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8/3/2009 - 8/9/2009

Getting to Know the Aphids in Corn

By Erin Hodgson, Department of Entomology

There have been reports of aphids feeding on lower corn leaves this week. In July, I wrote an ICM article about corn leaf aphid thresholds prior to tasseling; however, there is still some concern about economic loss in August. People have noticed aphids moving down the plant and forming colonies on the stalk. Many are wondering if they are corn leaf aphids or something else?

There are at least two different species that can be found in corn this time of year, but the identification may not be critical to management.

General Description.  In general, aphids are soft-bodied and pear-shaped insects with walking legs. Usually they are wingless, but migratory, winged forms can be generated throughout the summer. The main diagnostic feature of aphids is a pair of cornicles that resemble tailpipes, towards the end of the abdomen. All aphids have a piercing-sucking stylet and feed on plant phloem.

Damage.  Aphids excrete a sugar-rich honeydew that can promote a sooty mold which can reduce photosynthesis. Some aphids are capable of vectoring plant diseases. Those species that vector disease are considered more economically important because low aphid densities can reduce quality and yield. Heavily infested plants will be discolored and stunted, and have curled and mottled leaves.

Corn leaf aphid.  This species prefers sorghum, but will also feed on corn, barley, millet and many different grasses. These aphids are more rectangular, or box-shaped, compared to other aphids. They are dark olive-green or blue-green in color, and sometimes cover themselves in a protective wax. The antennae are dark and short, and the cornicles are short, dark and surrounded by a dark patch. Adults range from 0.9-2.44 mm in length. Corn leaf aphids cannot overwinter in Iowa, but migrate on jet streams here every year.

corn leaf aphids

Corn leaf aphids have pear-shaped bodies that are more typical of aphids.

 

Bird cherry-oat aphid. This species prefers wheat, barley, oats, rye, and triticale, and is less commonly found forming colonies in corn. Their body shape is pear-shaped and is more typical of aphids. They can be variable in color, ranging from yellow-green, olive-green or black. Often they have a rusty colored patch around the cornicles. The antennae and cornicles are dark and average length. Adults range from 1.2-2.4 mm in length. Bird cherry-oat aphids have the potential to overwinter on chokecherry in Iowa.

bird cherry-oat aphids

Bird cherry-oat aphids are yellow-green or dark green in color and have round bodies.

 

Management.  Scouting for either aphid species should start before tasseling - probably too late for that this year. Corn leaf aphids generally start colonies deep within the whorl. Stop at five locations and examine twenty plants. Examine the ear, leaves and stalk. Use a hand lens and look for an orange patch between the cornicles to distinguish the species. Take note of beneficial insects (e.g., ladybeetles and lacewings) and/or parasitized mummies; consider not treating aphids if more than 20 percent are parasitized.

Spraying corn for aphids deep in the whorl is not effective. Treating for aphids after tasseling is usually not a cost-effective management decision because aphid populations will decline naturally. A well-timed spray while aphids are outside the whorl or on the tassel is recommended when they exceed thresholds. Corn plants can be particularly sensitive to aphid feeding if they are drought-stressed. If 50 percent of the plants have more than 100 aphids per plant, tassels are coated in honeydew and plants are under drought stress, treatment may be justified. There are several products registered in Iowa for corn leaf aphid (Table 1). Follow label directions and pay attention to spray guidelines.


 

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.

Hail Damage Disaster Recovery Web site Offers Crop Information

By Brian Lang, ISU Extension field agronomist

hail damage to crops

ISU Extension has set up a Hail Damage – Disaster Recovery Web site  in response to the needs of Iowa producers making hail damage decisions. This site includes information on topics listed below, with additional information to be posted as harvest approaches:

  • assessing crop damage
  • emergency forage
  • salvaging a hail damaged crop
  • foliar fungicide and hail damage
  • silage harvest issues
  • grain harvest issues

The severe hail storm that hit northeast Iowa July 24 left behind a six-county strip of major crop issues. “The storm caused damage over approximately 400,000 crop acres, with at least 10 percent of this acreage receiving around 100 percent yield loss,” said Brian Lang, ISU Extension field agronomist.

Other recent storm events left smaller pockets of severe damage across the state. These are occurring at a time in the growing season when there are few options. “Producers have some difficult decisions to make regarding the remaining crops,” Lang said.

Degree Days - We Don't Need Stress!

By Rich Pope, Department of Plant Pathology

It is beyond time to get hot summer weather started! Both corn and soybean are (finally) in reproductive stages, and parts of Iowa are over 200 degree days behind normal. 

But we don't want to make up that deficit with significantly above-normal temperatures, as high August temperatures mean stress that can cut yield potential dramatically. In 1992, we were nearly as far behind as now, and the cool August weather then produced an above-normal yielding crop, however the grain was wet and fall grain handling and dry down were significant issues. 

We cannot say yet if there will be a repeat of 1992. The ideal weather recipe now is not too cold and not too warm, with a little rain mixed in. Anything cooler and we delay maturation, anything warmer and we cut yields.  

 Accumulated degree days from May 1 through August 2, 2009

Observations indicate soybean aphid numbers are increasing, most notably in northern and central Iowa. Scouting fields is critically important. Foliar corn diseases also warrant monitoring -  eyespot in norther parts of the state and gray leaf spot towards the south.

There is an interesting graphic and story of the July 24, 2009 hail storm that struck northeast Iowa on the Iowa Environmental Mesonet (IEM) website. The IEM contains a weatlth of weather and climate related information for Iowans.

 

Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at ropope@iastate.edu or by calling (515) 294-5899.

Weekly Crop and Weather Report

The Aug. 3 crop and weather report includes Iowa State University Extension climatologist Elwynn Taylor, integrated pest management specialist Rich Pope, and corn agronomist Roger Elmore visiting with ISU Extension communication specialist Doug Cooper.

Taylor says July was officially the second coldest month on record (1891 was the coldest by one tenth of a degree). The forecast for August is mixed, rain in the east and dry in the central and western areas. Taylor discusses the major hail storm that hit northeastern Iowa on July 24 and the related images available at http://mesonet.agron.iastate.edu/

Soybean aphids are reaching threshold in some areas, according to Pope; he recommends scouting for the pest. The key from now until harvest is to avoid stressing crops.

Corn agronomist Elmore says the cool or cold July can have a negative impact on yields. He says a late frost would be very helpful in getting the crop to reach maximum yield potential.

Common Corn Nematode Characteristics

By Greg Tylka, Department of Plant Pathology

Awareness of and interest in corn nematodes seems to be growing along with the crops this season. Growers and agronomists are asking lots of good questions about how nematodes feed on corn, what soils they prefer, how much corn yield loss they cause, etc.  Answers to those questions can be confusing and frustrating because there are more than a dozen different types of corn nematodes and the different species vary in their biology and behavior. The table below lists the basic characteristics of different types of corn nematodes.

* The damage thresholds listed in the table above are expressed as numbers of nematodes per g of root for lance and lesion nematodes; all others are numbers of nematodes per 100 cc soil.

These damage thresholds were established in the 1970s and 1980s and have not been verified with modern corn hybrids. The thresholds should not be considered absolute values for modern corn production systems. Also, threshold values will vary among states and universities; the values presented are those used by Iowa State University.

 

 

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. Tylka can be contacted at gltylka@iastate.edu or by calling (515) 294-3021.  

Iowa Corn 2009 Outlook, as of August 1

By Elwynn Taylor and Roger Elmore, Department of Agronomy

Iowa average corn yield in 2004 was 181 bu/acre (BPA), the highest ever recorded. Our assessment of current crop conditions and weather reports indicates that the 2009 corn yield can possibly be every bit as high. 

Historical data (since 1952) show that late planting reduces crop yield potential but cooler than usual temperatures from planting to silking have little definitive impact on realized yields.  On the other hand, temperature from silking to R6, blacklayer, has a major impact.  Higher than usual daytime – and nighttime – temperatures often bring on water stress and reduced yields (e.g. 1988). Warm night-time temperature even when day-time heat is not excessive tends to reduce yield by shortening the filling period. Cool night-time temperatures after silking are associated with the higher yielding years in Iowa. 

Based on years with similar conditions through July as we have had this year, the chance of continued cool weather is more likely than a change to hotter than usual.  The weather and corn yield estimation model “Hybrid-Maize” shows a strong relationship between night-time temperature and relative yield (see figure below - the model allows for comparisons over years with weather as the only variable).  Much of the modeled impact on yield was related to cooler temperatures causing an extending of the period from silking to R6. Note: although the model shows a possible yield of 280 BPA this should be shifted to the reality of 2008 when the yield was 171 BPA (not 280 BPA).  The model assumes all factors other than weather are perfect.

Our assessment is: the chance of having corn yields as good as 2008 is very real for Iowa this year.

 

Predicted Yield and Average Minimum Temperature

Ames, IA.  - Negatively related, R = -0.53

Corn yield predicted* Yields predicted using Hybrid-Maize, Ames IA, 1986 -2008

 

 

Elwynn Taylor is a professor of agronomy with research and extension responsibilities in climatology. He can be reached by email at setaylor@iastate.edu or by phone at (515) 294-7839. Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Elmore can be contacted by email at relmore@iastate.edu or (515) 294-6655.



This article was published originally on 8/10/2009 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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