Vol. 1 No. 10

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue Office: 712-792-2364
Carroll, IA 51401 Cell: 712-249-6025
Email: vagts@iastate.edu Fax: 712-792-2366

Northwest IA Crop Update, June 18, 2001

In this issue

Western IA research farm field day

GDD accumulation and corn development

Alfalfa Insect control

Corn and Soybean insect pests

Introduction

The Western IA Research Farm is hosting an annual Crops Field Day on June 21 at 6:00 p.m. Please plan to attend if you are interested in learning about weed control, carbon sequestration, white grub control, obtaining agricultural weather data, and current pest and crop issues for northwest IA. The research farm is located east of Castana in Monona County. Contact myself or your local extension office for more information on the field day.

Last week’s warm temperatures helped with growth and development of the local crop, but now this week we are forecasted for cooler than normal temperatures once again. Corn and soybean fields have been growing well, yet I’ve been in many fields this last week with stunted, yellow corn plants resulting in very uneven fields. I attribute much of this to a combination (interaction) of seedling disease, environmental stress and (in some situations) herbicide. Once the plants get set back, they have a hard time catching up. Also look for white grubs feeding on the root system, grubs have been a very common problem this year. Many fields appear to be nitrogen deficient (yellow and stunted), yet the corn’s requirement for N has been minimal (20 lbs.) up to this point, so N should not be limiting in most situations. For information on nitrogen loss this spring and on the late spring nitrate test, refer to this week’s ICM newsletter. Insects continue to cause many problems across the area, Black cutworm, Variegated cutworm, Potato leafhopper, Alfalfa weevil and soybean aphid (northeastern IA) have been found. Even though the crop is behind in development, we still have a chance for excellent corn yields. The 50% silking date is the best indicator of crop yield potential. Normally, 50% of the corn is silking somewhere between July 21 - 23rd. Later silking dates compares with lower yields.

Crop Development
Figure 1 has accumulated GDD’s on the Y-axis and Date of Planting on the X-axis. The line with the "Squares" tracks GDD’s for the southern area of my eight county region and the line with the "Diamonds" tracks GDD’s for the northern area. The top line (-) shows estimated accumulated GDD’s in seven days (June 23). For example, corn planted on April 20 (currently V9 to V10) should be at the V12 leaf stage on June 17.

accumulated gdd graph

Most of the corn across the region is now at or past the V5 leaf stage. At the V6 stage the growing point and tassel are above the soil surface and the stalk is beginning a period of greatly increased elongation. At this point the nodal root system is now the major functioning root system. Nutrient demand by the plant greatly increases by V8. A corn plant at the V8 leaf stage has taken up about 10% of the total N and P and about 15% of the total K for the year. At V10, the time between the appearance of new leaf stages will shorten, generally occurring every two to three days.

Alfalfa insect pest control

Many area alfalfa fields have insect pest populations that are chewing away at the fresh regrowth. Feeding injury can occur from potato leafhoppers, alfalfa weevils, and a new pest, variegated cutworm. Dr. Marlin Rice summarizes potential problems with Variegated cutworms (see photo) in alfalfa in the most recent ICM publication. The article can be accessed online at http://www.ipm.iastate.edu/ipm/icm/2001/5-28-2001/alfalfacw.html.

Variegated cutworms feed on new leaves of stubble and can substantially delay regrowth of new stems. This feeding reduces dry matter yields and forage quality in the second and possibly third cuttings. An economic threshold calculation was developed by Larry Pedigo at Iowa State University. Depending on the value of the crop and the cost of treatment, most fields could only tolerate a few days of defoliation before economic yield losses would occur. If variegated cutworms are found defoliating alfalfa stubble and preventing regrowth, and most of the larvae are less than an inch in length, consider spraying the field if the economic threshold is exceeded. If most of the variegated cutworms are 1.5 inches in length, they have done most of their damage and should be finishing feeding; no control would be necessary in this situation.

Corn and Soybean Insects

Black cutworm continue to be problem in late planted corn and corn that is younger than V5. Some of these fields have cutworms of varying lengths, therefore cutting may occur for another 2-3 weeks. Cutworms can also be a problem in soybeans, so don’t forget these fields in your scouting process.

Corn rootworm. Dr. Jon Tollefson (ISU Entomologist) started to see recently hatched rootworm larva on June 5th in Clinton county. This is a few days earlier than normal. University entomologists in Indiana, Ohio and Illinois have also reported early hatches this year. The Nebraska rootworm egg hatch was about normal. Because of the early hatch in Iowa, it would probably not be economic to apply rootworm insecticides to corn planted the 2nd week of June or later. New emerging adults are predicted to show up between June 29th and July 2nd this season.

Soybean Aphid. The soybean Aphid was a newly discovered soybean pest to the United States last fall and it has been found again this spring in southeastern MN and northeastern IA. The aphid was not detected in western IA last year, but we should be on the lookout for it. More information about the soybean aphid can be obtained from your local Extension office or online at http://www.ipm.iastate.edu/ipm/icm/2000/9-18-2000/soybeanaphid.html

Weed management

Dr.’s Mike Owen & Bob Hartzler (ISU Extension weed specialists) suggest going to drop nozzles for post corn products for better weed control and crop safety. As always, be observant of the wind speed and direction when applying herbicides and keep your spray records up to date.

Fertility

Potassium (K) deficiency symptoms have been reported across IA corn fields. Anything that restricts early season root growth can cause these symptoms to show up. You can learn more about K deficiency symptoms in corn by going to this 6-26-01 ISU ICM newsletter article http://www.ipm.iastate.edu/ipm/icm/2000/6-26-2000/kdef.html

This page last updated on 02/03/04

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