Soybean aphid populations have not yet “crashed”. As of the week of August 18th, soybean aphid populations have continued to flourish and increase beyond threshold in some areas of Wisconsin.
Soybean Aphid Update for Late August 2008
Eileen Cullen, Extension Entomologist, UW-Madison
Soybean aphid populations have not yet “crashed”. As of the week of August 18th, soybean aphid populations have continued to flourish and increase beyond threshold in some areas of Wisconsin. Telephone calls over the last two days included R3 soybeans near West Bend (Washington County) with 500-1000 aphids/plant; R4 and R5 soybeans in western Wisconsin exceeding 500 aphids/plant, and finally a call this morning from Price County in Northern WI, soybeans well into R5 stage (R5.5) with seeds nearly filling the pods at the upper most four nodes of the plant; and over 1,000 aphids per plant.
FAQs about soybean aphid management in August
Q1: “What is the difference between the ‘white dwarf’ aphid morph and normal green colored soybean aphid?”
Soybean aphid populations statewide are now comprised of the typical green soybean aphid color morph, the "white dwarf" morph, or both forms on the same plant. The white aphid morph is smaller than the green color form. These are both soybean aphid. Research by Dr. David Ragsdale, University of Minnesota, has demonstrated that SBA "white dwarfs" live half as long and their lifetime nymph production is reduced 70%, compared to the normal green soybean aphid color morph. White dwarfs are formed in response to host plant quality during R3-R4 when host plant quality declines. During this time, white dwarf nymphs reproduce more slowly and are less likely to exceed Economic Threshold of 250 aphids/plant. Dr. Ragsdale reports that if white dwarves persist, they will begin to reproduce at a faster rate once late R5 stage is reached, when the soybean plant photosynthate is being mobilized in the lower leaves and used to fill seeds.
Q2: “Soybean Aphid populations continue to exceed threshold the 3rd week in August, should I treat?”
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Q3: “Spraying decisions past R4 or Aug 15th need to be clearly defined because some economic injury does occur after that time frame.”
The soybean aphid economic threshold of 250 aphids per plant applies to soybean growth stages R1 (beginning bloom) through R5 (beginning seed and early pod fill). This includes the R4 growth stage of course, and there is no predictable calendar cut off date for when soybean aphid becomes non-economic. (Although that does typically occur in late August, it’s a little different each year).
Usually soybean aphids are declining by now, and most of the state’s crop is well into the R5 growth stage. Typically, it is also much hotter late in August. This year is different in two ways. First, with re-planted and late planted soybeans due to spring flooding, there is significant acreage still between R3 and R5 throughout the state. This crop stage falls squarely within the R1 through R5 soybean growth stages during which yield loss occurs if soybean aphid populations exceed the Economic Injury Level. Remember the Economic Threshold of 250 aphids/plant is set below the Economic Injury Level. In fact all insect pest economic thresholds are set below the economic injury level to prevent pest populations from reaching economic injury level. Second, it is cooler than normal for mid to late August. Soybean aphid population growth is somewhat suppressed at temperatures in the upper 90F range, and this has not consistently been the case these past couple of weeks. Upper 80s but not upper 90s.
University research data on the soybean aphid Economic Threshold (ET) and Economic Injury Level (EIL) … See Question 5 below … show that soybean aphid is not expected to cause additional yield loss after soybeans reach R5.5. This is the stage between which the seed is 1/8-inch long inside the pod at one of the four uppermost nodes on the main stem, and the R6 stage, when the pod contains a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem. UWEX Price County Agriculture Agent Mark Kopecky sent in a very good picture today from an R5.5 field with soybean aphids (Fig. 1). In this field, the canopy is closed, and pods are almost filled well into R5.5, nearing the R6 growth stage. Given the fact that sprayer wheel tracks in the field would cause a couple of bushels of yield loss, and multi-state university research data that shows it is not economic to treat at R6, this field is beyond the treatment window.
However, fields still in the R3-R5 stage exceeding economic threshold may warrant treatment as soybean aphid populations are continuing to flourish on younger plants in relatively cooler August temperatures. Continue to scout through the R5.5 stage of soybean growth.
Fig. 1. R5.5 Soybeans, August 21, 2008. Price County, WI.
Photo: Mark Kopecky, UWEX Price County
Q4: When will soybean aphid populations “crash”?
There is a seasonal decline each year, regardless of what kind of soybean aphid populations existed for the season (high or low). This seasonal decline is triggered by environmental cues (day length and temperature). The soybean aphid female colonies on soybean plants pick up these cues. The Upper Midwest is now past the critical shortening day length (mid August) and once soybean aphid experiences temperatures below 45F, the next generation will be sexual and winged. Winged aphids will appear and fly to buckthorn where mating and overwintering eggs will be deposited.
A population decline could also occur due to entomopathogenic fungi (aphid-killing fungi) or predators and parasitic wasps that can cause aphid populations to decline. These are natural controls with a population suppressive effect.
Q5: “The price of beans needs to be factored into the number some way. Do $15/$16 soybeans mean we lower the economic threshold below 250 aphids/plant?”
The Economic Threshold of 250 aphids/plant has not changed.
For an excellent, clear explanation of the Upper Midwest research data behind the SBA economic threshold of 250 aphids/plant and Economic Injury Level, please see the Slide Set below with voice narration. . It takes about 26 minutes to view the slide set. The Economic Threshold is valid through the R5 stage. The research was conducted over 3 years, in 19 yield-loss experiments, across 6 states (including WI). This data was used to develop the yield loss equation across a range of aphid densities and soybean varieties.
To access the presentation by Dr. Dave Ragsdale, Univ. of MN, at:
http://www.plantmanagementnetwork.org/edcenter/seminars/SoybeanAphid/
From the link above. Click on “Part II: Soybean Aphid: Economic Threshold and Economic Injury Level”
Another key Extension newsletter to read on this topic is from Iowa State University (July 30, 2008) titled “Reviewing Decision Thresholds for Pest Insect Control”
http://www.extension.iastate.edu/CropNews/2008/0730tollefsonriceoneal.htm
In Summary:
The Economic Threshold (ET) of 250 aphids/plant is set below the Economic Injury Level (EIL). When the research was conducted 2004, 2005, 2006, soybean prices were $5.50-$6.50. At that time the EIL, or number of aphids that need to be present for the value of the lost yield to equal the costs of control, was approximately 674 aphids/plant.
Now that market value for soybeans has risen, a lowered EIL can be calculated (based on damage relationships between soybean aphids and soybean yield over a wide range of densities, over three years across 6 states). David Ragsdale, the lead author of the Economic Threshold study (Journal of Economic Entomology 100: 1258-1267) re-calculated the EIL. For example, for soybeans selling at $15/bushel, with $8/acre control costs, and anticipated yield of 50 bu/acre. The EIL is lowered from 674 aphids/plant to 450 aphids/plant.
The ET of 250 aphids/plant is still below this revised EIL of 450 aphids per plant. In the past, the ET of 250 aphids/plant allowed 7 days lead time to treat (when population density is increasing) before reaching the EIL of 674 aphid/plant. With a lower EIL (450 aphids/plant) given current prices , this effectively cuts the lead time down to 3-4 days to treat before reaching the EIL. The ET of 250 aphids/plant has not changed. It is still below the EIL and allows a treatment window of 3-4 days.
In our research across the Upper Midwest, using the common experimental protocol detailed in the slide set above, treating below 250 aphids/plant resulted in NO detectable yield increase. 250 aphids/plant is not where injury begins, it gives you a heads up treat the field within a few days to prevent it from exceeding the revised EIL of 450 aphids/plant.