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Lensing --Basic information regarding soybean aphid outbreaks Tuesday, June 23, 2009
By Jerome Lensing
Pioneer area agronomist
While there are no known outbreaks of soybean aphids at this time, we want to remind you of some basic information regarding SBA. Most SBA insecticides are labeled for application along with the second shot of glyphosate. While the two products are labeled for this type of application, you don't want to spray for SBA if levels are still at sub-economic levels.
SBA prefers cooler/drier conditions for giving birth to six to eight live young nymphs per day. SBA populations were at economic levels by mid-July 2008 along hot-spot overwintering sites in the Minnesota River valley. Due to the cooler growing season in 2008, SBA populations rebounded with vengeance by mid-August last year.
This created the situation where many fields across southern Minnesota needed a second SBA insecticide application. Some strip trials and yield monitor data showed up to 10 bushel per acre advantages with a second SBA application a year ago.
While we don't know what weather conditions we'll experience this growing season, we have good data and experience to predict SBA population dynamics depending on the temperature curve and insecticide products used.
The "shotgun" approach of indiscriminating spraying soybean acres should be avoided. Scout field by field and use the soybean antibiosis rating Pioneer breeders have assigned our varieties to help you in prioritizing which Pioneer brand varieties could be more or less attractive to SBA migrating populations.
Although sometimes debated by some sources, the standard threshold that most Midwest Extension Services share is when you observe 250 SBA per plant on 80 percent of the plants. The critical time to scout for SBA is when soybeans are in the R3-R5 (early pod fill to pod filling stages).
Late July to early August has historically been the window for SBA population increases and need for treatment.
The University of Minnesota developed a speed scouting method to reduce the complexity of actually having to count out the number of aphids per plant. The Web page link to this speed method is http://www.soybeans.umn.edu/pdfs/2007/aphid/speed_scouting_2007.pdf
Quick facts on corn growth
• A new leaf will fully expand roughly every three days during early stages of growth or about every 82 GDUs up to V10. From V11 to final leaf stage, a new leaf will emerge every 50 GDUs. A field is defined as being at a "given" growth stage when over 50 percent of the population shows the given leaf collar being counted.
• The plant height method of staging growth will most often be between one to two leaf stages greater than the collar method, simply because several leaves are counted prior to a visible collar.
• Prior to the V6 stage (collar method); the growing point is still below or just at the soil surface.
With the plant height method, simply measure from the ground to the arch of the uppermost leaf that has its tip pointing downward. The problem with this method is that height doesn't accurately indicate the growth stage of the plant. Cooler weather and shorter season hybrids will tend to produce shorter plants with the same amount of collars. This could create some off-label herbicide applications on corn and increase the risk of crop injury.
Hoop method for soybean stand counts: Sometimes it is difficult to count plants by row -- particularly in solid-seeded soybeans. The hoop method usually works well.
Make your own hoop. Use anhydrous or other stiff hose using a hose barb and a couple of clamps: This has the advantage of allowing you to predetermine the size of the hoop, making the math conversion easier.
For example, if you want to create a hoop that is 1/10,000 of an acre (4.356 square feet) you need a hoop with a circumference of about 89 inches. Connect a length of hose of about 89 inches and you have a hoop of 1/10,000th acre. Count the plants inside the hoop and multiply by 10,000 and you have plants per acre.
Sidedressing Nitrogen: These days, growers are trying to find ways to cut fertilizer costs while maintaining optimum corn yields. Split nitrogen application is a way to do both, plus it results in better timing and plant uptake of nitrogen. The ideal timing for supplemental applications of nitrogen is when corn is between V4-V6 (6-12 inches).
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