Soybean Aphid Populations Reaching and Exceeding Thresholds

We are starting to get reports of soybean aphid populations that are exceeding the economic threshold of 250 soybean aphids per plant in parts of South Dakota. So far, the areas where spraying for aphids is occurring are located in the northern counties, but we are also observing populations that are near threshold near Brookings and Volga. Soybean aphid scouting should be conducted throughout the eastern side of the state to ensure that economic loss due to the soybean aphids doesn’t occur. Soybean aphids that reach and exceed the threshold can be successfully treated through the R5 growth stage. Once soybeans reach the R6 growth stage (full seed) there generally isn’t a yield benefit to reducing the soybean aphid populations. In addition, many insecticides have a pre-harvest interval of 21-30 days, which may delay harvest if they are applied later than R5.

 

Soybean aphids can be scouted for using two methods. Both of these methods are based on research that has determined that the economic threshold for soybean aphids is 250 soybean aphids per plant. Although economic thresholds generally fluctuate with the market value of soybean and the expense associated with insecticide application, the 250 threshold remains relevant due to a lack of observable yield loss when soybean aphid populations that are less than 500 per plant. The 250 threshold provides a seven-day window for insecticide application before observable yield loss will occur.

 

The issue with the 250 threshold is that it requires quite a bit of time commitment for each field. To use this method, 20-30 plants should be chosen at random in a large U or X pattern in the field. Examine each plant and record the number of soybean aphids present. By examining plants spread throughout the field, soybean aphid “hot spots” won’t lead to a treatment decision when the overall threshold hasn’t been reached. The decision to treat occurs when 80% of the plants inspected have 250 or more soybean aphids on them.

 

An alternative to this is referred to as Speed Scouting. This method of scouting was developed due to the time intensive nature of traditional scouting. Speed scouting only requires 40 soybean aphids per plant to be counted, and requires fewer plants. To use the speed scouting method, print out a special speed scouting chart, or download the Aphid Speed Scout app. Scout a broad portion of the field, just as you would for traditional scouting. If there are fewer than 40 soybean aphids on the entire plant, put a minus (-) on the speed scouting chart line for plant 1. If there are more than 40, stop counting and put a plus (+) on the line for that plant. Walk 30 rows or paces before examining a second plant, and continue this for a total of 11 plants. Add up the pluses. If there are 6 or fewer plants with over 40 aphids (infested), resample the field in a week. If 7 to 10 plants were infested then sample 5 more plant and continue sampling according to the directions on the sheet until a decision can be made.