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Maturing Soybean Defoliation and Insect Feeding: How Much Matters?

Maturing Soybean Defoliation and Insect Feeding: How Much Matters?
By John Tooker
 
In the past week or so, we have seen an increase in calls asking about the potential yield effects of insect pests feeding on maturing soybean plants, particularly those in growth stages R5 plants or later. The general response to this question is that the window of opportunity for insects to cause yield loss this time of year slowly closes as plant maturity approaches. Around this time of the season, soybean yields are established in plants and, generally speaking, there is not that much that insects can do to reduce that yield. As soybeans progress through the remainder of the season, growers should keep plant growth stage in mind before making any management decisions.
 
For example, let’s consider general defoliation of soybeans; this damage can be caused by a suite of insects including bean leaf beetle, Japanese beetle, grasshoppers, and caterpillars. Accepted economic thresholds for leaf-feeding pests in soybeans are: 25% defoliation during vegetative stages; 15% defoliation for bloom to pod-fill; and 25% pod-fill to maturity. Remember that a synonym for economic threshold is action threshold, meaning that when these levels of defoliation are found it is time to do something to stop that damage. However, late in the season other details need to be considered, particularly the growth stage of plants, the general health of the bean, and the cost of doing something. If soybeans are in R5 and defoliation is around 20% across an entire field, I would hesitate to recommend an insecticide treatment because that field will be in R6 soon and yield will be even better established, despite relatively high levels of defoliation. Moreover, it is likely that driving through the field to apply the spray will cause a greater yield loss than the spray will potentially save. Driving down the crop becomes even more difficult to justify if the crop looks like it is thriving—deep green color and large healthy plants.
 
Similar reasoning can be applied to soybean aphid, which is unlikely to influence soybean yield after growth stage R6. If aphids have reached the economic threshold of 250 aphids per plant and the field is in the R5 growth stage, I would bet that treating that field would not provide much of an economic return because R6 is approaching and there is only a small amount of time for that insecticide to provide a benefit. Keep in mind that the economic threshold of 250 aphids is the time to act to avoid aphid populations from reaching the economic injury level (EIL), which is when plants will start to lose yield; for soybean aphids the EIL is close to 1000 aphids per plant. So the question to ask is: will that aphid population increase around four times before the plants hit R6? I think the chances are small, but they are not zero, which is why continued scouting should be part of any decision process. Continue to check the field and to try to determine the cost of driving through the beans with a sprayer. And of course, the case for not spraying for aphids becomes even easier to justify when lady beetles and other predators start colonizing fields to feast on those pesky aphids.
 
One final detail to keep in mind: control decisions may be different for a pest like brown marmorated stink bug or even grasshoppers, both of which can feed directly on mature pods. This sort of damage will only be detected by scouting and if it is occurring heavily, the argument for protecting plants with a spray becomes easier. Good luck with your decisions!