Author Archives: David Holshouser

About David Holshouser

David serves as Associate Professor & Extension Agronomist at Virginia Tech’s Tidewater Agricultural Research and Extension Center. He provides leadership for agronomic extension and research programs that lead to profitable and environmentally-responsible agriculture.

Corn earworm AVT results, BMSB and Kudzu bug update….nearing the end (Dr. Ames Herbert)

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Our final batch of corn earworm moths showed only 26% survivorship, down from last week. This season results showed a 37% survival rate for the seasonal total which exceeds all previous years. We are now up to 33 counties where brown marmorated stink bugs were/are present in soybean fields. Most are at pretty low levels compared with last year, but they are much more widespread. This week n some have been found in soybean fields in north central North Carolina. We are taking as much data as we can to help with answers for next season. And, the first kudzu bug nymphs were found in Charlotte County. We are nearing the end of this season in terms of insect pests and advisories…a long summer for sure.

Brown Marmorated Stink Bugs Infesting Soybean Fields – Not Playing By the Rules (Ames Herbert, Extension Entomologist)

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With our full complement of field scouts in place, more soybean fields are being found with brown marmorated stink bug (BMSB) infestations. We are up to 20 counties in Virginia spread over a very large area of the state. Infestations were a little slow to develop compared with last year but we are predicting that by the end of the season, many more fields will be infested compared with last year. So far, most infestations are at low to moderate levels (from 2-3 to 4-6 per 15 sweeps) but a few are in the high infestation range 8-10 or more/15 sweeps. We know that sweep netting is not the ideal way to sample for these insects, but there is no other method other than simply easing into the field edge and counting what you see. Basically, if you can stand in one spot, do a 360 degree turn and count more than 5 or 6, that area needs protection. So, how are they not playing by the rules?  Last season, our first with treatable levels, bugs stayed on field edges and a single insecticide edge treatment provided season long control. This is true for many fields this year but in a few we are finding them deeper into fields, and, there are some cases where re-infestations are occurring in fields previously treated. The good news is that 1) based on our insecticide trials it is not difficult to kill them with several common products, and 2) many fields, especially the full season crop fields, are rapidly approaching the safe stage which based on previous field cage studies happens after R6 (full seed) when pod walls begin to toughen. Many growers are treating field edges this year and we are going to follow as many as possible to determine if those treatments worked and held. More on this as we progress.

This report was taken directed from Dr. Herbert’s Weekly Virginia AG Pest Advisory.  To access this advisory, go to http://www.sripmc.org/Virginia.

 

Soybean Crop is Still at Risk to Drought & Pests

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Our soybean crop is moving right along.  Much of our May-planted crop has hit or it approaching the R6 stage.  Our double-cropped soybeans are in the R4-R5 stage depending on their maturity, planting date, and whether or not they experienced early-season drought.  There’s a tendency for us to believe that the crop is made after the seed have met in the pod.  However, as shown below, only 50% of our yield has been made at the R6 stage. 

Dry weight will continue to accumulate in the seed for the next 3 weeks until the crop reaches the R7 stage (physiological maturity), which is defined as one pod on the plant that has reached its final mature color.  Much of this seed weight is due to translocation from other plant parts, especially the leaves and petioles (remobilization).  Sometimes soybean is called a self-destructing plant.  This is largely true.  As photosynthate is moved from the leaves to the seed, the leaves will turn yellow and drop from the plant.  If the leaves are yellowing and dropping due to this natural process, then we shouldn’t be concerned.  However, if the leaves are dropping due to another reason such as dry weather or disease, we should take notice.  It is important to keep the leaves green as long as possible to maximize yield.

It takes about 2 weeks to move from the R5 to R6 stage and another 3 weeks for to move from R6 to R7.  This is a long time and much can happen during this period.  First, we must protect the developing seed from insects such as corn earworm and stink bugs.  After R6, these two pests are not as much of a concern, but defoliators such as soybean looper can still remove green leaf area, which I’ve already stressed, is vital for maximum yield.  We also need to be concerned about foliar disease, which can also rob us of green leaf area

Foliar Fungicides for Soybeans

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Ten years ago, few farmers considered applying foliar fungicides for soybeans.  These crop protection products had their home with higher-value crops such as vegetables and peanuts, where a return on investment was more likely.  But with soybean futures topping $16, more growers are using fungicides to protect their soybeans from common diseases and hopefully to increase yields.

The question that is still plaguing nearly everyone is will the fungicides pay for themselves this year?  The answer is really quite simple – We Don’t Know!  Why don’t we know?  First, the fungicides that effectively control our most common soybean diseases, the strobilurin fungicides (e.g., Headline, Quadris, etc.) are preventative.  In other words, they must be applied before the disease develops.  So, we’re applying a chemical to prevent a disease that may or may not progress to yield-reducing levels.  Unfortunately, our ability to predict soybean disease development is not very good.  Therefore, we rely on results from applied research, experience, and the probability of getting a response.

How likely are we to get a response to fungicides in Virginia?  We regularly test fungicides on soybean and have built up a fairly large database.  While disease incidence is reduced with fungicide application, a significant yield response occurs only about 1/3 of the time.  The average yield response is 3 to 4 bushels per acre, regardless of whether we average over only experiments with significant responses or over all experiments.  We’ve seen yield increases up to 12-14 bushels, but that type of response is rare.  So, if you can average 3 to 4 bushels per acre over all acres, then a fungicide will likely pay for itself – unless we experience one of those years where disease is nearly absent (remember how dry it was last year?).  If there’s no disease, we’ve wasted our money.

Although we do not yet have an effective weather model (such as is being used in peanut) that will guarantee success, we can make an informed, although not perfect, decision.  We must remember that three conditions must be present in order for a disease to develop: 1) a susceptible host; 2) the pathogen; and 3) a conducive environment.  This concept is commonly referred to as the plant disease triangle.  If any of these three is missing, then the disease will not develop.  We have the host – soybean.  And we usually have some yield-robbing pathogens present – especially the Cercospora species.  But the variety used must be susceptible to the pathogen.  This includes two things: genetics and stage of soybean development.  From the genetic standpoint, we have many varieties with resistance to frogeye leaf spot (Cercospora sojina), probably the most damaging disease that is annually present in Virginia.  So, if you’re using a frogeye leaf spot resistant variety, response to fungicides is less likely.  Certain varieties are more susceptible to cercospora blight (Cercospora kikuchii); some are less susceptible.  But few have true resistance.  Cercospora blight is likely most responsible for yield loss to to foliar disease and the reason that fungicides prevent soybean yield losses in Virginia.  Regarding stage of development, soybean disease usually appears after full flower (R2) and the soybean are just beginning to pod (R3).  This can be attributed to a larger canopy at this stage (which is related to the environment side of the triangle – see below discussion), but may also be related to physiological changes taking place in soybeans as plants transition from vegetative to reproductive growth.  So if the pathogen is present on a susceptible host, there is a greater likelihood that a disease will develop.  Still, the environment has to be right.

What is the right environment?  Temperatures ranging from 60 to 85 F in periods with moisture provided by rainfall, dew, or high relative humidity is usually required.  The greater number of days that these conditions are met, the more disease there will be. The weather service is pretty good at predicting temperatures, but not so great with rainfall.  Additionally, even if rainfall is predictable, the environment within the soybean canopy (micro-environment) is usually what matters most.  A tall soybean crop that has completely closed its canopy and is growing in good soil moisture will result in a micro-environment more conducive to disease development than a short crop that has not closed the canopy and is growing in a relatively dry soil.

In summary, all three conditions shown on the disease triangle must be present for a disease to develop to yield-robbing levels.  We do not yet have a good model to predict common soybean diseases – but we are working on it.  Until such a model is developed, Dr. Pat Phipps and I suggest the following guidelines on whether or not to spray a foliar fungicide on soybeans:

  • Soybeans are growing well, have a full canopy, and are in the R3 (beginning pod) to R5 (beginning seed) stage.  Research indicates that applying fungicides at R3 is better than later stages.  Note that most fungicides cannot be legally applied after they have reached the R6 (full-seed) stage.
  • Daily air temperatures averaging between 60 and 77 F and accumulations of rainfall were ≥ 0.5 inches in the previous 5 days or ≥ 1.0 inches in the previous 10 days, or periods of relative humidity were ≥ 95% for ≥ 12 hours per day.  The greater number of days that these conditions are met, the greater likelihood for a response to fungicide application.

When soybeans were $5 to 6 per acre, very little foliar fungicide was being applied to soybean.  However with today’s prices, yield is king; therefore, a small yield increases resulting from an external input such as foliar fungicides can be cost effective.

Corn earworm moths already showing high levels of pyrethroid resistance in Virginia

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From Ames Herbert via the Virginia Pest Advisory (http://www.sripmc.org/Virginia/)


As of the end of this week (June 29, 2012) we have tested 372 corn earworm (CEW) moths for pyrethroid susceptibility and have a season average of 31.2% surviving the AVT (adult vial test) challenge (see the attached line graph). We had one sample with over 40% survivorship. These are high numbers for the beginning of the season and compare pretty well to what we had at this time in 2011, if not a bit higher. What does this tell us? We cannot claim pyrethroid resistance based on this kind of random survey of moths, but historically, when we see survival numbers of about 25-30% or higher, we can expect some pyrethroid control problems, especially if moth fights are heavy, and the weather turns dry. That combination would almost guarantee control problems. But, if CEW populations reach only low to moderate numbers and the season continues to get plenty of rainfall, field failures will not be nearly as common. With loss of Larvin, an effective non-pyrethroid for controlling CEW, growers will need to turn to other non-pyrethroids like Belt, Coragen (Prevathon**), Steward, or combinations that include a pyrethroid plus a non-pyrethroid either tank mixed (like a pyrethroid + Orthene) or as a product (like Besiege** which contains Karate and Coragen). (**note, the registration status of these products is not certain at this point)

Make Timely Postemergence Weed Control A Priority

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Roundup Ready and LibertyLink soybean varieties have made weed control much simpler.  In addition to being easier, we are generally doing a better job at controlling all weeds, especially those that could not be controlled well with traditional herbicides.  Perennial weed control is especially better with glyphosate.  However, weed resistant to glyphosate continues to get worse.  To date, we have glyphosate resistance in marestail (horseweed), lambsquarters, Palmer amaranth, common ragweed, and johnsongrass in Virginia.  To date, no resistance has been documented for glufosinate (Liberty or Ignite herbicides).  I strongly encourage using preemergence residual herbicides with glyphosate and glufosinate, even in double-cropped soybean.  With glyphosate, tank-mix partners are also in order due to the increasing weed resistance issue.

It is worth noting that glufosinate will not kill as big of weeds as glyphosate, nor is it as effective on grasses.  To me, this may be an advantage of the LibertyLink program.  Why?  First, it almost requires one to put down a preemergence herbicide, if nothing else to buy one a little time for the subsequent postemergence glufosinate application.  Secondly, you generally need to make the glufosinate application before the weeds reach 4 inches in height (height requirements depend on the weed to be controlled; see the label for weed-specific height requirements).  While these two “advantages” may not seem to be advantages, they will help insure that weeds do not become resistant to glufosinate.  In addition, if we let the weeds grow in competition with the soybean much longer, we’ll lose yield (see the discussion below).

On the other hand, since glyphosate will kill larger weeds that glufosinate, we tend to wait too late to spray.  It has been shown that spraying weeds that are too large can encourage herbicide resistance.  That in itself should make timely weed control a priority.  But other than herbicide resistance, I see two main problems with relying on glyphosate weed control systems.  First, we tend to spray later than we should because we want to kill all of the flushes of weeds with one spray.  This is a mistake!  As I said, glyphosate will kill larger weeds than most of our other postemergence herbicides.  So, why not try to wait and only spray once?  While that approach will save time and money if two applications would otherwise be necessary, it could be costing you more money in the long run by letting weeds rob the crop of essential light, water, and nutrients.  The figure below shows the effect of weed competition with soybean as related to application timing and row spacing.  In general, we need to spray our herbicides within 2 to 3 weeks after the weeds emerge.  If we don’t, we’ll suffer the consequences of lower yields and possibly less control.

The second problem is related to the first.  Weeds become more difficult to control the bigger they are, even with glyphosate.  Most of our control failures (and glyphosate resistance problems) are due to allowing weeds to get too large.  Weeds such as marestail, lambsquarters and morningglory are much more difficult to control when they are large.  Again, spray within 2 to 3 weeks after weed emergence.

Plant As Soon As Possible For Maximum Double-Crop Yields

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Early planting and good stands are the foundation of high double-crop soybean yields.  Small grain harvest is very early this year.  If we can get soybeans planted by the first week or two of June, there will be very little yield loss due to late planting.  Good wheat and soybean yields plus high prices can make the wheat/soybean cropping system the most profitable in Virginia.

I stress that it is very important that we forge ahead with soybean planting.  After mid-June, we lose about ½ bushel per acre per day with every day delay in planting.

This is not to say that double-crop yields will always be less than full-season yields.  Rainfall during pod and seed development is most important.  For two of the last three years, double-crop yields were equal to or greater than full-season yields.  But, we can’t count on late-season rains every year.  Therefore, it is important that we get the double-crop soybean set up for maximum yields.  That means planting as soon as possible to maximize leaf area production.

For those of you that do not have adequate soil moisture, planting into dry soils is OK, but only if the soil is completely dry.  If the soil has enough moisture to swell but not germinate the seed, the seed may die.  If the soil has enough moisture to initiate germination, but not enough to send a root into soil moisture, it will definitely die.  Considering the cost of seed, my suggestion is not to plant into these dry soils; wait for a rain.  Only if you are sure that there is absolutely no soil moisture in the field and you have hundreds (or thousands) of acres to plant, would I suggest planting into dry soils.  At $40 to 50 per bag, we don’t want to waste any seed.

Keep in mind that my seeding rate recommendation for double-crop soybeans is to plant enough seed to insure a final plant population of 180,000 plants per acre.  Assuming 80% emergence, that’s 225,000 seed per acre (~75 lbs of seed).  Do not attempt to lower seeding rates in double-crop settings. Furthermore, if planting moves into July, we’ll need to increase that seeding rate to 250,000 seed per acre.  On droughty soils and July planting, yield increases will take place up to nearly 275,000 seed per acre.

Slugs Remain in High-Residue Fields

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I never expected to be writing an article on slugs in soybean in June , but this year is continuing to bring surprises.  After a generally warm winter and spring, we’ve suddenly cooled off in the first week of June.  And the slug activity in fields with a lot of residue has picked up.  Usually, our slug problem disappears by the time warmer temperatures set in.

The photo to the right was taken 5 days after planting soybean in a no-till field in Suffolk that contains lots of corn and rye cover crop residue.  Stand was only about half of what was expected and feeding scars could be seen on the hypocotyl and cotyledons.  When I dug in the seed furrow, I found slugs more often than not.  Since it was a cloudy afternoon when I was in the field, I even found slugs feeding on soybean plants that were still partially covered by residue (slugs usually only feed at night).  In an adjacent field that had not yet been planted, I found slugs on the underside of corn residue.

Will the soybean crop survive this late-season infestation?  If the crop has emerged and has a couple of leaves on it, I’d say it will.  But if you’re just planted or are now planting soybean and you’re finding slugs, I’d suggest using slug bait/molluscicide.  The only one available (other than in small packages in home improvement/gardening stores) is Deadline®, which contains the active ingredient, metaldehyde.  It is sold at Deadline®M-Ps™ Mini-Pellets (colored with a blue dye) and Deadline®Bullets (dye-free).

What if you have damage and are considering replanting?  Usually, severe damage is only in part of a field, usually wetter areas.  If this is the case and the damage is severe, you may want to consider replanted these areas.  But if it’s the entire field, I’d suggest evaluating the stand of undamaged or slightly-damaged plants, and then following my guidelines for replanted poor stands (in this issue).

For more info, see Purdue Univ. website:

http://extension.entm.purdue.edu/fieldcropsipm/insects/soybean-slugs.php or

Ohio State’s pub, Slugs on Field Crops at

www.Ohioline.osu.edu/ent-fact/pdf/0020.pdf.

Marestail Management in Wheat (Chris Drake, Southampton County Agent)

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Over the last couple weeks, I am seeing a lot of Marestails poking through the top of wheat fields throughout the county. This is a major concern for the farmers that have to manage this weed in their bean crops. Once these weeds get the point of being 30 inches or greater in height, control is difficult with any herbicide.

There are a couple options for control after the wheat harvest. One is the use of mechanical cultivation prior to planting beans. Disking, turbo-tilling, or running a dyna-drive will eliminate these weeds that are present. This option does not appeal to many producers due to moisture loss and conservation tillage program payments. The second option is using glufosinate (Liberty formerly called Ignite). An application of Liberty should control or suppress these weeds, but at these sizes two applications may be necessary for complete control. A second application of Liberty will not be possible unless Liberty Link beans are planted. In either scenario, the use of a residual material at planting such as Envive, Prefix, or Boundary is strongly encouraged.

What is causing these escapes? Two things are the likely culprit. One is the lack of use of a burndown/ tillage before planting wheat and two is inadequate weed control in the growing season. Harmony is widely used for weed control in wheat but is not a great product on Marestails. Many applications were made after the weeds were 8 inches in height or greater. Harmony will simply not control this species at this height.

So, what do we do next year to prevent these problems? BURN DOWN the land before planting wheat if you don’t till before planting. Also, in season Marestail control can be obtained using 2-4D after wheat is fully tillered but before jointing. Scouting your fields in late February or early March when you start to see winter annual weeds may be necessary to determine if control measures are needed.

Early-Season Insect Pests

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Potato Leafhopper. This insect overwinters in gulf-coast states and migrates northward each year, typically arriving in Virginia between late April and early June. Adults and nymphs injure the plant by inserting their piercing-sucking mouthparts into plant tissue and removing liquids. High populations can result in visual injury (cupping of leaves) and under drought conditions, can stunt growth, Injury is more severe on varieties with little leaf pubescence. But, the injury will not necessarily result in yield loss. Very dry conditions will increase injury and likelihood of yield loss. The insect can be controlled with pyrethroid insecticides.

Thrips may be the most abundant insect pest species on soybean.   But, the feeding alone will not usually cause yield reduction.  Under favorable environments, soybean will outgrow thrips damage.  However, if high numbers of thrips coincide with droughty conditions early in the season (seedling plants), then growth can be severely stunted and yield loss might occur.  Thrips feed by rupturing the cell walls of leaf cells and sucking the exudates.  Leaves will take on a silvery appearance from thrips feeding.  The insect can be controlled with insecticides from several chemical classes.  Early-season control can be obtained with insecticide seed treatments.  Ames Herbert is updating thrips counts in cotton and other crops on a regular basis in his Virginia AG Pest Advisory found at http://www.sripmc.org/Virginia/.

Bean leaf beetle is a common pest through all soybean production areas and has become more of a concern in the Midwest in recent years.  These beetles are defoliating insects, whose injury is easily recognized by small round holds between major leaflet veins.  The insect can also feed on the surface of soybean pods, leaving the seed vulnerable to excess moisture and secondary pathogens.  The insect can feed all year, but most concern is during the early vegetative stages.  However, soybean can normally grow out of this injury, without yield loss.  This insect can transmit the virus, bean pod mottle virus.  However, viruses have not traditionally been a problem in soybean.  There is resistance and/or tolerance in many varieties.  However, we suspect that some newer varieties have less tolerance.  The insect can be controlled with insecticides from several chemical classes.

Soybean Aphid.  Soybean aphid is a relatively new pest, first discovered in Virginia just 10 years ago.  It feeds by sucking plant sap, which can cause leaf curling and plant stunting and pod abortion.  At high levels, yield can be seriously reduced.  While an early-season pest in the Great Lake states, it has never occurred in Virginia before July, and rarely before August.  In addition, it only reaches threshold levels on relatively few acres in Virginia each year.  We only mention this pest here because some companies are promoting early-season control of aphid with soil insecticides.  Although soil insecticides may provide some control to seedling soybeans, this is not an issue in Virginia.  Management of this pest depends on regular scouting and applying insecticides when threshold levels are reached (250 aphids/plant before R5).

White Grubs.  With less tillage and more residue buildup on our soils, grubs have become more of a concern.  White grub damages soybean by feeding on soybean roots, killing young plants, and reducing stands.  Insecticide seed treatments have some, but limited effect on grub.

Wireworms.  As the name implies, wireworms are wire-like worms that feed on soybean seed, preventing germination.  This leads to poor and spotty stands when populations are high.  They may also feed on the underground base of the plant.  Later, they may feed on roots.  To determine if wireworms are a problem, bait stations can be employed.  Seed treatments are effective against wireworm.

Lesser cornstalk borer can be a problem on seedling soybean; problems on older soybean are infrequent.  Outbreaks are more likely under hot, dry conditions and in sandy fields with weedy hosts.  Larvae of this insect bore into the main stem at or just below the soil surface.  Numerous seedlings can be injured by a single larva.  Seedlings can be cut off at the soil surface or the tunneling can cause wilting and death.  Surviving plants may lodge and be lower yielding.  Insecticide seed treatments or other applications are not effective.

Insecticide seed treatment to soybean is of limited value in Virginia.  Seed treatments can reduce feeding by some species of insects early on the season, for the first 3 to 4 weeks after plant germination.  However, we do not typically treat for insects early, nor is there data to support the value or need.  Early season insects include thrips (various species) and bean leaf beetle.  Ames Herbert, Extension Entomologist, spent several years doing tests across the state trying to determine the value of treating for thrips and was never able to find a yield advantage.  Bean leaf beetle can feed on seedling plant leaves, but he has never seen a yield reduction from the feeding.  In the north central US, growers use seed treatments to reduce first generation soybean aphid.  In Virginia, we do not see aphids until late July or August long after any seed treatment would be out of the plant system.  Seed treatments may have some utility for wireworms and grubs.

Kudzu Bug.  Although not necessarily an early-season insect, this new pest is showing up early this year just to our south.  Little is known about this insect, but we are learning quickly.  This insect was discovered in Georgia in 2009, moved into South Carolina in 2010 and through North Carolina in 2011.  It was also found in Patrick County, Virginia in 2011.  As of May 2012, it has already been found in at least six N.C. counties and in Greensville County, VA.  It feeds on wide range of legume hosts including kudzu, wisteria, some vetches, and soybean.  It has several generations per year, moving from sheltered areas such as bark or rocks in the winter to kudzu and then on to soybeans.  Like an aphid, it has piercinig and sucking mouthparts, therefore does its damage by sucking juices and nutrients from the plant.  Of the studies conducted in 2010 and 2011, it has reduced soybean yield by an average of 21%.  We need to track this insect, so timely spray recommendations can be implemented.  So if you see this insect, please notify your County Extension office or you can contact Ames Herbert directly at the Tidewater AREC.  You will be hearing more about this insect, so stayed tuned.