Yield Loss from Defoliators in Soybean and Insecticide Termination

Now that early planted beans are drying down quickly and insect pressure is increasing on late planted soybeans in some areas, many are asking when it is safe to quit spraying insect pest in soybeans. For most pest in soybean the general rule is R7 growth stage for insecticide termination. R7 is one brown pod anywhere on the plant and occurs approximately 12-21 days after R6 stage depending on planting date and maturity group. This is important to know, since it gives you an idea of how long you will need to protect foliage after R6. I have received numerous calls about loopers and bean leaf beetles in R6 stage soybeans and wondering whether or not they need to treat.

Defoliator Complex: Bean Leaf Beetle, Soybean Looper, Velvetbean Caterpillar, Green Cloverworm, etc.

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. Another important finding is that yield loss can occur even as late as R6 stage from foliage loss, however, it takes at least 50% defoliation before we start seeing significant yield loss. Once we get 7-10 days past R6 it would be very difficult to show a yield reduction from defoliating pest of soybean. For example, looking at the graph below, 60% whole plant defoliation at R6 would result in a 10% yield loss.

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Stink Bugs

Stink bugs cause the most yield loss during the R5 growth stage. They will feed on developing seed and cause shriveled seed that can result in yield loss. After R6 stage is reached, the biggest concern with stink bugs is quality. It is not uncommon for stink bug stink bug numbers to reach very high levels after R6 and can still affect quality even at the R7 growth stage. We recommend at a minimum doubling the threshold at R7. In research conducted by Dr. Fred Musser on the quality affects of stink bugs at R7 growth stage, it typically took 3-4 x thresholds to have an affect that caused dockage at the elevator and in some cases even at 3-4X threshold there was no effect on quality. The point is quality loss from stink bugs is real and can happen but its not very predictable. Also, keep in mind the preharvest (PHI) intervals for soybean insecticides when making an application past R6.