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Catchot, A.

Terminating Insect Sprays in Mississippi Soybeans

We are quickly approaching that time of year where some of the earlier planted fields are turning color and others are approaching R6 very quickly. Often times, many of the insect pests we deal with “pile in” about this time and growers face the tough decision to treat when it may not pay back or try and race them to the end. We have devoted a lot of time and effort in recent years to evaluate and validate thresholds in soybeans under the current growing practices in Mississippi. Through these research efforts, we have made several changes to insect thresholds and termination points for insect pest of soybeans in MS.

Growth stage R6.5 is the time we are recommending terminating all insect sprays in soybeans (unless there is some extraordinary circumstance or rare event that cannot be accounted for). The question is what is R6.5 really? It is easy to identify the R6 growth stage (when the beans have completely filled up the inside of the pod) and we know through years of research by Dr. Larry Heatherly that the time between R6 and R7 is roughly 14-20 days across numerous maturity groups and planting dates, therefore R6.5 is roughly 7-10 days past R6.

Foliage Feeders: This group includes anything that feeds on leaves. Soybean loopers, velvetbean caterpillars, green cloverworm, bean leaf  beetle, grasshoppers, etc. Terminate all treatments for foliage feeders at R6.5. In fact, research at LSU indicates it is in fact safe to apply a desiccant at R6.5 Harvest aids in indeterminate and determinate soybeans – application timing and value. This further supports our position that terminating treatment for foliage feeding insect pests will not result in yield loss.

Seed Feeders: This group includes primarily stink bugs and bollworms. Extensive work has been done on late season stink bug damage and yield loss at Mississippi State University. Essentially yield loss in terms of bushels per acre from stink bugs is over at R6. An R6 soybean that is punctured by a stink bug will not shrivel to nothing but rather can cause small necrotic lesions which could be a deduct at the elevator. However, stink bug dockage at the elevator is not nearly as common as folks may think. In a 5 year study by Dr. Fred Musser at MSU, in only one year did we ever see a quality dockage with populations as high as 6X threshold (54 per 25 sweeps). Those numbers did not even result in dockage in 4 out of the 5 years. I am not playing down the reality that it can happen under certain environmental conditions, but it is not as common as many think. This is why we recommend doubling the threshold at R6 and terminating sprays 7-10 days later. The question is often asked, “what if rain sets in and the pods are full of stings, can disease be worse?” In short, yes. Diseases do in fact like avenues of entry and we had a student work on this very thing. He did show that fungi were more likely to enter a seed that had punctures verses those that did not. However, in bad years like 2009 even non punctured seed rotted.

Decisions to terminate sprays are not taken lightly. We feel that the research strongly backs these recommendations and that late sprays are not offering returns. Please call if you have any questions any time.

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