay eggs that are pale green and include 14 to 19 vertical ribs. 

Once hatched, thistle caterpillars could be creamy white to gray-brown. It’s possible they will have a yellow stripe extending from the top of their body. Caterpillars will also have distinctive branching spines or hairs. 

Finally, chrysalids are often very striking in the field. Easy to notice, they could be metallic green, brown or blue.

Where are thistle caterpillars? 

Thistle caterpillars aren’t known to overwinter in the Midwest. However, they usually migrate north from Mexico and the southern U.S. each year. This migration typically occurs in mid-June. 

Adults will lay eggs, and each year usually produces two generations. After butterflies emerge, they’ll migrate back south in September. 

Within individual fields, thistle caterpillars usually gather around the edges. If fields include Canada thistle (the caterpillars’ favorite feed), they will likely congregate near those plants.

How to scout 

Plant defoliation, particularly in soybean fields, will be the top sign of damage. Examine at least a dozen leaves for holes left behind by the feeding caterpillars. 

Leaf tissue consumption usually occurs during the caterpillars’ feeding period in July and August.

What to do about thistle caterpillar

If there’s been 30% or more of defoliation during the vegetative stages, or 15 to 20% during the reproductive stages, a foliar insecticide treatment is warranted. 

If fields have been extensively damaged by thistle caterpillars, reach out to your local extension specialist or AgriGold agronomist to determine the best management plan. 

Planting specific soybean varieties that offer tolerance against thistle caterpillars can also be a proactive way to manage risk. Check with your trusted AgriGold agronomist to choose the best varieties for your soil and planting conditions.