April 23, 2004: Focus on soybean pest management

As farmers rushed to get fields planted before predicted rains, clouds just hovered. Across the state corn planting was well underway this week with soybeans expected to follow next. This producer was planting northeast of Lincoln on Tuesday, April 20. (Photo by Brett Hampton)

Major soybean viruses vectored by insects

In recent years Bean pod mottle virus has been found in up to 91% of surveyed soybean fields. This disease tends to be more of a problem in early planted fields which attract overwintered bean leaf beetles.

Plant exhibiting symptoms of virus infection which could be due to Bean pod mottle virus or Soybean mosaic virus.

Resistance to this disease is not currently available, but varieties will respond differently. In yield comparisons for current commercial varieties, there is an average 18% reduction when plants are inoculated at the V2-3 growth stage. Your entire field will not be inoculated at this stage, and some spread will occur later in the year when F1 and F2 bean leaf beetle populations emerge. The impact of spread by the F1 and F2 beetles will not be as significant as early season infection. This is why our research efforts have focused on developing strategies for timing the initial applications of the insecticide early and/or the use of seed treatment insecticides. These experiments will be repeated again this year as the last two years have resulted in conflicting data.

In another project we are screening materials in the NU soybean breeding program to develop tolerant germplasm to Bean pod mottle virus. Again, no resistance has been observed for this virus.

The distribution of Soybean mosaic virus in Nebraska recently changed significantly. This disease was encountered only rarely prior to 2002 when it was observed in 30% of soybean fields surveyed. The increase in incidence is due to the presence of soybean aphids in the Midwest and the ability of Soybean mosaic virus to be transmitted in seed.

The level of seed transmission for Soybean mosaic virus is significantly higher than the level for Bean pod mottle virus and will result in Soybean mosaic virus being a long-term problem for soybean production across the Midwest. For Soybean mosaic virus there is resistance in some soybean varieties. As we do not know what soybean aphid populations will do this year, I have been recommending the use of varieties resistant to Soybean mosaic virus, assuming they contain good agronomic characteristics for your situation.

The current research literature suggests that one source of management for Soybean mosaic virus is to avoid late planting dates. In some areas of the United States late planting dates have resulted in higher soybean aphid populations; however, this may not be true in Nebraska, based on observations in the last year. At this time we are not recommending later planting dates to avoid this virus. Seed lots infected with the virus will increase in the percentage of infected seed as that lot is continually used as a seed source. For this reason, producers who hold seed should definitely start out with new seed lots every three to four years or at least have the seed lot tested for Soybean mosaic virus.

Remember that viruses cannot be differentiated based on plant or seed symptoms. The only way to identify them is by Elisa testing. (Elisa testing is available from the Cooperative Extension Plant and Pest Diagnostic Clinic.) If you observe a significant level of distorted leaves which cannot be linked to a chemical misapplication or drift event, I would encourage you to send a sample to the diagnostic clinic for identification.

Bean leaf beetles have two generations a year in Nebraska. However, since they over-winter as adults, three periods of beetle activity are seen in the growing season: Overwintering colonizers, F1 generation (offspring of the colonizers, the true first generation) and the F2 generation.

Bean leaf beetles overwinter as adults in leaf litter (woodlots) and soybean residue. They become active fairly early in the year (April-May) and often can be found in alfalfa prior to soybean emergence. As soybeans emerge, the beetles quickly move to seedling plants, feeding on cotyledons and expanding leaf tissue. These overwintered beetles, called colonizers, mate and begin laying eggs. Females live about 40 days and lay from 125 to 250 eggs. After egg-laying is complete, the colonizing population dwindles as the beetles die. A new generation of beetles (F1) will begin to emerge in late June to early July. The F1 beetles mate and produce a second generation of beetles (F2) that begin to emerge in mid August and feed on leaf and pod tissues. The pod-feeding F2 beetles are most likely to cause economic damage.

Bean leaf beetles vary in color, but are usually reddish to yellowish-tan. They are about 1/4 inch long and commonly have two black spots and a black border on the outside of each wing cover. These spots may be missing, but in all cases there is a small black triangle at the base of the wings near the thorax. Because they move to soybean fields so soon after seedling emergence, early-planted fields will usually have more beetles and suffer the most injury. This has become more of a problem in recent years because planting dates seem to be getting earlier each year.

Table 1. VC Economic thresholds (beetles per plant) Crop value Pest management cost $/bu $/acre $6 $8 $10 $12 $5 3 4 4 6 $6 2 3 4 5 $7 2 3 3 5 $8 2 2 3 4 $9 2 2 3 3 $10 1 2 2 3 Table 2. V1 Economic thresholds (beetles per plant) Crop value Pest management cost $/bu $/acre $6 $8 $10 $12 $5 4 5 7 8 $6 3 4 6 7 $7 3 4 5 6 $8 3 3 4 5 $9 2 3 4 4 $10 2 3 3 4

Although the defoliation the beetles cause can appear quite severe, research in Nebraska and elsewhere has shown that it usually does not result in economic damage. Soybean plants can compensate for a large amount of early tissue loss, so it takes a considerable amount of beetle feeding to impact yield. Generally, unless insect populations are large enough to cause more than 50-60% defoliation of seedling soybeans, it is unlikely that treatment would be economically justified.

Tables 1 and 2 show economic thresholds for bean leaf beetle on seedling soybean. Be aware that these thresholds are for defoliation of beans at VC-V1. If beetles enter the field right at or during seedling emergence, the thresholds will likely be lower because the beetles do not have leaf tissue to eat and will feed on the growing point, stem, and cotyledons. We do not have a good research base for bean leaf beetle injury to newly emerging soybean, but the thresholds are probably about 1.5 beetles lower than the VC thresholds.

Remember that early-planted soybeans are the most susceptible. If economic thresholds are reached, many insecticides are available for bean leaf beetle control. All will do an adequate job if applied according to label directions.

Another reason some producers treat bean leaf beetle on seedling soybeans is to reduce the pod-damaging F2 generation that emerges in August; however, UNL Extension does not recommend this practice. Many environmental factors can impact beetle populations throughout the growing season, making it impractical to use spring beetle numbers to accurately predict if beetle populations will reach economically damaging levels in August. Regular scouting and the use of the appropriate economic thresholds are the best means for managing late season bean leaf beetle in soybean. Late-season economic thresholds will be included in CropWatch later this summer.

Bean leaf beetles also vector bean pod mottle virus, which is discussed further in "Major soybean viruses. . ."

Soybean aphid description

The aphid is light green to pale yellow, less than 1/16 inch long, and has two black-tipped cornicles (cornicles look like tailpipes) on the rear of the abdomen. It has piercing-sucking mouthparts and typically feeds on new tissue near the top of soybean plants or on the undersides of mature leaves. Later in the season the aphids can be found on all parts of the plant. It is the only aphid in North America that forms colonies on soybean, so if you have aphid populations developing on soybeans, they are soybean aphids.

Nebraska observations from 2003

In June, extremely low numbers of aphids were found in Nemaha, Cass, Saunders, Douglas, and Burt counties. Aphids were found primarily in fields near wooded river bottoms along the eastern border of Nebraska. Buckthorn, an overwintering host of the aphid, was found in wooded river bottoms throughout the eastern half of Nebraska. In mid-July, soybean aphid infestations began to be reported from northeast Nebraska. By late July soybean aphids could be found in all soybean production areas of Nebraska (eastern half of Nebraska), with almost all economically damaging populations being in the northeast portion of the state. Populations peaked in mid-August. Peak populations in the northeast ranged from less than 100 aphids/plant to approximately 5000 aphids/plant (field averages). Most infested fields in the northeast had low to moderate populations.

In 2003 if aphid populations reached thresholds and farmers treated in late July or early August, they benefitted from treatment.

The pattern of soybean aphid colonization in 2003 was similar to 2002. Very few aphids were found until mid-July, with more fields per county infested in the northeast corner of the state. Mid-July colonization coincided with summer storm patterns having high northeast winds. However, population levels were much higher in 2003. In 2002 there were only two reports of fields being treated for soybean aphid. In 2003 many fields in northeast Nebraska were treated, although it is likely that many did not require treatment or were treated after economic damage had been done.

A possible explanation for higher numbers in 2003 may be that in 2003 temperatures in the second half of July through the first week of August were rather mild, which favors soybean aphid reproduction. In 2002 temperatures were high during this period.

Various studies were initiated to examine the effects of the aphid on soybean growth and yield, aphid ecology, etc. In general, if aphid populations reached thresholds and farmers treated in late July or early August, they benefitted from treatment. If treatment occurred in mid August, benefit was variable and depended on aphid population size, population dynamics, and predator levels (primarily lady beetles). Late August treatments likely resulted in no benefit, as aphid populations naturally declined.

Looking to 2004

It’s too early in the season and this insect pest is too new to Nebraska to predict its impact. We expect to see aphids in much of the Nebraska soybean production areas in 2004, probably starting in mid-summer (possibly some in June) in the northeast and eastern part of the state. Actual levels and impact will depend on weather and natural enemies.

While soybans were found in most of the state’s soybean production area in 2003, economic populations were generally limited to the northeast area of the state. This pattern is likely to continue with higher populations more often found in the northeast.

Currently soybean rust is in South America and working its way north. The most logical way for this disease to get to the United States is via the land bridge through Central America. The other way would be through an inadvertent introduction by someone traveling to South America. Given that the Midwest could be the first place it would appear if it is inadvertently introduced, it is critical that we are all on the lookout for this disease.

Soybean rust development is favored by temperatures ranging from 59-84°F, with relative humidity above 90% for more than 12 hours. In order for spores to germinate and germ tubes to develop and penetrate leaves, leaves must be wet for more than six hours. Environmental conditions in the Midwest are not as favorable as conditions in the southern United States for this disease, but it could still have a significant impact on soybean production in Nebraska.

Soybean rust symptoms

While soybean rust has not yet been identified in the United States, researchers say it's just a matter of time until it enters. The yellow areas indicate countries where soybean rust has been identified.

Lesions first appear as small yellow and irregularly shaped spots. Lesions appear most commonly on the leaves, but also can be found on petioles, pods and stems. As the disease progresses, lesions become polygonal in shape, and enlarge to 2 to 5 mm² (0.03-0.08 in²), and tan to red-brown in color. At this stage, symptoms can look like bacterial pustule, however, mature soybean rust pustules have a small opening at the top of the raised pustule for release of the urediniospores. Bacterial pustule lesions lack the opening on top and lack spores. These features can only be seen under magnification (20 X recommended). As rust severity increases, plants prematurely lose their leaves and commonly mature early. The defoliation and initial symptoms can also be confused with Septoria brown spot which is common in our soybean fields. Septoria can also be differentiated from rust based on the lack of pustules and urediniospores.

When rust arrives in the United States, our main form of management will be foliar fungicide applications. Currently, Bravo, Echo and Quadris are labeled for soybean rust and a special request has been filed for emergency use (Section 18) of the following fungicides: Bumper, Folicur, Headline, Laredo, Pristine, PropiMax, Stratego, and Tilt. Based on data coming out of South America, the use of the currently labeled products (active ingredients of chlorothalonil or azoxystrobin) for soybean rust after it is established will not provide adequate control and the chemistry provided by the Section 18 products will be needed. The Bottom Line: We are ready for this disease and should all be on the lookout for it. If we can detect it early enough, its impact can be significantly reduced.

Standard sample — $10: The standard sample fee is applied to all samples that can be diagnosed with only visual and/or microscopic examination.

Additional charges for services: Culturing for pathogen identification costs an additional $10. This fee helps cover the cost of media plates and the time involved in identifying the causal agent. Other additional costs include:

SCN Assay - $10

Plant Parasitic Nematode Assay - $15

Goss’s Culture - $10

Stewart’s Elisa - $15

Misc. Elisa Test - $15

Bacterial ID (Biolog System)- $20

Virus Screen (protein based) - $15

Rare Species ID - $10

Insect Culture - $10

When you have a plant or pest problem, first consult your local Extension Educator. Often he or she can provide a diagnosis or will have copies of the specimen identification form for submitting samples. Send samples and forms to:

University of Nebraska-Lincoln

Plant & Pest Diagnostic Clinic

448 Plant Sciences

P.O. Box 830722

Lincoln, NE 68583-0722

Tips for sample collection

1. Collect a sample representative of symptoms and include healthy tissue for comparison. For turf samples, collect a portion that includes a margin between healthy and diseased areas.
2. Send as much of the sample as possible. This means send the entire plant including the root ball if feasible. Also send multiple plants or multiple branches from a tree or shrub showing a range from healthy to unhealthy.
3. Provide as much information about the sample as possible (age and variety of plant, moisture availability, soil type, disease history of site, chemical history of site, description of symptoms, plant part(s) affected, time of symptom development, distribution of symptoms, occurrence of severe weather, and any other information that may be helpful in diagnosing the problem).
4. Include a picture of the distribution of symptoms, as this can be very helpful.
5. Plants submitted for horticultural and weed identification should include flowers and/or fruit, leaves and roots.

1. Keep samples cool before sending them.
2. Place sample in a plastic bag and include a dry towel if the sample is damp. If the roots are in soil, enclose them in a separate plastic bag with the soil intact. Place the sample into a sturdy box with packing material to take up excess space. A padded envelope can be used for relatively small and flat samples, such as some tree branches.
3. Do not mail samples late in the week, as the sample can deteriorate if the package sits in the post office over the weekend.
4. Include all sample information (see No. 3, Tips for sample collection, above), photographs if possible, and contact information such as phone numbers and mailing address.

All of us at the Plant and Pest Diagnostic Clinic look forward to serving you this summer for your diagnostic needs. We hope that your plant and insect problems are minimal and your harvest is plentiful.

WeedSOFT is a decision support system designed to help growers, consultants, and extension educators make both proactive and reactive weed management decisions. WeedSOFT provides the treatment information you need based on your specific field conditions while factoring in economic and environmental principles. Whether you are considering early season soil applied treatments, control of mid-season infestations, or comparing treatments requiring additional costs for herbicide resistant crops, WeedSOFT is a powerful tool in your weed management arsenal. Each year a new version is released to update database information and provide new features.

WeedSOFT consists of four modules; Advisor, EnviroFX, MapView, and WeedView.

ADVISOR is the heart of the WeedSOFT suite of decision-support tools. ADVISOR provides a bioeconomical analysis based on weed biology, weed management efficacy, and production costs. Through information provided by the user, the program generates a list of allowable treatments from an extensive database of possible treatments and control practices. The net dollar gain in expected yield resulting from the application of a particular treatment is determined and becomes the criteria used to rank the allowable treatments. Treatments may be ranked by expectations of percent maximum yield or “net gain”.

EnviroFX is intended to provide information on the potential environmental impact of specific herbicide treatments. EnviroFX estimates relative herbicide leaching and potential for groundwater contamination based on soil and herbicide properties and water table depth.

MapVIEW is a first step in the process of evaluating the risk of groundwater contamination by herbicides. This module includes digitized Nebraska county maps that are color coded to display the vulnerability of certain sites to groundwater contamination with herbicides. Once vulnerability of a site is determined, EnviroFX may be used to determine the relative potential of a specific herbicide to reach groundwater. These tools allow the user to make informed management decisions based on soil properties and depth to ground water. WeedVIEW is a visual library of color images and line drawings for each of 46 common weed species found in Nebraska. This module facilitates the correct identification of weed species.

The 2004 version of WeedSOFT 2004 includes several new features:

• State-specific versions of WeedSOFT^® are available for: Illinois, Indiana, Kansas, Michigan, Missouri, Nebraska, and Wisconsin.
• A Seed Calculator has been developed which calculates the amount of seed you need for a field and the cost
• A Herbicide Tank Mix Calculator has been developed to provide you with the amount of selected herbicide to add to a tank mix. The module also allows you to print a load ticket for easy reference
• A Record Keeping module has been developed which allows you to track the ADVISOR-recommended herbicide treatments you have applied to your fields.
• Several Learning Modules have been developed to provide more information about the types of information ADVISOR uses to make recommendations. These are readily accessible within ADVISOR
• ADVISOR can now make recommendations selectively if a herbicide resistant crop is planted.

For starters, prepare a strategy for using leftover hay. One of the better options is to feed hay a bit longer this spring before turning cows out to permanent pasture. Usually, this would be the exact opposite of what I would recommend -- feeding less hay and grazing more. Allowing pastures to accumulate more growth before grazing will provide more total grazable forage if drought prevents later regrowth. Leftover hay also can be used later during the grazing season to give pastures more time to recover between grazings. Another strategy is planting drought-tolerant forages for pasture or hay. Summer annual grasses like sudangrass, sorghum-sudan hybrids, and pearl millet are your best choices. Wait until soils are good and warm before planting these grasses. Late May or early June usually is best.

Reserve some ground for these drought-insurance grasses before you plant everything to corn, beans, and milo and who knows, maybe we’ll have some good rains before then. If we don’t, however, acting now to reduce potential forage losses can pay big dividends later this summer.

However, after a drought year, when plants have been weakened, early grazing must be done carefully to avoid lowering grass yields and yearlong carrying capacity.

If your pastures were in good shape last fall, you can carefully start grazing early this spring to avoid wasting good grass growth. However, make sure you don’t graze too close, otherwise, if it turns dry again like last summer, grass growth will stop prematurely.

Most pastures, though, were not in good condition last fall. Delay grazing in these pastures at least until your usual turn out date. With this year’s early spring, that should give pastures a strong start. Once you start grazing, quickly move animals through all your spring pastures once, taking about two weeks to finish this rotation. When you start your second round, slow down so each paddock has at least six weeks to recover before the third pass. After that, let rainfall and grass growth be your guide.

A. Bruce Anderson, Extension Forage Specialist: Net-wrapping can represent an investment -- equipment costs $3,000-$4,000 and plastic net is $0.75-$1.00 more per bale than twine. While net wrapping isn’t cheap, it does offer several advantages that can save money and time. Research from Wisconsin showed that net wrap reduces harvest losses about 1%. That’s how much you lose while bales are spinning and being wrapped with twine.

Storage losses are quite a bit less with net wrap because net wrapped bales shed water better. Under Wisconsin conditions, twine wrapped bales lost 11% of their weight but net wrapped bales only lost 7% during five to twelve months of outdoor storage. That’s an extra 4% feed from net wrapping and doesn’t even count the better forage quality in net wrapped bales.

A couple percent here and there may not sound like much, but if you add the harvest and storage losses together to save 5% of your hay and it costs a dollar to wrap each bale, hay only needs to be worth about $35 a ton to pay for the net wrapping material.

By far the biggest savings, though, may be time. Net wrapping only takes a couple turns of the bale compared to 15, 20 or even 30 turns for twine. Waiting to finish twine wrapping wastes time, burns fuel, and adds to tractor wear and tear. With net wrap, you can make 30% more bales per hour. Little things can make big differences. How you wrap your bales is one of those things.

Wheat condition rated 11% very poor, 17% poor, 38% fair, 33% good, and 1% excellent, below last year and average. Fields were 15% jointed, ahead of last year at 10% and average at 8%.

Corn planting had made a start in most areas around the state with 7% seeded to date, ahead of last year and average at 3%.

Oat planting progressed to 84% seeded to date, ahead of last year and average at 68%. Twenty-nine percent of the acreage had emerged, ahead of last year at 23% but the same as average.

Sugar beet planting continued to make good progress with 57% completed to date. A year ago 20% had been planted.

Alfalfa conditions rated 3% very poor, 9% poor, 44% fair, 39% good, and 5% excellent.

Feedlot placements in March totaled 295,000 head, down 8% from 2003 and 5% below March 2002. Marketings of fed cattle during March totaled 350,000, up 13% from last year and 17% above March two years ago. Other disappearance during March totaled 5,000 compared with 10,000 during March 2003 and 10,000 during March 2002.

National increase

Nationally, cattle and calves in feedlots with capacity of 1,000 or more head totaled 10.75 million head on April 1. The inventory was slightly above last year but 7% below April 1, 2002.

Feedlot placements during March totaled 1.80 million, 11% below 2003 and 8% below 2002. Marketings of fed cattle during March totaled 1.97 million, 9% above 2003 and 8% above 2002. Other disappearance totaled 67,000 during March, 8% above 2003 but 3% below 2002.

"Rarely in my career have I met someone with such a broad knowledge of plant nutrition," said IFA Director General Luc M. Maene. "We at IFA are pleased to see someone as deserving as Ken rewarded for his dedication to marrying the goals of global food security and environmental sustainability."

In his 27 years of research and extension work on plant nutrition, nutrient cycling and soil quality, Cassman has advocated the "crucial role of fertilizers in crop intensification, which he considers necessary to safeguard important natural ecosystems that contain much of the world's terrestrial biodiversity," IFA said in announcing the award. "He argues that in order to meet the dual goals of global food security and protection of natural resources, fertilizer use efficiency must increase and that current or emerging crop nutrient technologies provide effective options for doing so."

The major accomplishments for which Cassman was honored include:

1. Documenting the critical role of fertilizers, proactive nutrient management and soil quality maintenance as essential components of sustainable agriculture and global food security.
2. Increasing yields and fertilizer use efficiency of irrigated rice in Asia. Because rice systems account for the majority of nitrogen fertilizer use in Asia, enhancing nitrogen fertilizer use efficiency has a substantial positive impact on water quality and emissions of greenhouses gases throughout the continent.
3. Improving the basic understanding of cotton potassium requirements and translating this into better nutrient management at the farm level. This work has helped researchers and extension workers identify and correct potassium deficiency in Australia, Brazil, Egypt, India, South Africa, the United States and Zimbabwe.