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High commodity prices coupled with receding flood waters have some growers pushing their soybean planting dates well into late July. As these soybeans emerge and develop they will be greeted by an increasing population of soybean aphids. Dr. Eileen Cullen UW Field Crops Entolomogist suggests the following plan of action to manage the soybean aphid on late planted soybean.

From Dr. Eileen Cullen:

For vegetative stage soybeans, if aphids are colonizing, do not treat the soybeans below 250 aphids/plant. Data from yield loss experiments in WI and throughout the North Central region show that there is no yield benefit to decreasing the threshold, even on small soybeans (Journal of Economic Entomology 100: 1258-1267). Keep in mind also that 250 aphids/plant is not the Economic Injury Level (EIL). The EIL is the number of aphids that need to be present for the value of the lost yield to equal the costs of control. There is lead time of 7 days to reach that +/- 674 aphids per plant EIL.

Now that the market value for soybeans has risen, a lowered EIL can be calculated. David Ragsdale, the lead author on the paper describing the existing EIL for soybean aphids, calculated a new EIL for soybeans selling at $15 per bushel, with $8 per acre control cost, and an anticipated yield of 50 bushels per acre. With these values the EIL is lowered to 452 aphids per plant. However, this doe not lead to a decrease in the Economic Threshold of 250 aphids/plant.

Entomological research in six states (including WI) from 19 yield-loss experiments conducted over a 3-yr. period found no detectable yield differences attributed to treating soybean aphids at numbers below the Economic Threshold of 250/plant. Additionally, treating soybean aphids below the ET of 250 aphids/plant increases risk to producers by treating an aphid population that is growing too slowly to exceed the Economic Injury Level, eliminates generalist predator insects, and exposes a larger portion of the soybean aphid population to selection by insecticides, which could lead to development of insecticide resistance.

If the ET of 250 is reached/exceeded on vegetative stage soybeans, treatment is warranted on the double-cropped beans, due to winged soybean aphid dispersal flights and potential outbreak.conditions in affected areas such as Western Wisconsin. Continue to scout soybeans following treatment to watch for recolonization and soybean aphid resurgence.

Economic threshold is being exceeded in many fields in Western Wisconsin. Winged soybean aphids are dispersing from southern MN and westerly, northwesterly winds have the potential to move dispersal flights toward WI. (This is largely happening in parts of Western WI now). For full information on Soybean Aphid Scouting and management recommendations, please vist the Wisconsin Crop Manager Newsletter article “Soybean Aphid Increasing and Variable – Scout Fields Now” at http://ipcm.wisc.edu/WCMNews/tabid/53/EntryID/606/Default.aspx

Image 1. Winged soybean aphid colonizing V2 soybean plant.Image 2. Soybean aphids feeding on V2 soybean plant.

Photos by John Gaska

Tim Wood, the Director of the Lancaster Research Station, reported today (7/25/08) that a few wheat samples have tested positive for DON (vomitoxin) in SW WI. These samples were collected from wheat fields where low levels of Fusarium head blight (FHB) were noticed prior to harvest. The DON levels in the positive samples were 2.6 and 3.0 ppm. A usual cutoff for DON levels is 2 parts per million or lower in wheat. A local elevator was assessing dockage fees of $0.25/bu for DON levels over 2.0ppm.

(Photo courtesy of Laura Sweets)

As a point of reference the UW Winter Wheat Variety Trial located at the Lancaster Research station was our most uniform and prevalent FHB site. Our ratings at Lancaster showed a FHB index range from 0.6 to 8.8 among the varieties. This FHB index is based on a range from 0 to 100 where 0 indicates no signs of scab infection and 100 means all heads in the field are completely infected. The 0.6 rating was found in the public variety “Truman”, which has resistance to head scab.

(Photo courtesy of Laura Sweets)

As we continue with wheat harvest and grain delivery remember that having FHB does not automatically mean that the grain will have a concentration of DON, and second, there is evidence that healthy looking kernels can test positive for DON. Therefore, if you are concerned that there is mycotoxin contamination, consult the Pest Management in Wisconsin Field Crops – 2008 (UW-Extension, A3646) in the corn disease section where a list of laboratories that conduct mycotoxin tests is listed. DON infected wheat can be blended with non-infected wheat to reduce the overall concentration.

Please contact Paul Esker if you learn of positive finds in your county at: pde@plantpath.wisc.edu or 608-890-1999.

Across many areas of Wisconsin the soybean crop is just starting to bloom. According to the USDA crop progress report (7/6/08) the WI crop is slightly behind the 5 year average for bloom progress. This is due to two factors: fewer accumulated GDU’s in May and June and later soybean plantings. As we enter the soybean reproductive growth phase there are a few things to keep in mind. The first is that soybean will produce flowers for ~3 to five weeks, depending upon planting date and environment. Soybean will abort anywhere from 20 to 80% of the flowers that they produce. Generally it is the first and last flush of flowers produced that are most likely to be aborted.

Second of all the timing window for our glyphosate applications in soybean are quickly closing. Chris Boerboom, UW Extension Weed Scientist states that:

“Monsanto has clarified the definition of “throughout flowering” in regard to the latest stage that glyphosate can be applied to glyphosate-resistant soybean. Soybean flowering is defined as the R2 growth stage. The R3 growth stage begins when one of the four top nodes with a fully developed leaf has a 3/16 inch long pod. With this definition, glyphosate can be applied through the R2 growth stage, but not after the R3 stage begins. Applications made after the R3 stage are off-label applications.”

On average it takes ~ 4 days to move from R1 (beginning flower) to R2 (full flower) and ~10 days from R2 to the start of R3 (beginning pod). That means we have ~2 weeks for glyphosate applications to occur.

Lastly, wheel track damage made from ground applications may start to reduce yield. Sprayer wheel traffic from first flower (R1) through harvest can damage soybean plants and reduce yield (Hanna et al. 2008). Our research suggests that an adequate soybean stand (more than 100,000 plants per acre) planted in late April though mid-May can compensate for wheel tracks made when a field is sprayed at R1. Yield loss can occur, however, when wheel tracks are made at R1 or later in thin soybean stands (less than 100,000 plants per acre) or late planted soybeans. Regardless of stand, plants could not compensate for wheel tracks made at R3 (early pod development) or R5 (early seed development). The average yield loss per acre is based on sprayer boom width (distance between wheel track passes). In our trials yield losses averaged 2.5, 1.9, and 1.3% when sprayer boom widths measured 60, 90, and 120 foot, respectively. Multiple trips along the same wheel tracks did not increase yield loss over the first trip.

For more information on spraying fungicides in soybean please see my Extension article entitled: Managing Fungicide Applications in Soybean.

Image 1. Soybean crop beginning to bloom.

When scouting your soybean fields over the next few weeks pay close attention to any elevated ant activity as an indicator of soybean aphid arrival (Image 1). Krista Hamilton (DATCP Entomologist) reported finding soybean aphids in Columbia and Dodge counties last week. This morning John Gaska and Branden Furseth (UW Soybean Extension and Research Program) found soybean aphids in our soybean seed treatment experiment at the Arlington research station (Image 2) as well as at our soybean research trials located at the West Madison research station. Fall suction trap counts suggest that this may likely be an off year for aphids, however remember to scout and be vigilant as some fields may still reach threshold. For more information on soybean aphids please visit the Wisconsin Crop Manger newsletter or the Soybean Aphid publication authored by Dr. Eileen Cullen (WI Extension Field Crop Entomologist).

Image 2. Soybean aphids located at new soybean growth.