Soybeans damaged by thistle caterpillars

These soybeans have had about 15 percent defoliation, according to Mike Witt, Iowa State University Extension and Outreach field agronomist. The thistle caterpillars that caused the damage will soon become butterflies that won’t eat the crops. Younger caterpillars could cause more damage and might require spraying. Photo submitted

A second generation of thistle caterpillars is causing damage to soybeans in Crawford County and farmers may need to take steps to kill the pests, according to Mike Witt, Iowa State University Extension and Outreach field agronomist for Crawford, Carroll, Greene, Shelby, Audubon and Guthrie counties.

The first generation of the caterpillars appeared in western Iowa about a month ago and resulted in swarms of painted lady butterflies that made a mess of bumpers, grills and windshields of vehicles on the roads.

“They laid their eggs and this is the second generation,” Witt said. “In western Iowa and southwest Iowa it is widespread. I would say there is more of it than I have seen in my lifetime out there.”

The caterpillar form is the first part of the lifespan and is the stage when the pest eats soybean leaves.

Witt said he has seen soybean fields in which the caterpillars had caused about 20 percent defoliation, which is around the economic threshold for spraying for the pests.

“If you would have asked me two weeks ago if we’d ever get to that number, I would have said probably not, but there is a large volume out there,” Witt said.

He recommends that farmers scout fields for the caterpillars and assess the age of any found.

“If they are an inch to an inch and a quarter, then they are probably close to being full-size and full-grown,” he said. “They will probably be a little bit lighter in color and they are about ready to start pupating and changing into the butterflies.”

The butterflies will move on and cause no more damage to the plants.

“If they are smaller than that, and black, then you might need to think about a treatment more than you would if they are older,” Witt said. “If you have a bunch of little ones, obviously they still need to grow up so there’s going to be a significant amount of feeding.”

The good news is the caterpillars are not hard to kill.

“Just about any insecticide will get the job done,” he said. “If you are going to spray a fungicide anyway, it might make sense to include an insecticide if you have a high thistle caterpillar rate.”

Fields with 10 to 15 percent damage and lots of young caterpillars may need to be sprayed to keep the pests from causing significant damage.

Fields with damage similar to that in the image on page 1, at about 15 percent, will likely recover, but may lose some measure of yield, Witt said.

“The reason for that is the plant is down a lot of leaf tissue and it needs to recover that leaf tissue,” he said. “A lot of that energy will go toward leaf tissue recovery as opposed to yield.”

The plants still have time to put on more leaves and recover, he said.

Witt doesn’t think a third generation of the caterpillars will show up, but he can’t completely rule it out.

Soybeans are not doing as well as corn this year, but the soybeans in Crawford County look better than those in other parts of the state, he said.

“I’ve been all over the state and the crops in Crawford County look very, very good. Knock on wood,” Witt said.

The soybeans that were planted late look the worst.

With the recent rains in the area, the upper soil profile is in good shape, Witt said.

The later start for planting means the growing season needs to last longer.

“We need that frost date, which we can’t control, to be later in the season,” he said. “We don’t need a September 10 frost date this year. We need it to be later in the fall to extend that window so the crops have plenty of time to mature and get dried down.”

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