Without Adequate Crop Scouting, Pests Like Hungry Caterpillars Can Eat Through Farmers' Profits

Jul 29, 2019

Walking through rows of growing crops helps farmers monitor for harmful insects, leaves that are damaged by disease or other problems that could reduce their overall harvest at the end of the season. 

And this year in Iowa, there’s a menace that, left to its own devices, could munch farmers out of profit. 

Agronomist Meaghan Anderson shows participants the roots of a corn plant and explains how to evaluate its health during a crop scouting workshop in June in Runnells, Iowa.
Credit Amy Mayer / Harvest Public Media

In June and July, agronomist Meaghan Anderson held a series of workshops to teach crop scouting in corn and soybeans. She arrived at the second session with some young soybean plants she’d picked.

"Can everybody see these nasty caterpillars?” she asked, revealing the fuzzy black and yellowish critters that formed small webs in the leaves. Anderson, who works for Iowa State University Extension and Outreach, said they are thistle caterpillars. 

These become painted lady butterflies, and are not normally much of an issue in central Iowa. But this year is different.

“They are eating the heck out of our beans,” she said, “especially our early planted ones.”

In the right conditions — a certain amount of moisture when the plants are young and the temperatures are compatible with the caterpillar life cycle — thistle caterpillars can produce two generations within one summer crop season. That’s part of the reason why it’s important to be out in the fields looking for them, Anderson explained. 

A thistle caterpillar sits on the leaf of a soybean plant that's not yet decimated.
Credit Amy Mayer / Harvest Public Media

Not all farmers are in the habit of scouting. These days, the application of insecticides or fungicides may be scheduled for a predetermined date, independent of actual crop conditions. But there’s little value to spraying if you don’t have an active problem, Anderson said. 

In the workshop, she showed participants how to evaluate the approximate percent of damage to a plant and how to monitor a field by walking a certain pattern so the plants that you select to monitor might be representative of the condition of the whole field. 

“We don’t want to purposely avoid bad places, but we also don’t want to get in a panic just looking at all the edge of the field and thinking, ‘Oh, everything’s terrible,’ and not ever stepping into the center of the field and seeing how things look,” she said.

Seeking expert advice

Even when farmers do scout their crops, they can question their own objectiveness in deciding how bad things are. That’s where Anderson and her colleagues can help with on-farm visits. 

Shortly after the session where she showed the presence of thistle caterpillars, Anderson’s phone started ringing more and more frequently with questions from farmers about what to do. 

One of those calls came from Dan Schall, who farms near Ogden, Iowa, and planted seven fields of soybeans this year. 

He and his wife saw painted lady butterflies in mid-July, so Schall did some crop scouting.

“Just thought it was time I get out and look around,” he said. The ubiquity of thistle caterpillars worried him. “You can see the webbing … and defoliation. Some holes in the leaves.” 

Seed dealer Kacy Frantum and customer Dan Schall stand in a field of late-planted soybeans, surveying the damage from thistle caterpillars.
Credit Amy Mayer / Harvest Public Media

By the time he was walking between rows of beans toward the end of July with Anderson and his seed dealer, Kacy Frantum, the variation among different fields was drastic. Frantum had taken similar walks with other customers. 

“We have not had this much pressure on thistle since I’ve done this,” Frantum said. He’s been a seed dealer for 10 years and a farmer for half a century. 

Anderson counted the number of caterpillars on 10 plants and evaluated the amount of leaf damage. She offered Schall her evaluation of each field. 

“In this field, I would say we’re not at the threshold point, which would be like the tipping point to say you need to treat or in a couple days from now, a few days, you may be at the point where you’ve lost yield and you’re not going to recover it,” she said about a field where the soybean plants were nearly hip-high and the caterpillars were fewer than two per plant. 

There was visible damage on some leaves, but plenty of the plants weren’t touched. Schall took in the information, then everyone piled back into his pick-up truck to visit another field. 

One stood out from all the others. Even from the road, Anderson could see the soybean plants on the edge were decimated. The leaves looked like green lace and multiple fat, full-size caterpillars clung to almost every leaf. 

“Wow,” Anderson said, “They’re literally hanging off these plants. This is so bad.” 

Anderson takes photos of damage that thistle caterpillars wrought on soybean plants in one of Dan Schall's fields.
Credit Amy Mayer / Harvest Public Media

The size of the caterpillars indicated they would soon form cocoons, at which point they wouldn’t cause any more damage. But Anderson said the really intimidating thought was that with the current conditions, a third generation of the pests might be possible. 

In the end, Scall determined the cost of spraying insecticide on four of his soybean fields would be worth it. The application likely would prevent significant losses to his crop, meaning he should earn enough money to more than cover what he spent on the insecticide. 

Without scouting, he might have missed the damage and taken a sizeable hit to his bottom line at harvest. And an ill-timed insecticide application might not have wiped out enough of the caterpillars to justify the expense.

Meanwhile, Anderson continued to get calls about thistle caterpillars — even as she was in the field with Schall. The scourge has been seen in parts of Nebraska and Illinois this year, too. 

At one point after counting caterpillars, Anderson dropped a handful, declaring they’d peed on her and she was grossed out. She took out her smartphone and snapped many photos of the caterpillars and the damage they caused. 

“I hope this goes down in the history books as just the strangest year that we never experience again, ever,” she said.

“Would be nice,” Schall replied.

Follow Amy on Twitter: @AgAmyinAmes

Scores of thistle caterpillars, which become painted lady butterflies, have munched through the leaves on these soybean plants at the edge of a field in Boone County, Iowa.
Credit Amy Mayer / Harvest Public Media