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Thistle Caterpillar Adds One More Item To Soybean Farmers’ 2019 List of Woes

Amy Mayer
Thistle caterpillars eat soybean leaves and use them to create webbed cocoons. The bug, while common, doesn't pose a threat to soybeans every year. But this season seems to be one of concern in Iowa and eastern Nebraska.

On top of trade disputes, a wet spring and late planting, many soybean farmers face yet another hurdle: the thistle caterpillar.

Although it becomes the painted lady butterfly, which can bring a fluttering swath of color to backyards and gardens, this caterpillar can be a real pest in soybean fields..

Iowa State University Extension agronomist Meaghan Anderson said it’s the No. 1 pest she’s hearing about this season, and she plucked some from a field earlier this month. The caterpillars have spiny backs and although they can feed on hundreds of different plants, the soybean leaves are good for the type of webbed cocoon they make.

“This spring in particular, for some reason, these are causing a lot of issues in our soybean fields,” Anderson said at a workshop in Runnells, Iowa. “They are eating the heck out of a lot of our beans, especially the early-planted ones.”

The caterpillars, which have also been found recently in Nebraska, usually stick to outer edges of a field, where they aren’t a problem for farmers.

And insecticides — yet another input in a pricey spring for many farmers — are cost-effective only when the bugs are present and have already chewed up 30 percent of the leaf matter in an entire field, Iowa State entomologist Erin Hodgson said.

She advised farmers to thoroughly look around before deciding whether to spray an insecticide, which means getting deep into a field and estimating how much of the leaves the bugs have chewed through.

“Spend some time, walk into the field and see if there really is activity defoliation throughout the entire field,” Hodgson said. “This really depends on the size and the shape of the field, because some very large fields you walk 30 or 40 rows in and there’s virtually no activities.”

Golden Harvest agronomist Stephanie Porter said she hasn’t seen any thistle caterpillars in Illinois this year — yet.

In the areas where they’ve been spotted, though, their life cycle should allow a second generation to emerge later this season, at about the same time Japanese beetles, soybean aphids and other pests arrive to do their own damage.

Follow Amy on Twitter: @AgAmyinAmes

Amy Mayer is a reporter based in Ames