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8/15/2011 - 8/21/2011

Corn Aphids Explode

By Erin Hodgson, Department of Entomology

Beginning last week, I have seen and heard about an aphid explosion - in corn! For the last three to four years, certain areas in Iowa have had serious problems with aphids infesting corn in August. The areas having problems right now include the northeast and northwest corners of Iowa and west central Iowa. Some of these heavily-infested fields were sprayed with an insecticide earlier this year. From my observations this week, I noticed aggregated colonies at the end rows.

In the past, corn leaf aphid could be a problem during tasseling. This species aggregated around the ear and silks, and sometimes their honeydew production interfered with pollination. But natural enemies and the environment rarely let them build up past July. So economic thresholds have only been determined for aphids around tasseling and mostly targeted to fields during drought-stressed summers.

Now, it seems aphids are colonizing corn later in the summer and are building up to striking levels. They can be found at the base of the stalk, around the ear and sometimes building up colonies above the ear leaf.



Aphids in corn can build up large colonies, sometimes exceeding 2,000 per plant.

One important observation I've made is that most fields have two aphids species - corn leaf aphid and bird cherry oat aphid. They are closely related and look very similar in size and color. The bird cherry oat aphid has an orange-red saddle between the cornicles. Other aphid species can also be found, including greenbug and English grain aphid, but are not as common in corn this year. Species identification isn't that critical for management at this point (an aphid is an aphid). You can see more than one species in a field and even on a single plant.

All aphids have a piercing sucking mouthpart and feed on the sap from the plant phloem. They excrete sugar-rich honeydew that can cover the above ground portion of plants. The honeydew can promote a sooty mold that can interfere with plant photosynthesis. You probably remember seeing gray-looking soybean leaves when soybean aphids first hit Iowa about ten years ago. We know soybean plants covered with mold and aphids can have serious yield loss, but we don't know the extent of yield reduction caused by aphids in corn.


corn aphids
Corn aphids infesting the ear and above the ear leaf.

Currently, there are no treatment thresholds for aphids in corn past tasseling. But regular sampling will help you make educated decisions about a foliar application at this time. Here are some considerations to make before applying an insecticide for aphids in corn:

  1. Are 80 percent of the plants infested with aphids?
  2. Do most of the ears have aphids? What about the ear leaf and above?
  3. How long has the field been infested and is the density increasing? Sample field-wide (30 plants for every 50 acres) to determine the average density.
  4. Do you see honeydew or sooty mold on the stalk, leaves or ear?
  5. Are you seeing winged aphids or nymphs with wing pads? This may be a sign of migration out of the field.
  6. Is the field under drought stress? Dry weather will amplify potential feeding damage to corn.
  7. Do you see any bloated, off-color aphids under humid conditions? Natural fungi can quickly wipe out aphids in field crops.
  8. What is the corn growth stage? Fields reaching hard dent may be past the point of a justified insecticide.
  9. Some insecticides have a 60-day pre-harvest interval. Check the label and calendar.
  10. Get good coverage of the application - ideally droplets should make contact with the aphids for a quick knockdown. Don't expect residual to protect the corn from fluid feeders.


I strongly encourage you to leave an untreated check strip or two in fields that you spray. Try to leave a strip that is a fair comparison to the majority of the field - not just the stunted corn around the field edge. If you decide to treat for aphids in corn, I would like to hear about the yield comparisons. Your pooled data will help me formulate treatment guidelines for the future.

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone 515-294-2847.

Crop Minute - Goss's Wilt

Alison Robertson, ISU Extension plant pathology, is monitoring the development of Goss's wilt in Iowa corn fields. During the weekly crop minute, she talks about the steady increase, no known rescue treatment and how to identify Goss's wilt.

A New Bean Leaf Beetle Threshold Calculator is Created

Erin Hodgson and Mike McCarville, Department of Entomology

In April, an ICM News article predicted a slightly higher winter survivorship of bean leaf beetle in Iowa. Not surprisingly, this summer I have been hearing about a few soybean fields with bean leaf beetle. Most reports of overwintering and first generation beetles are at low densities with negligible defoliation. There are two generations of bean leaf beetle in Iowa (Fig. 1), with the second generation emerging in a few weeks. Scouting for bean leaf beetle and other defoliators should continue until seed set. 
 

Figure 1. This is an example of bean leaf beetle population dynamics in Iowa. The second generation can be an economically important defoliator in soybean.


Sampling

Soybean fields in the reproductive stages can be sampled for bean leaf beetle by using either a drop cloth or a sweep net. Scout each field and each variety separately and walk into the field at least 100 feet before sampling.

Drop cloth
• Place a 3-foot wide strip of cloth on the ground between the rows.
• Bend the plants on one row over the cloth, and shake them vigorously.
• Count the number of beetles that drop on the cloth.
• Repeat the procedure four times for every 20 acres of the field.
• Estimate the average number of beetles per 3-foot of row.

Sweep net
• Take 20 sweeps while moving forward. A sweep is defined as a 180-degree pass across two soybean rows or along 3 linear feet within a row.
• Repeat the procedure four times for every 20 acres of the field.
• Estimate the average number of beetles per 20 sweeps.


Sweep netting for bean leaf beetle in 30-inch rows (top) or narrow rows (bottom).
Photos by Marlin E. Rice.

 

Management

The overwintering and first generation populations do not typically cause economic defoliation, but can be a useful predictor of the second generation. Bean leaf beetle feeding on soybean pods caused by the second generation can lead to significant reductions in seed quality and yield throughout Iowa. It is important to recognize bean leaf beetle injury. Managing bean leaf beetles in soybean during the pod set and pod fill can be frustrating to growers and crop advisers because adults may be feeding on pods for a couple of weeks before the population reaches the economic threshold.

To help make treatment decisions easier for first and second generation bean leaf beetles, a dynamic Excel spreadsheet has been created. These calculations use the expected market value (bushels per acre) and cost of control (dollars per acre) to determine the treatment threshold. This calculation assumes bean pod mottle virus is not an issue for seed sale.

To make these calculations easier and always up-to-date with the changing market, follow this link to a downloadable spreadsheet. By saving the spreadsheet to your personal computer or tablet, it can be used repeatedly as market values fluctuate. There are a few examples to demonstrate the tools.

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone 515-294-2847. Michael McCarville is a Department of Entomology graduate student; he can be reached at 515-294-8663 or by email at mikemcc@iastate.edu.



This article was published originally on 8/22/2011 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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