CORN
Crop Observation and Recommendation Network

July 9 to July 15, 2001
C.O.R.N. 2001-21

In This Issue:

A) Alfalfa Weevil And Regrowth Following Harvest Of 1st Cutting
B) Monitoring For First Year Corn Rootworm
C) Late Postemergence Herbicide Applications In Soybeans
D) Soybean Aphid
E) Check Nutrient Status Of Corn At Silking
F) Crop Diagnostic Field Meeting And Grain Marketing Wrap-Up
G) State Agronomy Team Meeting

A) Alfalfa Weevil And Regrowth Following Harvest Of 1st Cutting (Bruce Eisley) CORN Questions

Alfalfa fields that had large numbers of alfalfa weevil larvae and were cut instead of being treated should have the regrowth check for alfalfa weevil feeding. In cases where regrowth appears to be retarded 7 to 10 days after harvest due to feeding activity of weevil adults or larvae, application of a stubble spray may be warranted.

Although a harvest of alfalfa will generally decimate the weevil population, surviving larvae will continue feeding on regrowth and show the second cutting.

For more information, visit the Fact Sheet on Alfalfa Weevil on the Web at:
http://ohioline.ag.ohio-state.edu/ent-fact/0032.html.

B) Monitoring For First Year Corn Rootworm (Bruce Eisley) CORN Questions

Monitoring of first year corn rootworm activity in soybeans with yellow Pherocon sticky traps should be initiated at the end of this week (July 13) or early next week. A minimum of four traps should be posted per soybean site monitored. Traps should be changed on a biweekly schedule at the end of July, at mid-August and finally the survey should be terminated at the end of August.

The potential for FYCRW can be predicted in part by monitoring adult WCR activity in soybean fields with yellow sticky traps from mid-July to late August. The most common trap used for monitoring adult WCR activity in soybean fields is the unbaited Pherocon AM yellow sticky trap. In the 1998, 1999 and 2000 Ohio FYCRW surveys, four traps were stationed in each soybean field monitored and serviced on a biweekly schedule from mid-July through late August. Based on studies from Illinois and Indiana, an average collection of two or more WCR adult beetles per trap per day should indicate a potential FYCRW problem for first year corn at a monitoring site. In the 1999 and 2000 survey, only one site each year actually exhibited an average WCR catch on the traps exceeding the level of two beetles per day. Seventy percent of sites monitored had an
average catch of less than 0.20 WCR per trap per day which is less than 10% of the threshold of two beetles per trap per day.

Although the use of the Pherocon AM yellow sticky traps in soybeans is currently recognized as the best way to evaluate the potential for FYCRW injury in first year corn, it should be noted that other methods are available for FYCRW detection and assessment.

1. First year corn root systems may be sampled to detect rootworm injury. Occasional detection of feeding scars is not unusual, but detection of significant rootworm pruning of root systems in first year corn indicates a potential FYRW problem. When significant rootworm injury exists, a proportion of the plants should exhibit some goose-necked lodging. However, where lodging is evident, it is important to distinguish lodging due to rootworm from that due to wind damage.
   
   
2. Detection of an unusual level of adult corn rootworm activity in first year corn also may indicate a potential FYRW problem. In general, visual counts of one or more adult corn rootworm in first year corn fields are very rare - especially if the first year corn field is not a late planting or is not adjacent to any continuous corn. Visual counts of adult rootworm in corn should include inspection of at least 10 or 20 plants at random.
   
   
3. Adult rootworm activity in soybeans may also be sampled using a sweep net. Given variations in daily activity, sweep net samples are not as accurate as the use of yellow sticky traps. However, where a potential FYRW problem exists, collection of adult WCR will likely exceed one or more adults per sweep.

In summary, rootworm management on corn, first year or continuous corn, should be based on observations of the prior growing season. Such observations must be implemented during the months of July and August when root system injury may be evident and adult WCR or NCR activity can be monitored. A number of preventive treatments (old and new) currently exist for preventing rootworm injury, but such treatments can be rather expensive if not warranted.

A fact sheet about first year corn rootworm monitoring is currently available on the web at: http://entomology.osu.edu/ipm/factsheets/0017.htm

C) Late Postemergence Herbicide Applications In Soybeans (Mark Loux) CORN Questions

At this point in the growing season, emergence of new summer annual weeds should be nearing an end. It is still possible to observe emergence of foxtails, black nightshade, burcucumber, and a few other weeds, but primarily in response to significant rainfall events and in areas where the soybean canopy is not adequately shading the soil surface. A well-developed soybean canopy allows soybeans to suppress and outcompete most late-emerging weeds, making it difficult to justify a late postemergence herbicide application in most soybean fields. One exception would be late-planted soybeans that are still relatively small, where weeds that emerged around the time of planting may have to be controlled. In soybeans that were planted by early June, the main purpose of a herbicide application at this time should be to prevent harvest problems, rather than reduce weed-crop competition. Late-emerging foxtail, nightshade, and vines can interfere with harvest. The need for a herbicide application at this time to reduce problems at harvest is a judgement call based on the conditions in individual fields. If nightshade is a problem, keep in mind that preharvest application of Gramoxone or glyphosate will dessicate the foliage but will not cause berries to fall off plants. Fields with significant nightshade infestations may therefore be candidates for a herbicide application at this time.

D) Soybean Aphid (Ron Hammond, Bruce Eisley) CORN Questions

Soybean aphids were found in Ohio last week. Surveying in Wood and Wayne Counties in NW and NE Ohio, respectively, revealed fields that had aphids. Populations ranged from extremely low in Wood County to low in Wayne County. In some fields in Wayne County, aphids were not that difficult to find. All aphids were found on the upper most leaves of the plant, mainly on the upper, expanding trifoliates. Of importance, the aphids are here, and more intensive surveying needs to begin throughout the state.

Surveyors should use a technique that has been agreed on by entomologists throughout the Midwest--examining 30 plants, each for about 45 seconds, and recording the relative numbers of aphids found. For each plant record whether aphids are present and the relative population density using the scale of 0 rating = no aphids, 1 rating = 1-10 aphids, 2 rating = 11-100 aphids, and a 3 rating 100+ aphids per plant. In the Wood County fields, 1 field had 1 plant with aphids with a rating of 1, while another field had about 3-4 plants with a rating of 1. In Wayne County, more plants with aphids were found with a rating of 1. However, a single plant was found with an upper expanding leaf with nearly 100 aphids on it, thus a rating of a high 2 to a low 3. This was the highest density yet observed in Ohio.

If you look for soybean aphids, please make certain to accurately identify any insects you find. Full size aphids are about the size of first-instar potato leafhoppers, so you need to make sure you identify the correct insect. A good hand lens will aid in your identification. Any significant aphid problems, where plants are covered with aphids or have extremely large populations, should be reported to Ron Hammond at 330-263-3727 or hammond.5@osu.edu. No other aphid would be colonizing soybeans, so any colonies of aphids should be the soybean aphid. Anyone finding aphids should notify their extension agent.
No one is certain whether soybean aphids will build to economic levels this year. However, based on the time of year and frequency that they are being observed, close observation is warranted. We are trying to determine the role of natural enemies including lady beetles, parasitoids, growing practices, and environmental conditions. This is the first full summer we will be monitoring the situation, so we do not know what will happen. Of interest, it has been suggested that the presence of lady beetles might indicate high numbers of aphids. The leaf found in Ohio with nearly 100 aphids on it was found by having noticed 2-3 lady beetles on the leaf.

Our recommendation at this time is to watch what happens in the fields. Entomologists across the Midwest are keeping taps on the aphid, and will be sharing information on what is happening, when they might need controlling, and what insecticides will be best to use. Ohio is part of this group, and will do research as the situation demands, including examining insecticides if higher populations are found. A good web site for information on the soybean aphid is http://www.pmcenters.org/Northcentral/Saphid/aphidindex.htm.

E) Check Nutrient Status Of Corn At Silking (Peter Thomison) CORN Questions

Leaf tissue analysis, used in conjunction with other data (e.g. soil test results) and visual observations, can be an effective aid in evaluating the mineral nutrient status of the soil-plant system. It can be especially useful in determining plant micronutrient status. The silking stage in corn is a good time to collect ear leaf samples for tissue nutrient analysis. Fields should be sampled when 50% or more of ears show silks. However, don't sample after silks turn brown. The entire leaf directly below the ear is the part of the plant to be sampled, i.e. broken off at its base (do not include portions of the leaf sheath or collar).

The number of plants to sample depends on the general condition of the plants, soil uniformity, and purpose of the plant analysis. However, to minimize variation in nutrient concentrations from plant to plant, sampling should be from no less than 20 plants randomly selected in the area or field represented by the analysis. When a nutrient deficiency is suspected (even without visual plant symptoms) or there is a need to compare different areas in a field, ear leaf samples should be collected SEPARATELY from both the affected plants and adjacent normal plants.

Knowing what not to sample is just as important as knowing what should be sampled. Don't sample plants that -

• are diseased, insect damaged, or dead,
  
• are mechanically injured,
  
• have been stressed by too much or too little moisture (i.e. flooding or
  drought),
  
• have been stressed by abnormally high or low temperatures, and
  
• show nutrient stress (e.g. severe stunting) unless being comparatively
  sampled with more normal areas.

When leaf nutrient analysis is necessary to perform on plants which were stressed by drought or other environmental conditions prior to sampling, record weather and other conditions so later interpretation of the nutrient analysis can take environmental conditions into account.

Brown paper bags are ideal for collecting and drying tissue samples. Leaf tissue should be loose and remain loose in bag for adequate drying. Do not wad leaves tightly or tie in a bundle since heating and molding may occur. Allow about 48 hours for drying.

Sufficiency ranges for corn ear leaves sampled at silking are shown in the table below.

Plant Analysis for Nutrient Levels in Corn (ear leaf sample taken at silking)

F) Crop Diagnostic Field Meeting And Grain Marketing Wrap-Up (Howard Siegrist) CORN Questions

Cost--$5.00, which includes a light supper. Two hours of CCA Continuing Education Credits for those registered. For additional information, contact Howard Siegrist at 740-349-6900 or e-mail siegrist.1@osu.edu

G) State Agronomy Team Meeting CORN Questions

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Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Editor: David Jones        Web Editor: Tom Rosati

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

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