Iowa Field Reports: Weed Challenges; Thistle Caterpillars in Soybeans

Thistle caterpillar in soybeans. Photo by Erin Hodgson, Iowa State University

The wet weather, weed management challenges in corn and soybeans, and thistle caterpillars in soybeans continue to be common issues that Iowa State University Extension and Outreach field agronomists are hearing about and seeing in fields across the state. Read on for more information about what’s happening in specific regions across the state.

Northwest Iowa

Paul Kassel (Region 2): “A stretch of about three weeks of relatively dry weather came to an end this past week with 2 to 3 inches of rain falling in parts of my region. Farmers and applicators will turn their focus to postemergence herbicide applications in soybeans this week. Some postemergence herbicide applications have already been made on soybean acres, specifically acres that did not receive the intended pre-emergence herbicide program.

“Corn development is in a wide range across the area with late April and early May planted corn at the V9 stage and early June planted corn at V3.”

Central Iowa

Meaghan Anderson (Region 7): “My region received anywhere from about 0.5 inches to 3 or more inches over the past week. Seeing a lot of early season insect feeding in soybeans with thistle caterpillars being the main culprit. I haven’t seen a soybean flower yet, but I expect many soybean fields to be flowering soon. This is a good reminder to double check crop growth stages before making herbicide applications as some herbicide labels restrict application after R1 or R2 in soybeans.

“Over the last week I’ve been hearing about weed control issues in both corn and soybean fields. In addition to weed control issues I’ve also gotten calls about nitrogen and side dress applications and my first dicamba drift on a soybean field.”

Southwest and West Central

Aaron Saeugling (Region 6): “Most early corn has benefited from the sunshine, warmer temperatures, and some rainfall. The early planted corn is V7 to V10. Most post herbicide applications have been applied to corn. Soybeans range from V1 to V4. I have seen a few blooms, so some of the early planted fields are not far from reaching R1. Thistle caterpillars continue to do some feeding in soybean fields.

“Additionally, soybean gall midge adults have been confirmed in Cass County. Herbicides are being applied to soybean fields as the weather allows. Questions on thistle caterpillars dominated phone calls this past week. I’ve also gotten my first drift calls of the season.”

East Central, Southeast, and South Central:

Rebecca Vittetoe (Region 8): “Corn mostly ranges from V2 to V10. Most of the early planted corn is out of the ugly duckling stage and has gotten its nice dark green color. If you look closely you can still see some of the unevenness across corn fields. I have noticed some pale or yellowish “flag leaves” scattered in corn fields. Soybeans mostly range from V1 to V4.

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“I did see flowers starting to appear in early planted soybean fields last week, so some fields may be at R1 now. Thistle caterpillars have been the main concern in soybean fields. Weed control issues in corn and soybeans, weed identification, tissue testing for corn and soybean, and thistle caterpillars dominated calls this past week.”

Virgil Schmitt (Region 9): “Rainfall this last week ranges from 2 to 4 inches roughly south of Highway 92, 1 to 2 inches between Highway 92 and Highway 30, and a 0.5 inch north of Highway 30. Most people had from zero to three days to do any fieldwork. Corn planting has mostly ended. April planted corn is V9 to V10, mid-May planted corn is mostly V7 to V8, and June planted corn is mostly V3.

“On Friday, I noted some pale “flag leaves” in my travels in some fields in eastern Jasper and western Poweshiek Counties along I-80. Soybeans are from V4 down to still in the bag. Soybean planting is about 90% complete. Inquiries about herbicides, potential herbicide drift, and cover crops dominated calls last week.”

Josh Michel(Region 11): “Rainfall across the region put a halt to field operations last week. Much of southern Iowa received anywhere from 1.5 to 4.5 inches of rain. Planting progress is unchanged from two weeks ago. Corn remains at around 80 to 85% planted, and soybeans are about 60 to 65% planted. Corn ranges from V8 for the early planted corn to V1.

“Earlier planted soybeans are V4, with many fields possibly reaching R1 by the end of this week. Newly planted soybeans continue to have some emergence issues. Bean leaf beetles, Japanese beetles and thistle caterpillars are causing some minor concerns with soybean feeding. Common questions this past week have been on nitrogen management, uneven corn stands, late season weed management, and prevented planting options.”


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