Soybean Loopers Showing up in Late Soybeans:

Soybean loopers are starting to show up in high numbers in many areas of the state. In many cases, soybeans are past the point where yield loss can occur, but there are many later planted soybeans that need to be protected for a while longer. Below are a few tips on when to treat and what to treat with.

Understanding Yield Loss from Defoliation:

We recently had a student, Dr. Lucas Owen, who conducted an extensive research project on effects of defoliation on soybean yield at different growth stages and where the defoliation occurred in the plant canopy. The graph below shows expected yield loss at R3, R5, and R6 for whole plant defoliation. Whole plant defoliation is what our thresholds are based on. We looked at the effects of the defoliation in the top half or bottom half only of the plant and they were not significantly different than each other. In fact, if you had 100% defoliation in the top of 50% of the plant, the yield was the same as 50% defoliation in the whole plant. This is important because bean leaf beetles are feeding in top half of the plant and we must remember to average it out across the whole plant, and soybean loopers generally start in the bottom half of the canopy and move up.

Terminating Defoliator Sprays:

Once we get 7-10 days past R6 (R6.5) it would be very difficult to show a yield reduction from defoliating pest of soybean even if it was 100% defoliation. At R6.5 the seed is no longer held to the pod wall with any membranes and is easily separated when shelled by hand. Therefore it is in the drying down process and is no longer getting nutrients from the pod wall, so foliage is no longer playing a role in yield. For example, looking at the graph below, 60% whole plant defoliation at R6 would result in a 10% yield loss. So even at R6 stage it takes a tremendous amount of defoliation to cause yield loss. However, high levels of defoliation between R3 and R5 stages can be catastrophic. The graph below is shown as yield loss as a percentage of the untreated check.

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Thresholds: In Mississippi we recommend treating soybean loopers when numbers reach 19 loopers per 25 sweeps 1/2″ or longer or 20% defoliation in reproductive stage soybeans.

Control Options:  Soybean loopers have a long history of resistance to pyrethroids, organophosphates, and carbamates insecticides. Until recently we relied heavily on a short list of products such as Steward or Intrepid to control this pest. These products do indeed still work but have almost exclusively been replaced by the Diamide class of chemistry because of the long residual and broad spectrum control of nearly all caterpillar pests in soybeans.  Products in the Diamide group labeled in soybeans include Belt, Besiege, and Prevathon. This year, Dow AgroScience also received registration on Intrepid Edge which is a premix of methoxyfenozide and spinetoram. These products have also shown very good performance with extended residual on soybean loopers. Each of these products work extremely well but are more costly. Most growers feel the extra cost is worth the money due to extended residual control. It is highly recommended to avoid pyrethroids and/or acephate for soybean looper control. I have seen these products actually flare loopers. On occasion, I have seen suppression when tank mixing the two together but this is a risky game to play.

Use Rates. See product labels for more information.

Intrepid 4-6 oz. per acre

Steward 5.6-11.3 oz. per acre

Belt – 2-3 oz. per acre

Besiege 7-9 oz. per acre

Prevathon 14-20 oz. per acre

Intrepid Edge 4-6 oz. per acre