To Understand The Impact Of Defoliation On Maturing Soybean

On August 26, 2019

To Understand The Impact Of Defoliation On Maturing Soybean

In the past week or so, Extension Entomologist John Tooker reports he has seen an increase in calls asking about the potential yield effects of insect pests feeding on maturing soybean plants. This is particularly a concern for the soybeans 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, Tooker 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, Tooker would expect 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? Tooker thinks 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. 

To Understand The Status Of The Crops Across The Nation

If you travel anywhere across Pennsylvania, you will be impressed with the crops that are maturing in the fields. We have experienced an almost perfect growing season which leads to an expectation of record crops in 2019. We all know farmers in other states and folks who have traveled and the reports from many areas are very promising as well.

However, the most recent crop report from the USDA’s National Agricultural Statistics Service (NASS) forecasted soybean and corn production down from 2018. Soybean production is down 19 percent from 2018, forecast at 3.68 billion bushels; corn growers are expected to decrease their production 4 percent from last year, forecast at 13.9 billion bushels.

Area for soybean harvest is forecast at 75.9 million acres with planted area for the nation estimated at 76.7 million acres, down 4 percent from the June estimate, and down 14 percent from last year. Soybean yields are expected to average 48.5 bushels per acre, down 3.1 bushels from last year. If realized, the forecasted yield will be a record high in Arkansas, Kentucky, and Tennessee.

Average corn yield is forecast at 169.5 bushels per acre, down 6.9 bushels from last year. NASS forecasts record-high yields in Kentucky and Tennessee. Acres planted to corn, at 90.0 million, are down 2 percent from NASS’ June estimate but up 2 percent from 2018. As of August 4, 57 percent of this year’s corn crop was reported in good or excellent condition, 14 percentage points below the same time last year.

To Learn From The 2019 Barley and Winter Wheat Performance Trials

Over 80 small grain varieties from more than a dozen seed companies were evaluated in two regions of Pennsylvania in 2019.  Both malting and feed barley were evaluated, along with winter wheat in Centre and Lancaster Counties.  The results and details of the variety trials can be found here: .

Seed companies who are interested in variety evaluation for 2020 should contact Mark Antle (email: for an entry form.

Quote Of The Week: “Every father should remember that someday his sons will follow his example instead of his advice.” Amish Proverb by Suzanne Woods Fisher