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Soybean Aphid Thresholds: When does It Pay To Invest In Management Of This Insect?

The tiny soybean aphid has been bothersome to U.S. soybean farmers since it was first detected in the United States about 13 years ago. Left untreated in soybeans, this native-Asian insect can transmit viruses, as well as spread sooty mold, and cause yellow and wrinkled leaves, stunted plants and aborted pods, all of which can result in significant yield reduction.

After its rapid spread across most areas of the Midwestern soybean-growing region, researchers established an economic threshold for management of this insect. In 2005, they recommended that farmers treat their fields when infestations reach an average of 250 aphids per plant, when 80 percent of the plants have aphids, and if aphid populations were increasing.

In a recent webcast, Kelley Tilmon, Ph.D., soybean extension specialist at South Dakota State University, said she’s often asked if the economic threshold should be lower since soybean prices are now twice as high as when that threshold was developed.

To answer this question, Tilmon first explains the following:

  • An economic threshold is the point where farmers make a decision to invest in management of an insect, disease or other pest to keep it from reaching the economic injury level (EIL).
  • It is sometimes known as the “action threshold,” since this is when farmers should begin to take action against the insect or disease.
  • The EIL is when the crop value lost to plant injury caused by the pest equals the cost of the treatment.
  • The EIL depends on crop values and management costs.

Using the original economic threshold levels, which includes older economic variables, the EIL was approximately 675 aphids per plant; leaving seven days to treat the crop. According to Tilmon, the EIL is now around 450 aphids per plant because of higher crop values and higher control costs. If farmers use the same action threshold with the new EIL, they now have only about three or four days to treat their crop.

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