Volume 3, No. 2 , July 2000

Index


Unusual Insect Problems in 2000: What Are the Causes?

Ames Herbert, Virginia Tech Extension Entomologist

Is it the increasing amount of cover cropping and minimum tillage? Is it the mild winters wešve had in recent years? Whatever is responsible, some unusual insect pests are showing up in our soybean fields.

We are seeing a lot of minimum tillage soybean fields with two unusual insects. A dark bronze, metallic colored, fast moving ground beetle has been collected from several fields. Eric Day at the Virginia Tech Insect ID Lab has identified it as a member of the genus Amara. A large genus with over 100 species in North America, the adults is reported to be plant feeders. Are these beetles responsible for feeding damage to soybean seedlings? We are not sure. And, why are we seeing so many this year, especially in minimum tillage fields? Again, we are not sure.

Another new insect found in several soybean fields has been identified by Eric as Sehirus cinctus, or, white margined burrower bug. It is a small (4-7mm), shiny blue-black insect with a white margin at the posterior end. Unlike most burrowing bug species (some of which can be agricultural pests in crops grown in sandy soils), Sehirus cinctus lives its life aboveground, feeding on the developing seeds of mints and nightshades. It is frequently seen feeding on henbit (a common weed in the mint family) and horsenettle (a member of the nightshade family). Despite its sometime alarming numbers, this tiny bug is reportedly not harmful and will not affect the growth or development of its host plants. Is it a new pest of soybean? Will it be a problem in future years? Why is it showing up this year, especially in minimum tillage fields? We have no answers as yet.

Thrips have also been more abundant this year compared with past years and damage to soybean seedlings seems to be more extensive, at least in some areas, than in previous years. Why do thrips populations seem to be on the increase? Is it because of mild winters that favor their survival and the plants they over-winter on? Is it because of the increase in winter cover crops and small grains, which they develop on before migrating to spring planted crop seedlings such as cotton, peanuts, and soybeans? More and more producers are spraying insecticides on soybeans for thrips control. Is thrips damage to soybean seedlings really a threat to yield? Are the sprays paying off? Are thrips responsible for vectoring viruses into the soybean crop? These are all good questions, but ones that will require a lot of field research before we can make good management recommendations. We do have some preliminary field tests in place to begin gathering information. But it will take time.

New pests and old pests on the increase ­ are we seeing these changes due to changes in our agricultural practices, coupled with changes in weather patterns? Food for thought ­ and new work for those of us in pursuit of good pest management programs.

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Estimating Stands / Replant Decisions

Double-crop soybeans should have emerged by now and evaluation of the stand is needed. Poor stands will likely occur at one time or another and producers need to be aware of the potential yield loss from those deficit stands and the cost of replanting. Soybeans have a high capacity to compensate for low plant populations and gaps in the row. However, little time exists for this compensation with late-planted soybeans. Still, replanting is not usually the best option because on average, soybean yield is now decreasing at a rate of approximately _ bushel per acre with each day delay in planting. Therefore, in most cases, I would not advise replanting. Use the following guidelines when evaluating the stand. Because there are interactions between soybean maturity group, row spacing and soil type, the yield loss tables shown below are only estimates; but, they should serve as good guidelines.

  1. Determine the cause of the poor stand. Was the poor stand the result of poor seed quality, cold wet soils, hot dry soils, planting too deep or shallow, soil crusting, herbicide injury, insect feeding, poor soil to seed contact, or disease infection? Determine if the cause can be corrected to avoid a similar situation.

  2. Estimate the stand and percent stand loss due to gaps. Pace off the sections of row 20 paces long in at least 6 areas of the field. Determine (in number of paces) the total length of row lost to gaps. Then determine the percent of row lost to gaps. In addition, count and determine average number of plants per foot in sections of row not reduced by gaps. Use Table 1 to determine remaining plant population. Or use "hula hoop" method to determine population if rows cannot be distinguished. This involves placing a circular measuring device such as a hula-hoop on the ground and counting the plants contained within. To determine the plant population per acre, use Table 2.

  3. Estimate the yield of the poor stand. Use Table 3 to determine percent of full yield potential. Multiply this percentage by the expected yield. This is the yield to expect from the deficient stand.

  4. Estimate the yield from replanting. Up until mid-June, decrease the expected yield 1 bu/A per week from the original planting date. After mid-June, decrease the expected yield an additional 1/2 bu/A per day. This is the yield to expect from delayed planting.

  5. Determine the gain or loss from replanting. Subtract the expected yield of the poor stand (step 3) from the yield expected from delayed planting. This is the gain or loss in bu/A from replanting. Multiply this number by the expected price ($/bu) to obtain gain or loss in $/A.

  6. Estimate the cost of replanting. Include per acre cost of tillage, herbicide, seed and labor.

Determine profitability of replanting. Subtract your cost of replanting from your estimated gain from replanting.


Table 1. Plant populations of different row spacing with different plant counts per foot.

Plants/footRow Spacing (")
24 15 7.5
(Plant Population in 1,000's/acre)
1 22 35 70
2 44 70 140
3 65 105 210
4 87 139 278
5 109 174 ---
6 131 209 ---
7 152 244 ---


Table 2. Hula-hoop method for determining drilled soybean populations.

No. of
Plants		 Inside Diameter of Hula Hoop
30" 32" 34" 36" 38"
(Plants in 1,000's per acre)*
6 53 47 41 37 33
10 89 78 69 62 55
14 124 109 97 86 77
18 160 140 124 111 100
22 196 172 152 136 122
26 231 203 179 160 144
* Plants/acre = no. plants ÷ (3.14 * r2 ÷ 43,560 ft2)
where r = radius of hula hoop in feet.


Table 3. Yield response of double-crop soybeans to deficit stands.

% Stand
lost to gaps		Remaining Plant Pop (1,000's/A)
180 140 100
0 100 95 88
10 98 93 86
20 96 90 83
30 93 87 80
40 89 83 76
50 84 78 71
60 78 73 65
*Yield losses assume that soybeans were planting soon after wheat harvest (mid- to late-June).

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Micronutrient Management

Manganese. Unless soils have been excessively limed, applications of micronutrients (Mn, Fe, Cu, Zn) are not usually needed. Soils in the pH range of 5.6 - 6.3 can usually supply enough nutrients to meet soybean requirements. However, over-liming (pH > 6.5) can induce deficiencies. Manganese (Mn) deficiencies are the most common. Mn deficiencies are characterized by interveinal chlorosis occurring in the new growth and are generally restridted to the top of the plant. To overcome Mn deficiency, apply _ lb. chelated Mn (elemental basis) or 1 lb. inorganic Mn (elemental basis) per acre to the foliage upon appearance of symptoms and prior to bloom. More than on application may be needed. If fields have a consistent history of Mn deficiencies, it may be wise to make the application even before symptoms appear. This will prevent any growth cessation and potential yield loss between the time the deficiency is observed and when the problem can be corrected.

An additional word of caution is needed regarding tank-mixtures of Mn with Roundup herbicide in Roundup-Ready soybeans. Henry Wilson, Weed Scientist at the Eastern Shore AREC has found that tank mixtures of Roundup and Mn lower the effectiveness of the herbicide on several weed species. Higher rates of Roundup will be needed to obtain adequate weed control if the combination is used. Therefore, one should make separate applications or adjust the Roundup rate.

Boron. We have conducted extensive tests on the potential for increasing soybean yield with R3 (pod initiation) and R5 (early seed fill) applications of boron (B) to the soybean foliage. In the lower Southeast, Georgia in particular, they are seeing consistent benefits from such an application. However, 3 years of experiments in Virginia have not revealed any response to these late-season applications, regardless of yield potential, B rate, row spacing, or variety selection. We concluded that native B levels of 0.1 to 0.2 ppm as determined by the Mehlich I extraction appeared to be adequate to achieve high yields in Virginia. This was validated by tissue analysis showing B content within or above the sufficiency range set forward for soybean production. Therefore, I see no reason to apply B to soybean in Virginia unless tissue analysis indicates a deficiency.

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Will Late-Season Nitrogen Increase Soybean Yield?

Recent research in the Mid-West and Great Plain states have indicated that reproductive-stage applications of nitrogen to soybean holds promise for increasing yield, especially under high-yield potentials. The theory is that under high yielding conditions, the soybean's nitrogen-fixing mechanism cannot provide all the nitrogen that is needed to produce maximum yield. In order to verify this theory in Virginia, we have just completed 3 years of research evaluating nitrogen (N) applied at the R3 and R5 soybean development stages. In these experiments, we attempted to remove our most yield-limiting factor from the equation via irrigation. We also evaluated multiple rates of N, different row spacing, and different varieties. We were able to obtain a broad range of soybean yield ranging from 40 to 80 bu/A. However, in none of our experiments did we see any significant increase in yield from the N applications. Tissue analysis of N content was always within the sufficiency range set forward for soybean production. Therefore, we concluded that nitrogen from N-fixation and soil reserves appears to be adequate to achieve high yield in non-drought stressed soybean production systems of the Coastal Plain soils of Virginia. Due to this work and the work of others, I believe that if the nitrogen-fixing mechanism of the soybean crop is functioning properly, there no reason to add additional N to the soybean crop.

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Plant Tissue Testing

Generally, if tissue analysis indicates that nutrients are within the sufficiency range set forward for soybean production, no benefit from applying additional nutrients is needed. Our N and B experiments showed this. However, deficiencies will sometimes occur and need to be remedied. Diagnosis of nutrient deficiencies can be aided by the use of plant tissue analysis. Samples taken prior to or at initial bloom from good and bad portions of the field in conjunction with soil test results from each area can provide more information and permit accurate nutrient diagnosis. Follow the below steps in sampling soybean leaves for analysis:

  1. Either collect 20 to 30 samples from random sites throughout the field regardless of how uniform the field is or collect 20 to 30 separate samples from good and bad portions of the field. For soybeans in the seedling stages (V1 to V3), collect samples of the entire above ground portion of the plant. Do not include roots. After these early stages and prior to bloom, collect samples of the most recent, fully developed trifoliate leaf (including the petiole) from the top of the plant. Do not collect samples from field edges or near dusty roads as the dust may contaminate the sample.

  2. Don't wash or freeze samples. Place samples in paper bag, not in an airtight bag that may cause molding or rotting. Do not add water.

  3. Mail the samples early in the week to insure that they do not sit in a post office or mailroom over the weekend. There are several private labs that can analyze the samples. Virginia Tech does not offer this service.

The following table indicates sufficiency ranges for most of the nutrients required by soybeans. If the sample results fall within or above these ranges, no action is needed.


Nutrient Low High
Macronutrients --------(%)--------
N 4.25 5.00
P 0.30 0.50
K 1.75 2.50
Ca 0.50 1.50
Mg 0.25 0.80
S 0.20 0.60
Micronutrients ------(ppm)------
Mn 20 200
Fe 50 300
B 25 60
Cu 6 30
Zn 20 50

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2000 Virginia Ag Expo: Renwood & Riverside Farms - Charles City

The 2000 Virginia Ag Expo "Strategies for Change - 2000 and Beyond" will be held at Renwood and Riverside Farms in Charles City, VA on August 10 from 7:30 am - 3:30 pm. These farms are well known for their innovative continuous no-till practices and conservation efforts. For individuals or groups planning to attend Virginia's premier agricultural field day and trade show, applications to purchase tickets ($5 per person) will be made available in the Country Folks Farm Chronicle beginning with the June 26 issue. This change in ticket distribution will give greater exposure to the event. There will be no direct mailing, therefore if you do not receive the Farm Chronicle, please contact me for applications. The cost of the ticket includes admission and a barbecue luncheon served by Bill Ellis' Barbecue of Wilson, NC. This year's event will attract over 80 exhibitors along with many in field plots coordinated with Virginia Cooperative Extension. In addition to corn hybrid and soybean variety plots, information presented during the morning tour includes: "Comparison of 20- versus 30-inch corn row spacing", "Soybean leaf area index (LAI) and its relationship to seeding rate and insect defoliation", "Cotton variety selection and its relationship to insect and weed management", "Perennial Weed Control", and "Effect of tillage system on soil and nutrient runoff using a rainfall simulator." There will also be soybean inoculant and fungicide seed treatment plots on display. In the afternoon, there will be a sprayer and spray nozzle demonstration, an irrigation injection demonstration, and a presentation on the application of biosolids to no-till land. The Ag Expo should have something for everyone. For more information, contact the Virginia Ag Expo show management at 1-800-511-0671 or David Holshouser at 757-657-6450 ext. 123.

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American Soybean Association News

Soybean Can Reduce Heart Disease
The FDA has published its final rule that soy protein included in a diet low in saturated fat and cholesterol may reduce the risk of coronary heart disease by lowering blood cholesterol levels. ASA along with other soy industry partners was successful in petitioning FDA for this new labeling claim. Now, food labels may contain messages such as "25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease." It is estimated that the use of soybeans in food will increase at a rate of 10% per year for the next 5 years, up from about 37 million bushels to more than 60 million bushels.

House passes Permanent Normal Trade Relations (PNTR) with China!
After working very hard to convince the House of Representatives to pass PNTR with China, ASA is urging the Senate to quickly approve PNTR. Passage will pave the way for China to join the World Trade Organization, which will provide U. S. producers with greater access to China's vast markets. It will also level the playing field for U. S. agricultural products in markets where China has been heavily subsidizing their commodity exports.

Of the $13 billion worth of all U. S. products exported to China in 1999, nearly $900 million (> 6% of total U. S. exports) was soybeans, soybean meal, and soybean oil. According to industry estimates, demand for soybeans in China could double in the next 5 to 10 years. Strong grassroots membership support can give ASA the leverage to increase markets for U. S. soybeans. Thanks to all of you who contacted your congressmen in support of passage of PNTR!

ASA Working To Secure Oilseed Payments for 2000
In 1999, a total of $462 million dollars will be paid out to 601,000 oilseed producers as a result of ASA's lobbying efforts. As growers continue to struggle with low prices, ASA is continuing to work with Congress to authorize a $500 million payment program for the 2000 soybean crop. Such efforts should entice you to become or remain an active member in the ASA.

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Sincerely,

David L. Holshouser


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