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Ohio State University Extension


C.O.R.N. Newsletter 2005-19

Dates Covered: 
June 28, 2005 - July 6, 2005
Greg La Barge

Soybean Aphid Threshold and Treatment Recommendations

Authors: Bruce Eisley, Ron Hammond

We are receiving reports of soybean aphid (SA) from many areas of Ohio, ranging from difficult to find, at most a few aphids per plant, to easily finding aphids with numbers ranging from a few to 50-100 aphids per plant. Of importance is that we will be entering July when most soybeans will be starting to flower. This is growth stage R1/R2, when the plants become more susceptible to aphid injury. From flowering through the R5 stage, the threshold for treatment is an AVERAGE of 250 aphids per plant with rising population numbers. The only way to determine if aphid numbers are rising is to begin sampling weekly in your fields to see how the population changes from week to week. When the average reaches 250 aphids per plant, treatment is warranted. During the last soybean aphid outbreak in 2003, this mostly occurred in late July and early August. Whether these levels are reached sooner in 2005 is why sampling should begin within the next week. We recommend RANDOMLY sampling about 30 plants in different locations in your field, counting the number of aphids per plant, and calculating the average per plant. At this time, aphids will normally be seen on the new growth at the top of the plant; however, all aphids over the entire plant should be counted.

For those with some experience in sampling and making recommendations, we would suggest taking a look at a new technique from the University of Minnesota, Aphid Speed Sampling. This technique takes into account that when the average density of aphids is 250 per plant, that the lowest number of aphids on any of the sampled plants will be 40 or higher. Thus, if all RANDOMLY sampled plants have over 40 aphids, you can assume that the average number, if counted, would be at least 250 aphids, and that treatment is warranted. NOTE: this means that ALL plants have to have over 40 aphids; 40 aphids is NOT the average number. If you have more than 1 or 2 plants with less than 40, the assumption is that the threshold has not been reached. See the following website for more information on this technique, including the basis behind it and access to a worksheet In Ohio, we would recommend it only for those who have experience in sampling for aphids and using the regular threshold of an average of 250 aphids per plant after taking total aphid counts.

When treatment becomes warranted, there are a number of insecticides that can be used. There are a number of older materials, including Lorsban 4E at 1 to 2 pts per acre, Furadan 4F at 0.25 to 0.5 pt per acre, Penncap-M at 2-3 pts per acre, and dimethoate at various formulations and rates. Then there are the pyrethroids, Asana XL at 5.8 to 9.6 fl oz per acre, Baythroid at 2.0 to 2.8 fl oz per acre, Mustang Max at 3.4 to 4.3 fl oz per acre, and Warrior at 1.92 to 3.2 fl oz per acre.

If problems occur in early July that require treatment, and soybeans are mostly in the flowering or R2 growth stage, growers must take care because bees might be actively working a soybean field. Although bees are not usually associated with soybeans, they do forage in soybeans and problems could occur if hives are nearby. Label directions pertaining to most of the insecticides and state regulations state that the insecticide should NOT be applied to blooming or flowering crops if bees are actively working in the target area. Thus, most materials for soybean aphid control should be applied in early morning or late day when bees would not be active in fields.

Ohio regulations state in 901:5-11-02 (Trained Servicepersons, Safety and Restrictions) of the Ohio Administrative Code " No person shall: (15) Apply or cause to be applied any pesticide that is required to carry a special warning on its label indicating that it is toxic to honey bees, over an area of one-half acre or more in which the crop-plant is in flower unless the owner or caretaker of any apiary located within one-half mile of the treatment site has been notified by the person no less than twenty-four hours in advance of the intended treatment; provided the apiary is registered and identified as required by section 909.02 of the Revised Code of Ohio, and that the apiary has been posted with the name and telephone number of the owner or responsible caretaker. (16) Apply pesticides which are hazardous to honey bees at times when pollinating insects are actively working in the target area, however, application of calyx sprays on fruits and other similar applications may be made." The names and addresses of registered apiaries are available at the Ohio Department of Agriculture at 1-614-728-6270 (Division of Plant Industry – Apiaries). Growers or applicators should contact this number as early as possible prior to a potential treatment to give them enough lead time to respond to your requests. Growers should be advised to follow all label directions and state regulations; it is the soybean grower's responsibility.

Finally, we have been asked numerous times the past week what this hot and dry weather means for aphid problems. Past thoughts and discussions have suggested that the soybean aphid is a cooler temperature pest, and that hot weather will inhibit its growth. However, because (1) this insect is a relatively a new pest and there is still much to learn, and (2) it is hard to predict the weather for more than a few days at a time, we recommend continuing to weekly sample your soybeans and use the current threshold of 250 aphids per plant. We would rather see growers make their decisions to treat or not to treat based on insect numbers, rather than what weather history or predictions might say!

Two-spotted Spider Mites Diagnosis and Control

Authors: Bruce Eisley, Ron Hammond

If aphids were not enough of a problem for soybean growers, we are now receiving as many calls about two-spotted spider mites (TSSM), a problem we have not seen this early in the summer since 1988 and 1993. The cause of these mite problems is the hot and very dry conditions many parts of Ohio are experiencing. We should note that other Midwest states with similar conditions are also experiencing mite problems.

While sampling your field for soybean aphids, which should be started this week, you should also begin examining plants for two-spotted spider mites. The easiest way to locate mites is to look for telltale signs of their injury, yellow spotting or stippling on the upper side of leaves. Here are two pictures showing this stippling, both on a unifoliate leaf ( and a trifoliate leaf ( There are no thresholds available for mites on soybeans. Treatment is recommended when mites become numerous in the field and begin to cause discoloration of the plants, especially the lower leaves. If this damage is not controlled, the result can be the early death of the plant. Growers should also take into account future weather predictions, because we do know that continued hot and dry weather will aggravate the mite problem, while cool and wet weather will tend to lesson it. However, if cooler and wetter conditions arrive, we would still suggest keeping a close eye on this pest.

Although the two-spotted spider mite usually occurs later in the summer, especially along field edges, past experience has suggested that early in the summer, we can often find mite populations in spots across the field. Thus, before field edges alone are sprayed, growers should walk the field to determine if the mites are present throughout. If mites are indeed present across the field, an entire field treatment would be justified.

There are only two materials, which are currently available for mite control on soybeans, Lorsban 4E (chlorpyrifos) at 1 to 2 pt per acre, and dimethoate at various formulations and rates. Of these two, the label for Lorsban states that “When large numbers of eggs are present, scout the treated area in 3 to 5 days. If newly hatched nymphs are present, make a follow-up application of a non-chlorpyrifos product that is effective against mites”. It should be mentioned that Warrior is labeled for mites, although it is for suppression only. Also note that if treatment becomes necessary within the next few weeks, that soybeans most likely will be in the flowering stage, which brings up the same concern mentioned about bees as in the soybean aphid article. Please read the section on spraying during soybean flowering and concerns about bees in the article on soybean aphid.

What If The Field Has Both Soybean Aphids And Twospotted Spider Mites?

Authors: Ron Hammond, Bruce Eisley

One of the problems that we may encounter this year is the possibility of some fields having problems with both soybean aphid and twospotted spider mite at the same time. In these particular cases, the choice of insecticides to use becomes a little more difficult. For example: most of the pyrethroids are labeled for soybean aphid but are not labeled for the mite (as mentioned, the exception is Warrior which is labeled for twospotted spider mite for suppression only) and would not do a good job on mites. The other example is dimethoate, labeled and does as good job against twospotted spider mite but does not work as well as some other materials against soybean aphid according to previous studies from other states. The one compound that is labeled and does do a good job against both pests is Lorsban 4E. The point we are trying to make is that the pest makeup in the field will be important in deciding which compounds to use.

Don’t Worry About Soybean Rust For The Short-Term

Authors: Anne Dorrance

All of the sentinel plots from the gulf states through Ohio and Indiana are negative for soybean rust. This hot dry weather is not favorable for soybean rust development. In order for us to have rust in Ohio it has to be in high numbers some place close to here and that has not happened. A very cold winter reaching down to the gulf as well as unfavorable weather conditions for disease this spring in Florida and Georgia – this thing still has not gotten started. At this point, save your fungicide for later in the season or more likely another year.

Soybean Cyst Nematode is one of several things that we do need to worry about. Dry conditions will enhance the SCN feeding injury that soybeans receive. In years where dry conditions prevail, more stunting of plants can be observed. As you are scouting for aphids and mites, pull a few plants and look for those white pearls on the roots. Those will be the SCN females producing the next generation. If you can find SCN readily – note the size of the pockets where the plants are stunted. Check the variety, if it is a SCN resistant line – it is time to switch sources of resistance or even better, plan on a wheat crop for the winter of 2006. See more information.

Lambsquarters, Lambsquarters, And More Lambsquarters

Authors: Mark Loux

This appears to be a great year for lambsquarters, and control of this weed has been less than ideal in a number of fields across the state. Reduced postemergence activity of glyphosate on lambsquarters seems to be one of the more common issues, but it is also escaping preemergence treatments and presenting problems in non-Roundup Ready soybeans and corn. Corn is approaching or past the 6-collar stage in many cornfields, which greatly limits the selection of herbicides that can be applied for late control of lambsquarters. Table 7 on pages 74-75 of the current Weed Control Guide for Ohio and Indiana lists the maximum corn size for various herbicides and herbicide combinations. Use of drop nozzles will greatly reduce the risk of injury to large corn and increase the number of herbicide options, compared to broadcast applications.

In non-Roundup Ready soybeans, Harmony GT, Synchony, and Raptor are the only herbicides with significant activity on lambsquarters. All three are most effective on small lambsquarters, but they can sometimes prevent larger plants from further growth, even if they are not killed. The activity of Raptor will be maximized when applied with methylated seed oil, but this can also increase the injury to soybeans.

As we outlined in a previous C.O.R.N. article (Lambsquarters control issues in Roundup Ready soybeans, January 2005, lambsquarters can be a difficult weed for glyphosate to control. Size and age of the lambsquarters plants are two factors that influence control, but environmental conditions appear to have a role also. Judging by the control in our research plots and the calls about lambsquarters control, the combination of less than favorable environmental conditions this season and fairly high lambsquarters populations is resulting in reduced control with postemergence glyphosate treatments. This problem is being compounded by the delay in postemergence applications in many Roundup Ready fields (what are some of you waiting for, anyway?), and the failure to increase glyphosate rate to compensate for larger weed size. Many university studies have shown that failure to apply glyphosate before most of the weeds exceed about 6 to 8 inches in height can result in significant soybean yield loss. Therefore, those producers who delay application until soybean canopy closure or until the need for an insecticide application risk a degree of yield loss that can exceed the cost of a second glyphosate application.

Another factor in the lambsquarters control problem is the apparent evolution of lambsquarters biotypes with reduced sensitivity to glyphosate. We have identified several biotypes of this nature in greenhouse research, and we are currently conducting large-plot studies to determine the response of these biotypes to two rates of glyphosate under field conditions. In an evaluation of one of these plots 14 days after treatment, approximately 25% of the plants appeared to be surviving 0.75 lbs of glyphosate acid. However, we observed complete control with a higher rate, 1.5 lbs of glyphosate acid.

The bottom line on glyphosate and lambsquarters at this point in the season based on our observations – glyphosate rates of at least 1.1 lbs of glyphosate acid should be used where lambsquarters are less than 6 inches tall, and the rate should be increased to 1.5 lbs of glyphosate acid where plants are larger than 6 inches tall. Ammonium sulfate should be included in all applications, and a low rate of surfactant (e.g. 0.125% v/v) can be added where the quality of the glyphosate product’s adjuvant package is unknown. Where the first application results in less then acceptable control, be sure to wait until the lambsquarters recover and new growth is evident before making another glyphosate application. This can be difficult to assess under the current dry conditions, because lambsquarters that escape treatment may remain at about the same size for an extended period of time, during which the plant neither dies nor regrows. Additional application of glyphosate to plants in this condition may not provide additional control.

Management Of Postemergence Herbicides Under Dry Conditions

Authors: Mark Loux

We are sometimes amazed that postemergence herbicides are as consistently effective as they are, given the range in environmental conditions, weed sizes, and occasionally poor management under which they are used. Drought conditions probably cause more problems with postemergence herbicide activity than many other environmental conditions, and proper management of postemergence herbicides is essential when rainfall is lacking. Some points to consider for postemergence applications under dry conditions:

- size matters - smaller is better. Weeds are most likely to be effectively controlled under dry conditions when they are small.
- herbicide rate matters – where possible, increase herbicide rate to compensate for reduced activity under dry conditions, especially if weeds are large. Based on dry weather and weed size, it may be wise to increase glyphosate rates to at least 1.1 lbs of glyphosate acid in all Roundup Ready soybean fields.
- adjuvant selection matters – see product labels for information on adjuvants that is specific to dry conditions. Many ALS inhibitors, including Pursuit, Raptor, and postemergence corn herbicides such as Accent, are most effective under dry conditions when applied with methylated seed oils. Postemergence grass herbicides in soybeans, such as Select and Fusion, are typically most effective when applied with crop oil concentrate. This can be critical to maintain their effectiveness on tougher grasses (e.g. yellow foxtail, crabgrass) or when tank-mixed with broadleaf herbicides. For many herbicides, increasing the rates of nitrogen fertilizer (28% or ammonium sulfate) to the maximum allowed by the label may also improve control.
- the type and rate of ammonium sulfate (AMS) material used could matter – there is a general lack of information on the ingredients and effectiveness of some AMS replacement products. Therefore, when maintaining glyphosate activity under dry conditions and on tough weeds (see lambsquarters article in this C.O.R.N.) is critical, it might make more sense to use real AMS instead of a replacement product, and to use the maximum rate equivalent of 17 lbs of dry AMS per 100 gallons of spray solution. Note, we are not referring to blends of ammonium sulfate plus surfactant or a drift control agent, which can be supplemented with additional ammonium sulfate.
- antagonism is more likely – consider separate applications of postemergence grass and broadleaf herbicides in non-Roundup Ready soybeans. An additional reason for this – the grass herbicides are typically most effective with crop oil concentrate, but this may not be the appropriate adjuvant for the broadleaf herbicides.
- use of non-standard spray components matters – be cautious about the inclusion of nutrients such as manganese in postemergence sprays. The addition of manganese can reduce the activity of glyphosate, and this is more likely to be a problem when the weeds are large or subject to drought stress. To maintain effective weed control, make separate applications of glyphosate and manganese, and apply the glyphosate at least one day before the managanese. When mixing glyphosate and manganese is unavoidable, use a fully-chelated form of manganese, increase the glyphosate rate, and use the maximum rate of AMS (avoid AMS-replacement products).

Drought and Heat Stress Effects on Corn Yield Potential

Authors: Peter Thomison

The combination of high temperatures and inadequate moisture has created severe stress in many corn fields across Ohio. As dry weather continues, more corn fields are showing signs of moisture stress with leaf rolling evident during midday hours. In many stressed fields, soil moisture is available but it appears beyond reach of most corn roots. Current weather conditions are inhibiting root development. In addition to water deficits, high soil temperatures are limiting root growth near the soil surface. The corn canopy shades the soil surface and moderates soil temperatures, but many corn fields have yet to canopy. Plants with root systems restricted to the upper four to five inches of the soil profile are stunted. Scattered thunderstorms during the past weekend brought relief to some localized areas, but cooler weather and the possibility of more storms forecast for the coming week may ease the stress somewhat.

To estimate the impact of dry hot weather on corn yield potential, let's review the effects of moisture deficits on corn growth and development from the late vegetative stages, prior to pollination, to the dent stage of kernel development. Yield losses to moisture stress can be directly related to the number of days that the crop shows stress symptoms during different growth periods. The following summarizes findings of Iowa research by Claassen and Shaw on effects of drought on grain yields in corn. This Iowa data is widely used in estimating the potential impact of water stress on yield potential.

Vegetative Stages: Drought stress during early vegetative growth usually has a negligible impact on grain yield. However, during later vegetative stages, when kernel numbers per ear are determined, plants are more sensitive to stress. According to Claassen and Shaw's findings, four days of stress (i.e. corn wilted for four consecutive days) at the 12th-14th leaf stage has the potential of reducing yields by 5-10 percent. Kernel row numbers on the ear are determined by the 12th collared leaf stage and the potential number of kernels per row is complete about one week before silking.

Tassel Emergence: As the tip of the tassel begins to emerge from the whorl, the upper stalk internodes rapidly elongate and the ears begin to expand. Silks from the base of the ears are also rapidly elongating. Four days of moisture stress at this stage has the potential to reduce yields 10 to 25%.

Silk Emergence to Pollen Shed: At this stage, leaves and tassels are fully emerged and the cobs and silks are growing rapidly. This is the most critical period in terms of moisture use by the plant. Four days of moisture stress at this stage has the potential to reduce yields 40-50%.

Blister Through Dent Stage of Kernel Development: About 12 to 36 days after silking, the cobs, husks and shanks are fully developed and the kernels are increasing in dry weight. Moisture stress will reduce kernel fill from the ear tip down. Four days of drought at the blister stage has the potential of reducing yields 30-40%, and at dough stage, 20 to 30%.

Where dry weather has contributed to corn stands with uneven emergence, and development, yield loss may range from 5 to 20% depending on various factors such as the length of emergence delays and the percentage and distribution of later emerging plants. Where there is considerable variability in plant size, smaller, stunted plants will be at a competitive disadvantage with larger plants for nutrients, water and sunlight.

How Long Can Corn “Hang On”?

Authors: Peter Thomison

As dry, hot weather continues, many corn growers are wondering how quickly their crop is losing yield potential. Dr. Emerson Nafziger addressed this question in a recent University of Illinois newsletter article (The Bulletin, No. 14, Article 12/3 online at ) The following is an excerpt from that article.

While research tells us that short periods of dryness are most detrimental at pollination time in corn, such research uses methods of quickly withdrawing water and then supplying it quickly at the end of the defined period [this was the approach used by Claassen and Shaw to simulate drought stress - described in the previous article]. That doesn't very well duplicate what we're experiencing now in our driest areas, where stress on the crop has been increasing for weeks. Our question really is this: how quickly is the crop losing yield potential as it continues without renewal of soil water?

We don't have a good answer for this, but the crop is probably "hanging on" better than we think it is. I say that based on similar patterns in previous years, in which dry conditions were relieved and high yields followed. The crop is still growing in most fields, which indicates that it is tapping soil moisture. Soil moisture supplies are still present, though in lighter soils they might be starting to deplete. Still, there's no question that the sooner we get relief, the more certain the recovery to produce high yields.
By the numbers, plant-available soil water storage in the top 5 feet of soil ranges from about 10 to 15 inches in most of our soil types, though the extent to which the roots can tap this varies based on factors like soil compaction, claypans, and how well the roots got started. Daily water use rates approach 1/4 inch under as canopies become complete, but when the crop goes under stress and leaves roll, this indicates that stomata are closing. This decreases water loss rates. If we estimate that the crop has used half of the available soil water, has access to the rest, and is using 1/5 inch per day, then we still should have 20 days or so of water, at least adequate to keep the plant functioning.

The numbers, however, are probably a little optimistic, and it's highly likely that 3 more weeks without rain will reduce yields in most fields. This is because pollination events will occur within 3 weeks, and developmental events during pollination are more sensitive to water stress than is vegetative development.

Farm Focus Field Day July 12

Authors: Andy Kleinschmidt

On July 12, 2005, the Farm Focus Committee in conjunction with Ohio State University Extension in Van Wert will host a Field Day at the Marsh Foundation Farm. The activities for the day will focus around “Soybean Pests – Detection and Treatment.”

Featured speakers for the event are Dr. Anne Dorrance, a Soybean Disease Specialist from The Ohio State University; Dr. Ron Hammond, an Ohio State University Soybean Insect Specialist and Dr. Erdal Ozkan, an Ag Engineering Spray Systems Specialist from The Ohio State University.

Registration for the day begins at 8:00 a.m. The three speakers will be conducting 45 minute in-field workshops in three concurrent sessions beginning at 8:30, 9:30 and 10:30 a.m. These workshops are designed to show the producer how to identify and where to look for the various diseases and insect pests of soybeans including Asian Soybean Rust, Phytophthora Rot, Aphids, Bean Leaf Beetles, and many more. The sessions will also cover treatment options. The Farm Focus Committee will offer shuttles to run between sessions as needed.

Dr. Ozkan’s workshop will address sprayer setups for optimizing application coverage of fungicides and insecticides. Included as part of Dr. Ozkan’s workshop, will be several sprayer companies demonstrating their setups for applying fungicides/insecticides on soybeans. This will also include several of the new air assisted sprayers that use air to help direct the spray to the target as well as reduce drift.

The day will also include lunch provided by sponsors of the Farm Focus Field Day. An open session after lunch for questions and visits with the speakers and sponsors will round out the day. Sponsors include Ag Credit, Bayer CropScience, Dow AgroScience, GVM West Inc.(Gregson sprayers), Hardi Inc. Sprayers, Jacto Inc. Sprayers, Kennedy-Kuhn Inc.(Fast sprayers), Ohio Soybean Council, Top Air Sprayers and Williamson Insurance Agency.

The Farm Focus Field Day on July 12 from 8:00 a.m. – 2:00 p.m. is free and open to the public.

For more information about the July 12th Field Day or the September 12th Used Equipment Auction, visit the Farm Focus website at Or call the Ohio State University Extension, Van Wert County office at 419.238.1214.

OSU Weed Science Field Day – July 6

Authors: Mark Loux

The OSU Weed Science field research tour will be held at the OARDC Western Agricultural Research Station on Wednesday, July 6. The tour, which is self-directed in nature, starts at 9 am and runs until noon. OARDC Western Agricultural Station is north of South Charleston on State Route 41, approximately 5 miles south of Interstate 70. There is a cost of $5 per person for the tour.

Archive Issue Contributors: 

State Specialists: Pat Lipps & Anne Dorrance (Plant Pathology), Peter Thomison (Corn Production), Mark Loux (Weed Science), Bruce Eisley (IPM) and Ron Hammond (Entomology) Extension Educators: Roger Bender (Shelby), Steve Foster (Darke), Greg La Barge (Fulton), Howard Siegrist (Licking), Mark Keonig (Sandusky), Dusty Sonnenberg (Henry), Keith Diedrick (Wayne), Gary Wilson (Hancock), Jim Skeeles (Lorain), Harold Watters (Champaign), Alan Sundermeier (Wood), Bruce Clevenger (Defiance) and Steve Prochaska (Crawford)

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.