Volume 2, Issue 6
June 19, 2019
This newsletter is an update on current topics and events in agriculture in Boone, Dallas, Jasper, Madison, Marshall, Polk, Story, Tama, and Warren counties.

Meaghan Anderson
Extension Field Agronomist

ISU Extension & Outreach
220 H Avenue
Nevada, Iowa 50201
Office:  515.382.6551
Cell:     319.331.0058

Upcoming Events

IDALS Apr 19 - Dec 19 Pesticide Testing Schedule

June 25 - Herbicide Resistance Mgmt. Field Day, McCallsburg

June 27 -
Weed Science Field Day, Ames

June 27 -
Iowa Swine Day, Ames

July 10 -
Conservation Field Day, Slater

July 10-11 -
Crop Management Clinic, FEEL (Boone)

July 16 -
Agronomy in the Field, Runnells

July 18 - Women Landowners meeting, Perry

August 28 - Ag Engineering and Agronomy Farm Field Day, Boone

Handy Links

Central Iowa Crop Update
Updates from an agronomist and weed nerd in central Iowa.

In this issue:

  • Two upcoming weed management field days
  • Thistle caterpillars and defoliation in soybean
  • Pests to keep an eye out for in corn/soybean
  • Keeping ahead of weeds and checking crop growth stages
  • Rainfall and nitrogen
  • Cover crop and forage options for prevented plant acres

Weed Management Field Days

We have two weed management field days coming up that may be of interest for you to attend.

Tuesday, June 25 at 5:30 p.m. at the A. Knaphus farm 1.5 miles south of McCallsburg on S27
Angie Rieck-Hinz and I will be hosting a Herbicide Resistance Management Field Day to discuss weed management strategies for resistant waterhemp in soybean. This is a unique set of demonstration plots to show the importance of managing weeds from a program approach and the effect of spray timing on weed management. This field day is free and open to the public; attendees can receive 2.0 pest management CCA CEUs at the evening meeting. Read more about this field day here.

Thursday, June 27 at 8:30 a.m. at the Curtiss Farm south of Ames on State Ave.
The ISU Weed Science team is hosting a free "self-guided" opportunity to tour the weed science plots at the Curtiss Farm near Ames. Registration for the field day begins at 8:30 a.m. with a short program at 9:00 a.m. All attendees will receive a field book with treatment descriptions. Read more about the field day here.


Thistle caterpillars and defoliation in soybean

My phone has been ringing off the hook with phone calls in the last several days, primarily with questions about thistle caterpillars that are showing up in soybeans. The adult butterflies (painted lady butterflies) fly up to Iowa in the spring, much like armyworm and black cutworm moths, and lay eggs. We've seen an inundation unlike anything I've heard of across many counties.

Images showing thistle caterpillar on soybean trifoliate leaf and the webbing the caterpillars create.

Some thoughts for those of you seeing these:
  • Treatment is recommended if injury reaches 30% defoliation in vegetative soybeans or 20% if soybeans are flowering.
  • Check out this resource to gauge your eye for defoliation percents!
  • Caterpillars quit feeding on soybean to pupate by they time they reach 1.5 inches in length
  • Caterpillars create web of soybean leaves and feed from tissue, but I've noticed lots of feeding with missing caterpillars now (likely went to pupate)
  • Read more about thistle caterpillars here
Other defoliators in soybean
There have been a number of other defoliating insects/pests present in soybean fields this spring, including bean leaf beetle, green cloverworm, armyworm (true and yellowstriped), and even slugs (not an insect). The wet, cool weather has slowed crop development and allowed plenty of time for these pests to feed. This is why it is important to evaluate defoliation as a whole, as well as the presence of defoliating insects before treatment. 


Pests to keep an eye out for in corn/soybean

As if we didn't already have enough pests to manage, a few more will soon be showing up in corn and soybean fields.
  • Soybean gall midge - recall that this is a western Iowa, Nebraska, South Dakota, and Minnesota pest so far. It has yet to reach central Iowa, but please keep an eye out along soybean field edges for dying plants and easily lodged plants. Give me a call if you'd like another set of eyes on a possible sighting!
  • Japanese beetles - these pests commonly defoliate soybeans and move into corn fields when they begin to silk. They are often worst along field edges and farmers should follow the same defoliation thresholds for other foliar insect pests of 30% defoliation prior to flowering and 20% after flowering.
  • Stalk borers - stalk borers should be moving from grasses along field edges to corn this week. We've heard of higher than normal infestations in grasses at field margins. A telltale sign of their presence this time of year is singular dead stems/inflorescences of smooth brome along field edges.
  • Corn rootworm - egg hatch is behind this year, but many parts of Iowa should have reached 50% egg hatch by now, so scouting of continuous corn fields or those with Bt performance issues for root feeding should begin in 10-14 days.

Keeping ahead of weeds and knowing your crop growth stages

Everything seems to be growing pretty slowly in the cool weather, but don't sleep on the weeds. Read up on some of our thoughts about managing weeds in this weird spring/summer here.

Soybean plant in SE Iowa flowering on June 19, 2019. Photo by Rebecca Vittetoe.

Also, remember that corn and soybean growth stage limits exist on herbicide labels for a good reason. Even very small vegetative soybeans will soon be flowering, if they aren't already, and many herbicide labels restrict applications when flowering begins. Read more about staging crops here


Rainfall and nitrogen

With the wet spring, I've received some questions about how we might determine whether the corn has enough nitrogen for the rest of the growing season. In conjunction with all the data collected for the Corn N Rate Calculator, John Sawyer has collected data to try to use rainfall amount as an indicator whether corn would benefit from additional nitrogen in a growing season. In central Iowa, the trigger that seems to work best is if we get more than 15.5 inches of rainfall between April 1 and June 30; I've put some weather station data from the Iowa Mesonet below so you can see where we're sitting so far. You can read more about this here.


Cover crop and forage options for prevented plant acres

Those with prevented plant acres may be starting to think about what kind of cover to put on the ground this summer. Along with what will do well as cover over the season, it's important to consider tolerance to any herbicides that may have been applied to fields. Feel free to reach out if I can help you with choosing a cover.

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